Amino-acids, Peptides, and Proteins Volume 14

A Review of the Literature Published During 1981

By J. H. Jones

The Royal Society of Chemistry

Copyright © 1983 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-124-1

Contents

Chapter 1 Amino-acids By G. C. Barrett,
Chapter 2 Structural Investigations of Peptides and Proteins IA : Protein Isolation and Characterization By M. D. Scawen, R. J. Sherwood, P. M. Hammond, A. Electricwala, and T. Atkinson,
Chapter 3 Peptide Synthesis By I. J. Galpin,
Chapter 4 Peptides with Structural Features not Typical of Proteins By P. M. Hardy,
Chapter 5 Chemical Structure and Biological Activity of Hormones and Related Compounds,

CHAPTER 1

Amino-acids

BY G. C. BARRETT

1 Introduction

As in the preceding volumes of this Specialist Periodical Report, the coverage of the chemical and biochemical literature on the amino-acids is intended to be thorough, but the biological aspects, such as metabolism, biosynthesis, and distribution in the bio- and geo-spheres of the common amino-acids, are only covered through representative citations.

Textbooks and Reviews. — A textbook and symposium proceedings, covering general aspects and analysis, respectively, have been published in 1981. The analytical theme also features as a component of a review of the analysis of amino-acids and other constituents of single cells. Physical and chemical properties of amino-acids and theoretical analysis of hydration of amino-acids and peptides have also been reviewed; other reviews appearing in the recent literature are cited in the appropriate sections of this chapter.

2 Naturally Occurring Amino-acids

Occurrence of Known Amino-acids. — The biosynthesis, occurrence, and properties of y-carboxyglutamic acid have been reviewed, and, on an extra-terrestrial level, the results for amino-acid content of moon soil and meteorites have been brought together.

In selecting papers from the 1981 literature for inclusion in this section, the broad routine literature on the distribution of the common amino-acids is largely excluded. As in earlier volumes, however, items of exceptional interest, usually relating to geological or extra-terrestrial sources, are cited. The distribution of common and uncommon amino-acids in the Isua rocks of south-western Greenland of age ca. 3800 My has been determined to counter a claim, based on laser-Raman spectroscopic analysis, that amino-acids in microfossil crystal regions (and therefore insulated from leaching and infusion processes) may be of ancient derivation. Since amino-acids that are unstable to geological processes were found in the crushed rock samples, it was contended that these were laid down since the last of the Isua metamorphic episodes; the extent of racemization is by no means complete, suggesting a date of a few ten thousand years and also implying that the amino-acid content is the result of continuous seepage into the rocks from lichen since the end of the last ice age. The structures of a group of amino-acids found in the Murchison meteorite have also been discussed in relation to inferences that might be drawn are they synthesized by the condensation of simple precursors under the influence of energy input and catalysts, or are they the remnants of living organisms, racemized and degraded or rearranged through the trauma undergone by the meteorite preceding its arrival on Earth? The presence of six of the seven acyclic six-carbon amino-acids in the Murchison meteorite has been confirmed, although their relative proportions differ from those for amino-acid mixtures formed through electric discharge and Fischer–Tropsch-type syntheses. The major structural type represented in the amino-acid content of this meteorite is the a-branched α-amino-acid; the positive identification of 2-amino-2-ethylbutyric acid, 2-amino-2,3-dimethylbutyric acid, pseudo-leucine (C-t-butylglycine), and 2-meth-ylnorvaline represents the first report of the ‘natural’ occurrence of these amino-acids. Leucine and isoleucine (also allo-isoleucine), reported earlier, were also confirmed to be present, and the presence of norleucine is described as ‘probable’.

Thirteen common amino-acids have been identified in the 6M hydrochloric acid hydrolysate of humic acids extracted from Spanish lignite.

