Sephton 1.4.1.3 Determination of Enantiomeric Ratios 18 1.4.1.4 Determination of Compound-Specific Stable Isotope Ratios of Hydrogen, Carbon, and Nitrogen 18 1.4.2 Synthesis of Meteoritic
Trang 2Edited by
Andrew B Hughes
Trang 3ISBN: 978-3-527-31243-6Demchenko, A V (ed.)Handbook of Chemical Glycosylation
Advances in Stereoselectivity andTherapeutic Relevance
2008 ISBN: 978-3-527-31780-6Lindhorst, T K
Essentials of Carbohydrate Chemistry and Biochemistry2007
ISBN: 978-3-527-31528-4
Trang 4in Organic Chemistry
Volume 1 - Origins and Synthesis of Amino Acids
Edited by
Andrew B Hughes
Trang 5A catalogue record for this book is available from the British Library.
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Trang 6List of Contributors XVII
Part One Origins of Amino Acids 1
1 Extraterrestrial Amino Acids 3
Z Martins and M.A Sephton
1.4.1.3 Determination of Enantiomeric Ratios 18
1.4.1.4 Determination of Compound-Specific Stable Isotope Ratios
of Hydrogen, Carbon, and Nitrogen 18
1.4.2 Synthesis of Meteoritic Amino Acids 19
1.5 Micrometeorites and IDPs 23
1.7 Delivery of Extraterrestrial Amino Acid to the Earth and its
Importance to the Origin of Life 24
Trang 7‘‘Terrestrial’’ Amino Acids? 442.2.1 The 21st and 22nd Genetically Encoded Amino Acids 45
2.2.2 Do other Genetically Encoded Amino Acids Await Discovery? 462.2.3 Genetic Engineering can Enlarge the Amino Acid Alphabet 472.2.4 Significance of Understanding the Origins of the
Standard Alphabet 48
2.3 What do We Know about the Evolution of the Standard Amino
Acid Alphabet? 49
2.3.1 Nonbiological, Natural Synthesis of Amino Acids 50
2.3.2 Biosynthetic Theories for the Evolutionary Expansion of the
Standard Amino Acid Alphabet 53
2.3.3 Evidence for a Smaller Initial Amino Acid Alphabet 55
2.3.4 Proteins as Emergent Products of an RNA World 56
2.3.5 Stereochemical Rationale for Amino Acid Selection 57
2.4 Amino Acids that Life Passed Over: A Role for
Part Two Production/Synthesis of Amino Acids 77
3 Use of Enzymes in the Synthesis of Amino Acids 79
Theo Sonke, Bernard Kaptein, and Hans E Schoemaker
3.6 Ammonia Lyase Processes 90
3.6.1 Aspartase-Catalyzed Production ofL-Aspartic Acid 91
3.6.2 Production ofL-Alanine from Fumaric Acid by
an Aspartase–Decarboxylase Cascade 92
3.6.3 Phenylalanine Ammonia Lyase-Catalyzed Production of
L-Phenylalanine and Derivatives 93
3.7 Aminotransferase Process 94
3.7.1 Aminotransferase-Catalyzed Production ofD-a-H-a-Amino Acids 97
Trang 84.1.1 Importance of b-Amino Acids and their Biosynthesis 119
4.1.2 Scope of this Chapter 119
4.2 Biosynthesis of b-Amino Acids 120
4.2.1 Biosynthesis of b-Alanine and b-Aminoisobutyric Acid 120
4.2.1.1 b-Alanine 120
4.2.1.2 b-Aminoisobutyric Acid 122
4.2.2 Biosynthesis of b-Amino Acids by 2,3-Aminomutases from
a-Amino Acids 122
4.2.2.1 b-Lysine, b-Arginine, and Related b-Amino Acids 124
4.2.2.2 b-Phenylalanine, b-Tyrosine, and Related b-Amino Acids 127
4.2.2.3 b-Glutamate and b-Glutamine 132
4.2.2.4 b-Leucine 132
4.2.2.5 b-Alanine 132
4.2.3 Biosynthesis of a,b-Diamino Acids from a-Amino Acids 132
4.2.3.1 General Biosynthesis of a,b-Diamino Acids 132
4.2.3.2 Structures and Occurrence of a,b-Diamino Acids in Nature 132
4.2.3.3 Biosynthesis of Selected a,b-Diamino Acids 135
4.2.3.3.1 Biosynthesis of b-ODAP 135
4.2.3.3.2 Biosynthesis of the a,b-Diaminopropanoic Acid Moiety in the
Bleomycins 136
4.2.3.3.3 Biosynthesis of the Penicillins 137
4.2.3.3.4 Biosynthesis of the Capreomycidine Moiety in Viomycine 137
4.2.3.3.5 Biosynthesis of the Streptolidine Moiety in Streptothricin F 137
4.2.4 Biosynthesis of a-Keto-b-Amino Acids from a-Amino Acids 139
4.2.5 De Novo Biosynthesis of b-Amino Acids by PKSs 139
4.2.5.1 Introduction 139
4.2.5.2 General Biosynthesis of Polyketide-Type b-Amino Acids 141
4.2.5.3 Structures and Occurrence of Polyketide-Type b-Amino Acids
in Nature 142
4.2.5.4 Biosynthesis of Selected Polyketide-Type b-Amino Acids 149
4.2.5.4.1 Long-Chain b-Amino Acids Occurring as Constituents of
the Iturins 149
4.2.5.4.2 Biosynthesis of the Ahda Moiety in Microginin 151
4.2.5.4.3 Biosynthesis of the Ahpa Residue in Bestatin 151
4.2.5.4.4 Biosynthesis of the Adda Residue in the Microcystins 152
4.2.6 b-Amino Acids Whose Biosynthesis is Still Unknown 152
4.3 Conclusions and Future Prospects 154
References 155
Trang 9Acids found in Natural Product Peptides 163
Stephen A Habay, Steve S Park, Steven M Kennedy,
and A Richard Chamberlin
6 Synthesis ofN-Alkyl Amino Acids 245
Luigi Aurelio and Andrew B Hughes
6.1 Introduction 245
6.2 N-Methylation via Alkylation 246
6.2.1 SN2 Substitution of a-Bromo Acids 246
6.2.2 N-Methylation of Sulfonamides, Carbamates, and Amides 2496.2.2.1 Base-Mediated Alkylation of N-Tosyl Sulfonamides 2496.2.2.2 Base Mediated Alkylation of N-Nitrobenzenesulfonamides 2506.2.2.3 N-Methylation via Silver Oxide/Methyl Iodide 252
6.2.2.4 N-Methylation via Sodium Hydride/Methyl Iodide 2536.2.2.5 N-Methylation of Trifluoroacetamides 257