Some other ‘first occurrences’ of amino-acids in plant sources have been reported. Nδ-Methylornithine has been found in Atropa belladonna and shown to be the biosynthetic precursor of atropine. N-Carboxymethyl-L-serine is present in asparagus shoots. Mugineic acid (2) (see Vol. 12, p. 3) and 2′-deoxy-mugineic acid (2) are iron(n)-chelating constituents of wheat and barley; the 2′-deoxy compound was isolated from root washings of water-cultured wheat (Triticum aestivum) under iron-deficient media. N-Malonyl-1-aminocyclo-propanecarboxylic acid has been recognized as a metabolite of 1-aminocyclo-propanecarboxylic acid in buckwheat, while α-(methylenecyclopropyl)glycine, formed from threonine and a one-carbon unit from methionine, is an intermediate in the biosynthesis of β-(methylenecyclopropyl) alanine (the toxic amino-acid, hypoglycin, present in Blighia sapida). Briefer mention can be made of other amino-acids in new locations: L-4-oxalysine in Streptomyces roseoviridofuscus, 2,4-trans-4,5-dihydroxypipecolic acid and cis -5-hydroxypipecolic acid from leaves of Calliandra angustifolia and the sap of Calliandra confusa, 4-methylene-glutamine and 4-methyleneglutamic acid in newly germinated peanut {Arachis hypogaea), γ-methyl-L-glutamic acid {erythro/threo mixture) in Mycenapura and Wynnea gigantea fruiting bodies, (2S,4S)-4-hydroxyglutamic acid as the predominant isomer of this amino-acid in Phlox species, the (2S, 4S) isomer also found there being the first finding for this isomer in this species, of which Ledenbergia rose-aenea and Pandanus veitcheii also contain 4-hydroxy-4-methylglutamic acid.

Although non-tolerant seedling roots of Armeria maritima contain 1.5 — 3.0% L-proline, copper-tolerant strains contain extraordinarily high levels (11.0%) of this imino-acid, which seems to be a feature of the tolerance and not accumulated as a response to the stress.

meso-Diaminopimelic acid predominates in the vegetative mycelium of the setamycin-producing actinomycete KM-6054, while the aerial mycelium contains predominantly the LL-diastereoisomer.

Unusual amino-acid residues in proteins include Nεn-trimethyl-lysine, in two histone-like proteins from wheat germ, and dihydroxylysinonorleucine, shown to be present in collagen when the other well established crosslinking residue pyridinoline is also present.

New Natural Amino-acids. — Several new amino-acids have been discovered in fungal and bacterial cultures, some with antibiotic properties [L-2-(1-methyl-cyclopropyl)glycine from Micromonospora miyakonensis and 4-oxo-5-amino-6 -hydroxyhexanoic acid from Bacillus cereus 102 804]. Clithioneine (3) is a new amino-acid betaine from Clitocybe acromelalga; polyacetylenic amino-acids (4) from Fayodia bisphaerigera, L-α-amino-γ-nitraminobutyric acid from Agaricus subrutilescens (β-nitramino-L-alanine is also present), and (5), a close relative of the palythene-mytilin group of amino-acid derivatives (see Vol. 12, p. 3, and Vol. 11, p. 4, respectively), are also new natural products. Root nodule hydrolysates from Lotus tenuis (hosts to two strains of Rhizobium bacteria) contain 2,4-diamino-3-methylbutanoic acid, tentatively assigned the (2 R,3S) configuration based on retention times on g.l.c. over a chiral stationary phase. N-Acetyl-L-2-amino-3-(o-tolyloxy)propionic acid has been confirmed by synthesis as a metabolite of mephenesin.

New Amino-acids from Hydrolysates. — The title of this section is to be interpreted this year to cover structure assignments to new amino-acid constituents of peptides and proteins. Micromospora chalcea produces the dipeptide N-(2,6-di-amino-6-hydroxymethylpimelyl)-L-alanine, while the novel antibiotic sulfazecin (from Pseudomonas acidophila) has the structure (6).

The new amino-acid in baker’s yeast protein elongation factor EF-2, which is ADP-ribosylated in the diphtheria toxin, has been christened diphthamide and assigned structure (7). A protein containing hypusine [Nε-(4-amino-2 -hydroxybutyl)lysine] has been isolated from human lymphocytes treated with mitogen and grown in the presence of [3 H]putrescine or [3H]spermidine.

3 Chemical Synthesis and Resolution of Amino-acids

General Methods of Synthesis of α-Amino-acids. — Standard general syntheses and newer methods of potentially general character are collected in this section, although later sections also provide details of specific synthetic objectives achieved through the use of generally applicable methods.

Amino-acids may be synthesized in satisfactory yields from ammonia and the corresponding halogeno-acid, even though textbook warnings, that mixtures of amines result from this approach, linger in the minds of all organic chemists. DL-Alanine is obtained in 76% yield when a 10:1 molar ratio of NH3 to α-chloropropionic acid is heated under pressure at 70 °C during 5 hours in aqueous solution. A related approach, reaction of an alkali-metal cyanate with the halogeno-acid ester at 80 — 100°C in dipolar aprotic solvents, has been developed further; hydrolysis with dilute hydrochloric acid completes the synthesis in this variation.

Dialkyl acylamidomalonates R1CONHCH(CO2R2) 2 continue to serve the need for reliable routes to α-amino-acids, in cases where the side chain to be introduced by alkylation of these substrates can withstand the conditions for cleavage and decarboxylation of the substituted malonate. Recent examples include a synthesis of 7-chloro-DL-tryptophan via AcNHC(CO2Me) 2CH2CH2CHO and of 4-methyleneglutamic acid from acetamidomalonate, HCHO, and diethyl oxalate.

General routes to amino-acids could not reasonably be expected to develop from the reactions based on simple reactants, which have been studied as models of prebiotic synthesis. However, glow-discharge electrolysis of aliphatic amines in aqueous solution gives amino-acids in addition to hydroxyalkylamines. While ethylamine gives glycine in this process, propylamine gives β-alanine and isoserine. Reasonable yields of α-amino-acid amides are formed from urethanes, by anodic oxidation to give α-methoxyurethanes, followed by reaction with phenyl isocyanide, then hydrolysis:

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Ugi four-component condensation synthesis is gaining in favour because of variations that simplify the release of the amino-acid from the initial condensation product. The product from N-benzyloxycarbonylglycine, 9-(amino-methyl)fluorene, benzaldehyde, and cyclohexylisocyanide carries the base-labile fluoren-9-ylmethyl group, removable with 1.1 equivalents 1,8-diazabicyclo-[5.4.0]undec-7-ene in pyridine. Use of standard conditions for the four-component condensation in the synthesis of pyridylglycine derivatives has been described.

The azlactone synthesis has been used for the preparation of metallocene analogues of phenylalanine, starting from the corresponding metallocene aldehydes. 4-Substituted oxazolin-5-ones give the corresponding 4,4-disub-stituted analogues through Michael addition to electron-deficient alkenes.

Schiff-base alkylation usually gives the α-substitution product, but the anion formed from N-benzylidenealanine benzyl ester in conditions established by Stork and co-workers in their general synthesis based on the glycine analogue (-78 °C, HMPA-THF, LDA) leads to a γ-alkylation product with t-butyl 2-bromo -3-phenylpropionate. Although the influence of steric hindrance, solvent, and other rate-controlling factors must account for the predominance of γ-alkylation in this case, this adds a further uncertainty to the method, which has already been shown to give αα-dialkylation products in various proportions, with the desired mono-alkylation product in the case of glycine Schiff bases.

Like the oxazolinones and analogous thiazolines, when considered only as the Δ-isomer (other tautomeric forms do exist), the benzodiazepinone (8) represents a cyclic SchifF base. Substitution products involving the methylene group have been reported, including 1H-2H exchanged amino-acids obtained through alkaline hydrolysis of the substituted benzodiazepinone, apparently with no isomeric substitution product or di-substitution product.