6.2.2.6 N-Methylation via the Mitsunobu Reaction 257
6.3 N-Methylation via Schiff s Base Reduction 259
6.3.1 Reduction of Schiff s Bases via Transition
Metal-Mediated Reactions 259
6.3.2 Reduction of Schiff s Bases via Formic Acid: The Leuckart
Reaction 260
6.3.3 Quaternization of Imino Species 261
6.3.4 Reduction of Schiff s Bases via Borohydrides 263
6.3.5 Borane Reduction of Amides 264
6.4 N-Methylation by Novel Methods 265
6.4.1 1,3-Oxazolidin-5-ones 265
6.4.2 Asymmetric Syntheses 272
6.4.3 Racemic Syntheses 277
6.5 N-Alkylation of Amino Acids 280
6.5.1 Borohydride Reduction of Schiff s Bases 280
6.5.1.1 Sodium Borohydride Reductions 281
6.5.1.2 Sodium Cyanoborohydride Reductions 281
6.5.1.3 Sodium Triacetoxyborohydride Reductions 282
6.5.2 N-Alkylation of Sulfonamides 282
6.5.2.1 Base-Mediated Alkylation of Benzene Sulfonamides 2826.5.3 Reduction of N-Acyl Amino Acids 283
6.5.3.1 Reduction of Acetamides 284
Trang 106.5.4.1 Asymmetric Synthesis of N-Alkyl a-Amino Acids 284
6.5.4.2 N-Alkylation of 1,3-Oxazolidin-5-ones 284
References 286
7 Recent Developments in the Synthesis of b-Amino Acids 291
Yamir Bandala and Eusebio Juaristi
7.1 Introduction 291
7.2 Synthesis of b-Amino Acids by Homologation of a-Amino Acids 2917.3 Chiral Pool: Enantioselective Synthesis of b-Amino Acids from
Aspartic Acid, Asparagine, and Derivatives 298
7.4 Synthesis of b-Amino Acids by Conjugate Addition of Nitrogen
7.5 Synthesis of b-Amino Acids via 1,3-Dipolar Cycloaddition 312
7.6 Synthesis of b-Amino Acids by Nucleophilic Additions 316
7.6.1 Aldol- and Mannich-Type Reactions 316
7.6.2 Morita–Baylis–Hillman-Type Reactions 321
7.6.3 Mannich-Type Reactions 324
7.7 Synthesis of b-Amino Acids by Diverse Addition or
Substitution Reactions 328
7.8 Synthesis of b-Amino Acids by Stereoselective Hydrogenation
of Prochiral 3-Aminoacrylates and Derivatives 330
7.8.1 Reductions Involving Phosphorus-Metal Complexes 331
7.8.2 Reductions Involving Catalytic Hydrogenations 333
7.9 Synthesis of b-Amino Acids by use of Chiral Auxiliaries:
Stereoselective Alkylation 334
7.10 Synthesis of b-Amino Acids via Radical Reactions 338
7.11 Miscellaneous Methods for the Synthesis of b-Amino Acids 340
7.13.3 Representative Experimental Procedure: Synthesis of b3-Amino Acids
by Conjugate Addition of Homochiral Lithium
N-Benzyl-N-(a-methylbenzyl)amide 352
7.13.4 Representative Experimental Procedure: Synthesis of Cyclic and
Acyclic b-Amino Acid Derivatives by 1,3-Dipolar Cycloaddition 353
Trang 11Butoxycarbonylamino-3-phenylpropionic Acid Isopropyl Ester using
a Mannich-Type Reaction 354
7.13.6 Representative Experimental Procedure: General Procedure for the
Hydrogenation of (Z)- and (E)-b-(Acylamino) acrylates by ChiralMonodentate Phosphoramidite Ligands 354
7.13.7 Representative Experimental Procedure: Synthesis of Chiral
a-Substituted b-Alanine 355
7.13.8 Representative Experimental Procedure: Synthesis of Chiral b-Amino
Acids by Diastereoselective Radical Addition to Oxime Esters 357References 358
8 Synthesis of Carbocyclic b-Amino Acids 367
Loránd Kiss, Enik}o Forró, and Ferenc Fülöp
8.1 Introduction 367
8.2 Synthesis of Carbocyclic b-Amino Acids 368
8.2.1 Synthesis of Carbocyclic b-Amino Acids via Lithium Amide-Promoted
Conjugate Addition 369
8.2.2 Synthesis of Carbocyclic b-Amino Acids by Ring-Closing
Metathesis 371
8.2.3 Syntheses from Cyclic b-Keto Esters 372
8.2.4 Cycloaddition Reactions: Application in the Synthesis of Carbocyclic
b-Amino Acids 375
8.2.5 Synthesis of Carbocyclic b-Amino Acids from Chiral Monoterpene
b-Lactams 377
8.2.6 Synthesis of Carbocyclic b-Amino Acids by Enantioselective
Desymmetrization of meso Anhydrides 378
8.4 Enzymatic Routes to Carbocyclic b-Amino Acids 393
8.4.1 Enantioselective N-Acylations of b-Amino Esters 394
8.4.2 Enantioselective O-Acylations of N-Hydroxymethylated b-Lactams 3948.4.3 Enantioselective Ring Cleavage of b-Lactams 395
8.4.4 Biotransformation of Carbocyclic Nitriles 396
8.4.5 Enantioselective Hydrolysis of b-Amino Esters 396
8.4.6 Analytical Methods for the Enantiomeric Separation of Carbocyclic
Trang 12en-7-one (223) by the Addition of Chlorosulfonyl Isocyanate
to Cyclopentadiene 399
8.6.3 Synthesis of b-Amino Ester Ethyl
cis-2-aminocyclopent-3-enecarboxylate Hydrochloride (223a) by Lactam Ring-Opening
Reaction of Azetidinone 223 400
8.6.4 Synthesis of Epoxy Amino Ester Ethyl (1R,2R,3R,5S
)-2-(tert-butoxycarbonylamino)-6-oxabicyclo[3.1.0]hexane-3-carboxylate
(225) by Epoxidation of Amino Ester 224 400
8.6.5 Synthesis of Azido Ester Ethyl (1R,2R,3R,4R
)-4-Azido-2-(tert-butoxycarbonylamino)-3-hydroxycyclopentanecarboxylate (229)
by Oxirane Ring Opening of with Sodium Azide 401
8.6.6 Isomerization of Azido Amino Ester to Ethyl (1S,2R,3R,4R
)-4-Azido-2-(tert-butoxycarbonylamino)-3-hydroxycyclopentanecarboxylate(230) 401
8.6.7 Lipase-Catalyzed Enantioselective Ring Cleavage of
4,5-Benzo-7-azabicyclo[4.2.0]octan-8-one (271), Synthesis of (1R,2R)- and
(1S,2S)-1-Amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic Acid
Hydrochlorides (307 and 308) 402
8.6.8 Lipase-Catalyzed Enantioselective Hydrolysis of Ethyl
trans-2-aminocyclohexane-1-carboxylate (298), Synthesis of (1R,2R)- and
(1S,2S)-2-Aminocyclohexane-1-carboxylic Acid Hydrochlorides
(309 and 310) 404
References 405
9 Synthetic Approaches to a,b-Diamino Acids 411
Alma Viso and Roberto Fernández de la Pradilla
9.1 Introduction 411
9.2 Construction of the Carbon Backbone 411
9.2.1 Methods for the Formation of the Cb–CcBond 411
9.2.1.1 Reaction of Glycinates and Related Nucleophiles with Electrophiles 4119.2.1.2 Dimerization of Glycinates 416
9.2.1.3 Through Cyclic Intermediates 417
9.2.2 Methods in Which the Ca–CbBond is Formed 420
9.2.2.1 Nucleophilic Synthetic Equivalents of CO2R 420
9.2.2.2 Electrophilic Synthetic Equivalents of CO2R and Other Approaches 4239.2.3 Methods in Which the CbCb0or CcCc0Bonds are Formed 424
9.3 Introduction of the Nitrogen Atoms in the Carbon Backbone 425
9.3.1 From Readily Available a-Amino Acids 425
9.3.2 From Allylic Alcohols and Amines 427
9.3.3 From Halo Alkanoates 428
9.3.4 From Alkenoates 429
9.3.5 Electrophilic Amination of Enolates and Related Processes 431
9.3.6 From b-Keto Esters and Related Compounds 433
9.4 Conclusions 433
Trang 139.5.1 (SS,2R,3S)-(
þ)-Ethyl-2-N-(diphenylmethyleneamino)-3-N-(p-toluenesul-finyl)-amino-3-phenylpropanoate (14b) 434
9.5.2 Synthesis of Ethyl (2R,3R)-3-amino-2-(4-methoxyphenyl)
aminopentanoate 32a via Asymmetric aza-Henry Reaction 434References 435
10 Synthesis of Halogenated a-Amino Acids 441
Madeleine Strickland and Christine L Willis
10.4.1 Halogenated Aspartic and Glutamic Acids 463
10.4.2 Halogenated Threonine and Lysine 465
References 466
11 Synthesis of Isotopically Labeled a-Amino Acids 473
Caroline M Reid and Andrew Sutherland
11.1 Introduction 473
11.2 Enzyme-Catalyzed Methods 473
11.3 Chiral Pool Approach 477
11.4 Chemical Asymmetric Methods 483
11.5 Conclusions 488
11.6 Experimental Procedures 489
11.6.1 Biocatalysis: Synthesis of [15N]L-amino Acids from a-Keto Esters
using a One-Pot Lipase-Catalyzed Hydrolysis and Amino AcidDehydrogenase-Catalyzed Reductive Amination 489
11.6.2 Chiral Pool: Preparation of Aspartic Acid Semi-Aldehydes as Key
Synthetic Intermediates; Synthesis of Methyl
(2S)-N,N-di-tert-butoxycarbonyl-2-amino-4-oxobutanoate fromL-aspartic Acid 48911.6.3 Asymmetric Methods: Asymmetric Alkylation Using the Williams
Oxazine and Subsequent Hydrogenation to Give the
a-Amino Acid 490
References 491
Trang 1412.2.2.1 Introduction of the Side-Chain 497
12.2.2.2 Modifications of the Side-Chain 500
12.5.1 Side-Chain Introduction with a Phase-Transfer Catalyst 516
12.5.2 Introduction of Nitrogen Through an Oxazolidinone Enolate
with a Nitrogen Electrophile 517
12.5.3 Asymmetric Hydrogenation with Knowles Catalyst 518
12.5.4 Asymmetric Hydrogenation with Rh(DuPhos) Followed by
Enzyme-Catalyzed Inversion of the a-Center 519
References 520
13 Synthesis of g- and d-Amino Acids 527
Andrea Trabocchi, Gloria Menchi, and Antonio Guarna
13.1 Introduction 527
13.2 g-Amino Acids 528
13.2.1 GABA Analogs 528
13.2.2 a- and b-Hydroxy-g-Amino Acids 534
13.2.3 Alkene-Derived g-Amino Acids 539
Trang 1513.3.2 d-Amino Acids as Reverse Turn Mimetics 554
13.3.3 d-Amino Acids for PNA Design 562
13.3.4 Miscellaneous Examples 564
13.4 Conclusions 566
References 567
14 Synthesis of g-Aminobutyric Acid Analogs 573
Jane R Hanrahan and Graham A.R Johnston
14.1 Introduction 573
14.2 a-Substituted g-Amino Acids 575
14.3 b-Substituted g-Amino Acids 579
14.3.1 Pregabalin 581
14.3.2 Gabapentin 584
14.3.3 Baclofen and Analogs 584
14.4 g-Substituted g-Amino Acids 592
14.4.1 Vigabatrin 597
14.5 Halogenated g-Amino Acids 599
14.6 Disubstituted g-Amino Acids 600
14.6.1 a,b-Disubstituted g-Amino Acids 600
14.6.2 a,g-Disubstituted g-Amino Acids 601
14.6.3 b,b-Disubstituted g-Amino Acids 604
14.6.4 b,g-Disubstituted g-Amino Acids 604
14.7 Trisubstituted g-Amino Acids 604
14.8 Hydroxy-g-Amino Acids 605
14.8.1 a-Hydroxy-g-Amino Acids 605
14.8.2 b-Hydroxy-g-Amino Acids 606
14.8.3 a-Hydroxy-g-Substituted g-Amino Acids 619
14.8.4 b-Hydroxy-g-Substituted g-Amino Acids 619
14.8.5 b-Hydroxy-Disubstituted g-Amino Acids 638
14.9 Unsaturated g-Amino Acids 640
14.9.1 Unsaturated Substituted g-Amino Acids 641
14.10 Cyclic g-Amino Acids 644
14.10.1 Cyclopropyl g-Amino Acids 644
14.10.2 Cyclobutyl g-Amino Acids 648
14.10.3 Cyclopentyl g-Amino Acids 653
14.10.4 Cyclohexyl g-Amino Acids 663
14.11 Conclusions 666
14.12 Experimental Procedures 666
14.12.1 (R)-2-Ethyl-4-nitrobutan-1-ol (36c) 666
14.12.2 N-tert-Butyl-N-(p-chlorophenylethyl) a-Diazoacetamide 66914.12.3 (R )-5-[(1-Oxo-2-(tert-butoxycarbonylamino)-3-phenyl)-propyl]-2,2-
dimethyl-1,3-dioxane-4,6-dione 670
14.12.4 (R )- and (S)-[2-(Benzyloxy) ethyl]oxirane 671
Trang 17List of Contributors
Amino Acids, Peptides and Proteins in Organic Chemistry Vol.1 – Origins and Synthesis of Amino Acids.
Edited by Andrew B Hughes
Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim
Department of Medicinal Chemistry
Victorian College of Pharmacy
381 Royal Parade
Parkville, Victoria 3052
Australia
Yamir Bandala
Centro de Investigación y de Estudios
Avanzados del Instituto Politécnico
6720 SzegedHungaryStephen FreelandUniversity of MarylandBaltimore CountyDepartment of Biological Sciences
1000 Hilltop Circle,Baltimore, MD 21250USA
Ferenc FülöpUniversity of SzegedInstitute of Pharmaceutical ChemistryEötvös u 6
6720 SzegedHungary
Trang 18Università degli Studi di Firenze
Polo Scientifico e Tecnologico
Dipartimento di Chimica Organica
Departamento de QuímicaApartado Postal 14-740
07000 México DFMéxico
Bernard KapteinDSM Pharmaceutical ProductsInnovative Synthesis & Catalysis
PO Box 18
6160 MD GeleenThe NetherlandsSteven M KennedyUniversity of California, IrvineDepartment of ChemistryIrvine, CA 92697USA
Loránd KissUniversity of SzegedInstitute of Pharmaceutical ChemistryEötvös u 6
6720 SzegedHungary
Z MartinsImperial College LondonDepartment of Earth Science andEngineering
Exhibition RoadLondon SW7 2AZUK
Trang 19Università degli Studi di Firenze
Polo Scientifico e Tecnologico
Dipartimento di Chimica Organica
Imperial College London
Department of Earth Science and
Lichtenbergstraße 4
85747 Garching bei MünchenGermany
Madeleine StricklandUniversity of BristolSchool of ChemistryCantock's CloseBristol BS8 1TSUK
Andrew SutherlandUniversity of Glasgow, WestChemDepartment of ChemistryThe Joseph Black BuildingUniversity AvenueGlasgow G12 8QQUK
Andrea TrabocchiUniversità degli Studi di FirenzePolo Scientifico e TecnologicoDipartimento di Chimica Organica
‘‘Ugo Schiff’’
Via della Lastruccia 13
50019 Sesto FiorentinoFirenze
ItalyAlma VisoCSICInstituto de Química OrgánicaJuan de la Cierva 3
28006 MadridSpainChristine L WillisUniversity of BristolSchool of ChemistryCantocks CloseBristol BS8 1TSUK
Trang 21Extraterrestrial Amino Acids
Z Martins and M.A Sephton
1.1
Introduction
The space between the stars, the interstellar medium (ISM), is composed of phase species (mainly hydrogen and helium atoms) and submicron dust grains(silicates, carbon-rich particles, and ices) The ISM has many different environmentsbased on its different temperatures (Tk), hydrogen density (nH), and ionization state
gas-of hydrogen (for reviews, see [1–3]); it includes the diffuse ISM (Tk 100 K,
nH 10–300 cm3), molecular clouds (Tk 10–100 K, nH 103–104
cm3; e.g., [4])[molecular clouds are not uniform but instead have substructures [5]– they containhigh-density clumps (also called dense cores; nH 103–105
cm3), which havehigher densities than the surrounding molecular cloud; even higher densities arefound in small regions, commonly known as hot molecular cores, which will
be the future birth place of stars], and hot molecular clouds (Tk 100–300 K,
nH 106–108cm3; e.g., [6]) Observations at radio, millimeter, submillimeter, andinfrared frequencies have led to the discovery of numerous molecules (currentlymore than 151) in the interstellar space, some of which are organic in nature(Table 1.1; an up-to-date list can be found at www.astrochemistry.net) The collapse
of a dense cloud of interstellar gas and dust leads to the formation of a so-called solarnebula Atoms and molecules formed in the ISM, together with dust grains areincorporated in this solar nebula, serving as building blocks from which futureplanets, comets, asteroids, and other celestial bodies may originate Solar systembodies, such as comets (e.g., [7] and references therein; [8]), meteorites (e.g., [9, 10]),and interplanetary dust particles (IDPs [11, 12]) are known to contain extraterrestrialmolecules, which might have a heritage from interstellar, nebular, and/or parentbody processing Delivery of these molecules to the early Earth and Mars during thelate heavy bombardment (4.5–3.8 billion years ago) may have been important for theorigin of life [13, 14] Among the molecules delivered to the early Earth, amino acidsmay have had a crucial role as they are the building blocks of proteins and enzymes,therefore having implications for the origin of life In this chapter we describedifferent extraterrestrial environments where amino acids may be present and
Amino Acids, Peptides and Proteins in Organic Chemistry Vol.1 – Origins and Synthesis of Amino Acids.
Edited by Andrew B Hughes
Copyright 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim
ISBN: 978-3-527-32096-7
Trang 24detected, their proposed formation mechanisms, and possible contribution to theorigin of life on Earth.
1.2
ISM
The search for amino acids, in particular for the simplest amino acid glycine(NH2CH2COOH), in the ISM has been carried on for almost 30 years [15–28].While in theory glycine may have several conformers in the gas phase [29], astro-nomical searches have only focused on two (Figure 1.1, adapted from [21, 29]).Conformer I is the lowest energy form, while conformer II has a higher energy, largerdipole moment, and therefore stronger spectral lines [30] Only upper limits of bothconformers were found in the ISM until Kuan et al [25] reported the detection
of glycine in the hot molecular cores Sgr B2(N-LMH), Orion-KL, and W51 e1/e2.This detection has been disputed by Snyder et al [26], who concluded that the spectrallines necessary for the identification of interstellar glycine have not yet been found
In addition, they argued that some of the spectral lines identified as glycine byKuan et al [25] could be assigned to other molecular species Further negative resultsinclude the astronomical searches of Cunningham et al and Jones et al [27, 28], whoclaim that their observations rule out the detection of both conformers I and II ofglycine in the hot molecular core Sgr B2(N-LMH) They conclude that it is unlikelythat Kuan et al [25] detected glycine in either Sgr B2(N-LMH) or Orion-KL No otheramino acid has been detected in the ISM Despite these results, amino acids wereproposed to be formed in the ISM by energetic processing on dust grain surfaces,which will then be evaporated, releasing the amino acids into the gas phase (solid-phase reactions), or synthesized in the gas phase via ion–molecule reactions (gas-phase reactions) These two processes will now be described in more detail (for areview, see, e.g., [31])
1.2.1
Formation of Amino Acids in the ISM via Solid-Phase Reactions
Several mechanisms have been proposed for amino acid formation in the ISM.These include solid-phase reactions on interstellar ice grains by energetic processing,
N
O
O
HH
H
I
NOOH
HH
H
H
II
Figure 1.1 Molecular structures of conformers I and II of glycine
in the gas phase (adapted from [21, 29]).
Trang 25which may occur in cold molecular clouds (e.g., [32] and references therein) In theseregions of the ISM, in which temperatures are very low (<50 K), atoms and molecules
in the gas phase will be accreted onto the surface of dust grains leading to theformation of ice mantles [33, 34] Diffusion of accreted atoms leads to surfacereactions, forming additional species in the ice mantles These interstellar ices aremainly composed of H2O, CO, CO2, CH4, CH3OH, and NH3, with traces of otherspecies (Table 1.2; [32, 34–38]) Once these ice grains are formed, energetic processes[e.g., cosmic rays and ultraviolet (UV) irradiation] may change the ice mantlecomposition
A range of interstellar ice analogs have been irradiated at low temperatures (10 K)
to produce a variety of amino acids Holtom et al [39] used galactic cosmic rayparticles to irradiate an ice mixture containing carbon dioxide (CO2) and methyl-amine (CH3NH2), which produced hydroxycarbonyl (HOCO) and aminomethyl(CH2NH2) radicals The recombination of these radicals would then form glycineand its isomer (CH3NHCOOH) Briggs et al [40] UV-irradiated a mixture of CO :
H2O : NH3(5 : 5 : 1) at 12 K for 24 h This resulted in the formation of an organicresidue, which included among other organic compounds, 0.27% of glycine Icemixtures containing H2O : CH3OH : NH3: CO : CO2 (2 : 1 : 1 : 1 : 1 molar composi-tion; [41]), and H2O with NH3, CH3OH, and HCN (0.5–5% NH3, 5–10% CH3OH,and 0.5–5% HCN, relative to H2O [42]) were UV-irradiated in high vacuum at below
15 K While Bernstein et al [42] obtained glycine and racemic mixtures (D/L 1) ofalanine and serine, a large variety of amino acids were found by Munõz Caro et al [41].These results were confirmed by Nuevo et al [43, 44]
The exact formation pathway of amino acids in interstellar ices is unknown, but theStrecker synthesis ([42]; for more details, see Section 1.4), reactions on the surface of
Table 1.2 Abundances of interstellar ices (normalized to H 2 O) in the
high-mass protostellar objects W33A and NGC758:IRS9, in the
low-mass protostellar object Elias 29, and the field star Elias 16.
Ice specie
W33A high-mass protostar
NGC758:IRS9 high-mass protostar
Elias 29 low-mass protostar
Elias 16 field star
Trang 26polycyclic aromatic hydrocarbonflakes [45], and radical–radical reactions [46, 47]have been proposed As radical–radical reactions can occur with almost no activationenergy [47], theoretical modeling suggests that glycine may be formed in interstellarice mantles via the following radical–radical reaction sequence:
or radical–radical mechanisms Ultimately, the need for high UV flux to produceamino acids in ice mantles contrasts with the low expected efficiency of UV photolysis
in dark molecular clouds [4] This, together with the fact that amino acids have lowresistance to UV photolysis [49], raises concerns about the amino acid formation ininterstellar ices by UV photolysis
1.2.2
Formation of Amino Acids in the ISM via Gas-Phase Reactions
Potential mechanisms alternative to solid-phase reactions include gas-phaseformation of interstellar amino acids via ion–molecule reactions Amino acids,once formed, could potentially survive in the gas phase in hot molecular cores,because the UV flux is sufficiently low (i.e., 300 mag of visual extinction) [31].Alcohols, aminoalcohols, and formic acid evaporated from interstellar ice grains(Table 1.2; [50–53]) may produce amino acids in hot molecular cores throughexothermic alkyl and aminoalkyl cation transfer reactions [52] Aminoalkyl cationtransfer from aminomethanol and aminoethanol to HCOOH can produce protonat-
ed glycine andb-alanine, respectively via the following reactions [52]:
NH2CH2OH2þþ HCOOH ! NH2CH2COOH2þþ H2O ð1:4Þ
NH2ðCH2Þ2OH2þþ HCOOH ! NH2ðCH2Þ2COOH2þ þ H2O ð1:5Þ
An electron recombination will then produce the neutral amino acids Furtheralkylation may produce a large variety of amino acids through elimination of a watermolecule [31, 52]
Trang 27Alternatively, Blagojevic et al [54] have experimentally proven the gas-phaseformation of protonated glycine andb-alanine, by reacting protonated hydroxylaminewith acetic and propanoic acid, respectively:
NH2OHþþ CH3COOH! NH2CH2COOHþþ H2O ð1:6Þ
NH2OHþþ CH3CH2COOH! NH2ðCH2Þ2COOHþ þ H2O ð1:7ÞNeutral amino acids could then be produced by dissociative recombinationreactions [54]
Independently of the mechanism of synthesis (solid-phase or gas-phase reactions),once formed, amino acids would need to be resistant and survive exposure to cosmicrays and UV radiation in the ISM The stability of amino acids in interstellar gas and
on interstellar grains has been simulated [49] Different amino acids [i.e., glycine,
L-alanine,a-aminoisobutyric acid (a-AIB), and b-alanine] were irradiated in frozenargon, nitrogen, or water matrices to test their stability against space radiation It wasshown that these amino acids have very low stability against UV photolysis.Therefore, amino acids will not survive in environments subject to high UVflux,such as the diffuse ISM This does not eliminate formation of amino acids in the ISM,but instead requires that amino acids are incorporated into UV-shielded environ-ments such as hot molecular cores, in the interior of comets, asteroids, meteorites,and IDPs
1.3
Comets
Comets are agglomerates of ice, organic compounds, and silicate dust, and aresome of the most primitive bodies in the solar system (for reviews about comets, see,e.g., [55–57]) Comets were first proposed to have delivered prebiotic molecules to theearly Earth by Chamberlin and Chamberlin [58] Since then, space telescopes (such
as the Hubble Space Telescope, Infrared Space Observatory, and Spitzer SpaceTelescope; e.g., [59–66]), ground-based observations (e.g., [67–69]), cometary fly-bys(Deep Space 1 mission, and Vega1, Vega2, Suisei, Sakigake, ICE, and Giottospacecraft missions; e.g., [70–76]), impacts (Deep Impact mission, which impactedinto the 9P/Tempel comets nucleus; e.g., [77–79]), collection of dust from the coma
of a comet (Stardust mission to comet Wild-2; e.g., [80–84]), and rendezvous missions(such as the Rosetta mission, which will encounter the comet 67P/Churyumov–Gerasimenko in 2014) advanced our knowledge about these dirty snowballs
Several organic compounds have been detected in comets (Table 1.3; for reviews,see, e.g., [7, 8, 85]) Fly-by missions have suggested the presence of amino acids oncomet Halley [71], but their presence could not be confirmed due to the limitedresolution of the mass spectrometers on board the Giotto and Vega spacecrafts Inaddition, only an upper limit of less than 0.15 of glycine relative to water has beendetermined in the coma of Hale–Bopp using radio telescopes (Table 1.3; [69]).Although several amino acid precursors (see Section 1.4), including ammonia,
Trang 28Table 1.3 Molecular abundances of ices for comets Halley, Hyakutake, and Hale –Bopp.
a Extended sources (the abundance is model dependent).
b Measured at 2.9 AU from the Sun.
c Abundance deduced from CS.
Trang 29HCN, formaldehyde, and cyanoacetylene, have been observed in the Hyakutake andHale–Bopp comets [7], only a very limited number of carbonyl compounds necessaryfor the synthesis of amino acids were detected in comets [7, 32, 86] The ultimateproof for the presence of amino acids in comets is a sample return mission such asStardust, which collected dust from the coma of the Wild-2 comet using a lightweightmaterial called aerogel [80] Analyses of comet-exposed aerogel samples show
a relative molar abundance of glycine that slightly exceeds that found in controlsamples, suggesting a cometary origin for this amino acid [83] Compound-specificisotopic analyses of glycine present in comet-exposed aerogel samples have not yetbeen performed and therefore it has not been possible to ultimately constrainits origin Other amino acids present in the comet-exposed aerogel samples includede-amino-n-caproic acid, b-alanine, and g-amino-n-butyric acid (g-ABA) The similari-
ty in the distribution of these amino acids in the comet-exposed sample, the witnesstile (which witnessed all the terrestrial and space environments as the comet-exposedsamples, but did not see comet Wild-2), and the Stardust impact location soilindicates a terrestrial origin (contamination) for these amino acids [83]
1.4
Meteorites
Meteorites are extraterrestrial objects that survived the passage through the Earthsatmosphere and the impact with the Earths surface Excepting the lunar and Martianmeteorites [87–91], all meteorites are thought to have originated from extraterrestrialbodies located in the asteroid belt (e.g., [92–98]) Although unproven, it was alsosuggested that they could have originated from comets ([99–102] and referencestherein) Meteorites can be divided into iron, stony-iron, and stony meteorites.They can be further divided into classes according to their chemical, mineralogical,and isotopic composition (for reviews, see, e.g., [103–105]) A very primitive class
of stony meteorites, named carbonaceous chondrites, has not been melted sincetheir formation early in the history of the solar system, around 4.6 billion years ago(for reviews, see, e.g., [9, 10]) Within the class of carbonaceous chondrites, there arethe CI-, CM-, CK-, CO-, CR-, CV-, CH-, and CB-type chondrites Chondrites are alsoclassified and grouped into petrographic types This refers to the intensity of thermalmetamorphism or aqueous alteration that has occurred on the meteorite parentbody, ranging from types 1 to 6 A petrologic type from 3 to 1 indicates increasingaqueous alteration A petrologic type from 3 to 6 indicates increasing thermalmetamorphism
Carbonaceous chondrites have a relatively high carbon content and can contain up
to 3 wt% of organic carbon More than 70% of it is composed of a solvent-insolublemacromolecular material, while less than 30% is a mixture of solvent-soluble organiccompounds Carbonaceous chondrites, as revealed by extensive analyses of theMurchison meteorite, have a rich organic inventory that includes organic com-pounds important in terrestrial biochemistry (Table 1.4) These include aminoacids (e.g., [106–108]), carboxylic acids (e.g., [109, 110]), purines and pyrimidines
Trang 30(e.g., [111–113]), polyols [114], diamino acids [115], dicarboxylic acids (e.g., [116–119]), sulfonic acids [120], hydrocarbons (e.g., [121, 122]), alcohols (e.g., [123]),amines and amides (e.g., [124, 125]), and aldehydes and ketones [123].
The first evidence of extraterrestrial amino acids in a meteorite was obtained
by Kvenvolden et al [121], after analyzing a sample of the Murchison meteoritewhich had recently fallen in Australia in 1969 These authors detected several aminoacids in this meteorite, including the nonprotein amino acidsa-AIB and isovaline,which suggested an abiotic and extraterrestrial origin for these compounds Sincethen, Murchison has been the most analyzed carbonaceous chondrite for aminoacids, with more than 80 different amino acids identified, the majority of which arerare (or nonexistent) in the terrestrial biosphere (for reviews, see, e.g., [107, 108]).These amino acids have carbon numbers from C2through C8, and show completestructural diversity (i.e., all isomers of a certain amino acid are present) They can bedivided into two structural types, monoamino alkanoic acids and monoaminodialkanoic acids, which can occur as N-alkyl derivatives or cyclic amino acids, withstructural preference in abundance order a > g > b Branched-chain amino acidisomers predominate over straight ones and there is an exponential decline inconcentration with increasing carbon number within homologous series
Amino acids have also been reported in several other carbonaceous chondritesbesides Murchison (Table 1.5) Within the CM2 group the total amino acid abun-dances and distributions are highly variable; Murray [126], Yamato (Y-) 74 662 [127,128], and Lewis Cliff (LEW) 90 500 [129, 130] show an amino acid distribution and
Table 1.4 Abundances (in ppm) of the soluble organic matter found in the Murchison meteorite.
Trang 34abundance similar to the CM2 Murchison While the CM2 Y-791 198 has anextremely high total amino acid concentration (71 ppm [131, 132]), which is about
5 times as high as Murchison (15 ppb), the CM2s Essebi, Nogoya, Mighei [133], AllanHills (ALHA) 77 306 [134–136], ALH 83 100 [130], Y-79 331, and Belgica (B-)
7904 [137] have much lower amino acid abundances, some being depleted in aminoacids (Table 1.5) For Essebi, Botta et al [133] consider that the high abundances(relative to glycine) ofg-ABA and b-alanine are derived from terrestrial contamination
at the fall site
CM1s chondrites were analyzed for the first time for amino acids by Botta
et al [138] ALH 88 045, MET (Meteorite Hills) 01 070, and LAP (La Paz) 0227 havetotal amino acid concentration much lower than the average of the CM2s According
to Botta et al [138], these results and the similar relative amino acid abundancesbetween the CM1 class meteorites and the CM2 Murchison are explained bydecomposition of a CM2-like amino acid distribution during extensive aqueousalteration in the CM1s meteorite parent body
The CI1 chondrites Orgueil and Ivuna have total amino acid abundances of about4.2 ppm, withb-alanine, glycine, and g-ABA as the most abundant amino acids, whileglycine and a-AIB are the most abundant amino acids in the CM2 chondritesMurchison and Murray [136] The CV3 Allende [133, 139] and the ungrouped C2Tagish Lake meteorites [133, 140] are essentially free of amino acids (total amino acidabundances of 2 and 1 ppm, respectively), with most of the amino acids probablybeing terrestrial contaminants
The highest amino acid abundances ever measured in a meteorite were found
on the CR2s EET 92 042 and GRA 95 229, with total amino acid concentrations of
180 and 249 ppm, respectively [141] The most abundant amino acids present inthese meteorites are the a-amino acids glycine, isovaline, a-AIB and alanine.The high d13C results together with the racemic enantiomeric ratios deter-mined [141] for most amino acids indicate an extraterrestrial origin for thesecompounds (see Section 1.4.1) In addition, these authors analyzed the CR1 GRO
95 577, which was found to be depleted in amino acids (1 ppb) Other CRs analyzedinclude the CR2 chondrites Renazzo [133] and Shi¸sr 033 [142] Renazzo has a totalamino acid abundance of only 4.8 ppm, which is similar to the CI chondritesOrgueil and Ivuna This meteorite has a distinct amino acid distribution, withg-ABA, glycine, andL-glutamic acid as the most abundant amino acids Only upperlimits for alanine and a-AIB were reported for Renazzo, while isovaline wastentatively identified [133] The most abundant amino acids in the Shi¸sr 033meteorite are glycine, L-glutamic acid,L-alanine, and L-aspartic acid In addition
to this, Shi¸sr 033 D/L protein amino acid ratios are smaller than 0.4 and inagreement with theD/Lamino acid ratios of Shi¸sr 033 fall-site soil These resultssuggest extensive amino acid contamination of the meteorite (see Section 1.4.1).However, Shi¸sr 033 contains a small fraction of extraterrestrial amino acids, asindicated by the presence ofa-AIB [142]
Apart from carbonaceous chondrites, amino acid analyses have also been carriedout on Martian meteorites As our present knowledge of amino acids potentiallypresent in Mars may be accessed from these meteorites (see also Section 1.6), amino
Trang 35acid analyses have been performed in the Martian meteorites EET 79 001 [143],ALH 84 001 [144], and Miller Range (MIL) 03 346 [145] In all three samples, themeteoritic amino acid distribution was similar to the one in the Allan Hills ice, whichsuggested that the ice meltwater was the source of the amino acids in thesemeteorites In addition, analysis of the Nakhla meteorite, which fell in Egypt, showsthat the amino acid distribution (including theD/Lratios) is similar to the one in thesea-floor sediment from the Nile Delta [146].
1.4.1
Sources of Meteoritic Amino Acids (Extraterrestrial versus Terrestrial Contamination)
In order to determine if the amino acids present in carbonaceous chondrites areindigenous to the meteorites, four approaches are generally applied: (i) detection
of amino acids that are unusual in the terrestrial environment, (ii) comparison ofthe absolute abundances of amino acids in the meteorites to the levels found in thefall-site environment (soil or ice), (iii) determination of enantiomeric ratios (D/Lratios), and (iv) determination of compound specific stable isotope ratios of hydrogen,carbon, and nitrogen
1.4.1.1 Detection of Amino Acids that are Unusual in the Terrestrial EnvironmentThe majority of the more than 80 different amino acids identified in carbonaceousmeteorites are nonexistent (or rare) in terrestrial proteins (for a review, see,e.g., [108]) Extraterrestrial meteoritic nonprotein amino acids such as a-AIB,isovaline, b-ABA, and b-AIB have concentrations usually in the order of a fewhundred parts per billion maximum (Table 1.5) However, Murchison [126] has ahigher abundance ofa-AIB (2901 ppb), while Murray and LEW90 500 [126, 129, 130]contain higher abundances of botha-AIB (1968 and 2706 ppb, respectively) andisovaline (2834 and 1306 ppb, respectively) The highest abundances of a-AIB,isovaline, b-ABA, and b-AIB were detected in the CR2 chondrites EET92 042and GRA95 229 [141] The CM2 Y791 198 ([131, 132]) contained similar abundances
ofa-AIB, b-ABA, and b-AIB as EET92 042 and GRA95 229, but lower abundance ofisovaline (Table 1.5)
1.4.1.2 Determination of the Amino Acid Content of the Meteorite Fall EnvironmentSamples collected from meteorite fall sites have been analyzed for amino acids andtheir distribution compared to the one from the carbonaceous chondrites Ice fromthe Antarctic regions of Allan Hills [143, 144] and La Paz [130, 147] contained onlytrace levels of aspartic acid, serine, glycine, alanine, andg-ABA (less than 1 ppb oftotal amino acid concentration) No isovaline orb-ABA was detected above detectionlimits Only an upper limit ofa-AIB (<2 ppt) was detected in the Allan Hills ice [144],while a relatively high abundance (ranging between 25 and 46 ppt) ofa-AIB wasdetected in the La Paz Antarctic ice [130, 147]
Soil samples from the Shi¸sr 033 fall site show that the most abundant aminoacids areL-glutamic acid, L-aspartic acid, glycine, andL-alanine, with nonproteinamino acids absent from the soil [142] In addition, comparison of the protein
Trang 36amino acid enantiomeric ratios of Shiịsr 033 to those of the soil (D/L <0.4) showsagreement, indicating that most of the amino acids in this meteorite are terrestrial
in origin On the other hand, a soil sample collected close to the fall site of theMurchison meteorite showed much smaller amino acid relative concentrations(glycineỬ 1) when compared to the Murchison meteorite, indicating that themajority of the amino acids present in this meteorite are extraterrestrial inorigin [133]
1.4.1.3 Determination of Enantiomeric Ratios
Chirality is a useful tool for determining the origin (biotic versus abiotic) of aminoacids in meteorites On Earth most proteins and enzymes are made of only the
L-enantiomer of chiral amino acids; however, abiotic synthesis of amino acidsyields racemic mixtures (D/L 1) If we assume that meteoritic protein amino acidswere racemic (D/L1) prior to the meteorite fall to Earth, then theirD/Lratios can beused as a diagnostic signature to determine the degree of terrestrialL-amino acidcontamination they have experienced In fact, racemic amino acid ratios for protein(and nonprotein) amino acids in carbonaceous chondrites indicate an abioticsynthetic origin Although racemic mixtures have been observed for most nonproteinchiral amino acids (Table 1.5), smallL-enantiomeric excess for some nonproteinamino acids has been reported in the Murchison and Murray meteorites [148Ờ150].Six a-methyl-a-amino acids unknown or rare in the terrestrial biosphere (bothdiastereomers ofa-amino-a,b-dimethyl-pentanoic acid, isovaline, a-methylnorva-line,a-methylnorleucine, and a-methylvaline) hadL-enantiomeric excesses rangingfrom 2.8 to 9.2% in Murchison and from 1.0 to 6.0% in Murray [148, 149] Morespecifically, Murchison has shown to have an L-enantiomeric excess of isovalineranging from 0 to 15.2% with significant variation both between meteorite stonesand even within the same meteorite stone [150] The meteoritic enantiomeric excess
of a-methyl-a-amino acids and the absence for the a-H-a-amino acids may beexplained by the resistance to racemization of a-methyl-a-amino acids duringaqueous alteration in the meteorite parent body, due to their lack of ana-hydro-gen [149, 151, 152] Another explanation could be a different amino acid formationprocess, namely pre-solar formation for thea-methyl-a-amino acids and subsequentincorporation into the parent body, followed by parent body formation of thea-H-a-amino acids [149, 152]
1.4.1.4 Determination of Compound-Specific Stable Isotope Ratios of Hydrogen,Carbon, and Nitrogen
For meteoritic nonchiral amino acids, such as glycine,a-AIB, b-ABA, and b-alanine,compound-specific stable isotope measurements are the only means to establishtheir origin The abundances of stable isotopes are expressed ind values Theseindicate the difference in per mil (ơ) between the ratio in the sample and the sameratio in the standard, as shown by:
dđơỡ ỬđRsampleRstandardỡ
Trang 37where R represents D=1H for hydrogen, 13C=12C for carbon, and 15N=14N fornitrogen The standards usually used are standard mean ocean water for hydrogen,Pee Dee Belemnite for carbon, and air for nitrogen.
Stable isotope analyses of the total amino acid fractions of the Murchison meteoriteshoweddD ¼ þ 1370½, d15N¼ þ 90½, and d13C¼ þ 23:1½ [153], which werelater confirmed by Pizzarello et al [154, 155], who obtained dD ¼ þ 1751½,
d15N¼ þ 94½, and d13C¼ þ 26½ Stable isotope analyses were obtained forindividual amino acids in different meteorites (Table 1.6; [126, 141, 150, 156–159]).These values (with a few exceptions, in which there is terrestrial contribution) areclearly outside the amino acid terrestrial range (from70.5 to þ 11.25½; [160]) andfall within the range of those measured for other indigenous polar organiccompounds present in meteorites [161] The highly enricheddD, d15N, andd13Cvalues determined for the meteoritic amino acids indicate primitive extraterrestrialorganic matter
The deuterium enrichment of amino acids is thought to be the result of interstellarchemical reactions (e.g., gas-phase ion–molecule reaction and reactions on interstel-lar grain surfaces) which formed the amino acid precursors These reactions occur inthe low temperatures of dense clouds (T< 50 K) in which deuterium fractionation isefficient (e.g., [162–164]) Meteoritic amino acids would have then been formed fromtheir deuterium-enriched interstellar precursors and deuterium-depleted water([165] and references therein) by synthesis (aqueous alteration) in the meteoriteparent body (see Section 1.4.2) However,a-amino acids are more deuterium (and
13C)-enriched than a-hydroxy acids [117], which is inconsistent with a cyanohydrin-type synthesis from a common precursor [116, 166] Differences may beexplained by different reaction paths leading to different isotopic distributions [164].The15N enrichment of amino acids is also thought to be due to chemical fraction-ation in interstellar ion–molecule exchange reactions [167, 168]
Strecker-The hydrogen isotope composition of meteoritic amino acids follows a relativelysimple pattern, in whichdD varies more with the structure of their carbon chains (dD
is higher for amino acids having a branched alkyl chain) than with the chainlength [159] On the other hand,d13C ofa-amino acids (a-methyl-a- and a-H-a-amino acids) decreases with increasing carbon chain length (with thea-methyl-a-amino acids more13C-enriched than the correspondinga-H-a-amino acids), while
d13C for non-a-amino acids remains unchanged or increases with increasing carbonchain length [158] This suggests diverse synthetic processes for meteoritic aminoacids, in particular that the amino acid carbon chain elongation followed at least twosynthetic pathways [158, 159]
1.4.2
Synthesis of Meteoritic Amino Acids
Meteoritic amino acids are thought to be formed by a variety of synthetic pathways.Namely, it is suggested thata-amino acids form by a two-step process, in which the a-amino acid precursors (carbonyl compounds, ammonia, and HCN) were present (orformed) in a proto-solar nebula and were later incorporated into an asteroidal parent
Trang 40body [169] During aqueous alteration on the asteroidal parent body, cyanohydrin synthesis would have taken place to forma-amino acids (Figure 1.2;[108,
Strecker-116, 163, 166]) Since the carbonyl precursors (aldehydes and ketones) are thought to
be synthesized by the addition of a single-carbon donor to the growing alkane chain, adecrease of thea-amino acid abundances with increasing chain length is expected.Also, synthesis of branched carbon chain analogs is expected to be favored overstraight-carbon chain analogs (e.g., [108]), and this trend is observed in the EET
92 042 and GRA 95 229 meteorites [141] Additional support for this hypothesis isthefinding of a-amino acid, a-hydroxy acids [116, 117, 166], and imino acids [170]
in carbonaceous meteorites However, non-a-amino acids cannot be produced bythe Strecker-cyanohydrin synthesis Alternatively, meteoritic b-amino acids arethought to be synthesized by Michael addition of ammonia toa,b-unsaturatednitriles, followed by reduction/hydrolysis (Figure 1.3; e.g., [108] and referencestherein) These precursor molecules have been detected in the ISM (Table 1.1 andreference therein) and also in comets (Table 1.3 and references therein) A chemicalreaction such as a Michael addition could occur on the parent body of meteorites.For example, the extensively aqueous altered CI chondrites Orgueil and Ivuna arerich inb-alanine, but depleted in a-amino acids As suggested [138], this mightindicate that the CI parent body was depleted in carbonyl compounds (aldehydesand ketones) necessary for the Strecker-cyanohydrin synthesis to occur Additionalsynthetic pathways in the meteorite parent body have been proposed for non-a-amino acids (for a review, see, e.g., [108]) For example, hydrolysis of lactamsand carboxy lactams, which have been detected in carbonaceous meteorites [125],gives the correspondingb-, g-, and d-amino acids, and dicarboxylic amino acids,respectively
R2
R1R3
CHOR2
R1
OH
OCHOR2
Figure 1.2 The Strecker-cyanohydrin synthetic pathway for the
formation of a-amino a-hydroxy and imino acids (adapted
from [116, 133, 171, 247]) R 1 and R 2 correspond to H or
C n H 2n þ 1 If R 3 corresponds to H then a-amino acids are
produced; if R 3 is an amino acid then imino acids are produced.