γ-Oxoalkyl-α-amino-acids (10) may be prepared through an ene reaction involving a N-toluene-p-sulphonylimide (9), followed by cleavage of the N-protecting group.

General Methods of Synthesis of β- and Higher Homologous Amino-acids. — Routes to compounds carrying amino and carboxy groups separated by two or more carbon atoms are usually based on routine methods for introducing one of these functional groups into a compound already carrying the other. The Arndt–Eistert and Wolff rearrangement procedures provide an alternative approach, employing an α-amino-acid as starting material. New routes to β-amino-acids involve addition of a vinyloxyborane to a Schiff base, aminomercuration of α β-unsaturated esters followed by demercuration

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with NaBH4, and condensation of N-thioacylurethanes with ethoxycar-bonylmethylenetriphenylphosphorane

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followed by Pt-catalysed hydrogenation. Yields in all these new methods are good or excellent. Wolff rearrangement of N-phthaloyl-D- or -L-valine (successive treatment with SOC12, diazomethane, and u.v. light) provides the β-leucine enantiomers in up to 68% yield.

Asymmetric Synthesis of Ammo-acids. — Extensions of established methods provide most of the references for this section. Enantioselective catalytic hydrogenation of α-acylamino-acrylates and -cinnamates continues to be studied by research groups already publishing prolifically in this area, which is also attracting newcomers. Enantiomeric excess levels are remarkably uniform, around 75 — 80%, in all these reports in which open-chain substrates are used (although 84 — 99% levels were reported in one study), in which there is the common feature of Rh-chiral phosphine complex homogeneous catalysis. More rigid systems (cyclodipeptides of α-aminoacrylates with an L-amino-acid) give 66 — 90% chiral induction of hydrogenation over palladized charcoal, but reductive methanolysis of 2-methyl- and 2-phenyl-4-benzylideneoxazolin-5-ones in the presence of PdCl2 and (S)-phenylethylamine gives less satisfactory results. The Z-isomer leads predominantly to the N-phenylalanine derivative through this procedure, whereas hydrogenation of Z-α-benzamidocinnamic acid in the presence of (S)-phenylethylamine gives predominantly N-benzoyl-D-phenylalanine. The chiral Schiff base formed between (S)-1-ferrocenylethylamine and pyruvic acid gives a mixture of N-substituted alanines in which the L-isomer predominates, through hydrogenation over palladized charcoal. In an alternative approach based on Schiff bases, the glycine derivative (11) yields threo-β-hydroxy-α-amino-acids of high optical purity through addition of the derived anion to an aldehyde.

Further uses of chiral dihydropyrazines (12) in the synthesis of a wide variety of α-amino-acids through enantioselective alkylation and the methyl homologue of (12) in the synthesis of chiral α-methyl-α-amino-acids have been reported (see also Vol. 13, p. 5). Condensation of (12) with PhCOMe and cleavage with HC1 gives the βγ-unsaturated D-amino-acid (13) in 64% yield. Extension of this approach to 1-chiral-substituted 4-methyl-2-imidazolin-5-ones (14), as a route to L-α-methylphenylalanine and analogues, achieves asymmetric induction at levels better than 95%.

A mixed-ligand cobalt(III) complex of D- or L-alanine with (6R,8R)-6,8-dimethyl-2,5,9,12-tetra-azatridecane, formed from the corresponding α-amino-α-methylmalonic acid complex through enantioselective decarboxylation (cf. ref. 66; Vol. 7, p. 5), has been shown to release the amino-acid under conditions that do not cause racemization (a demerit of the earlier studies of this type).

Prebiotic Synthesis; Model Reactions. — Extensions of work featuring in all earlier volumes of this series are represented in near-u.v. irradiation of a suspension of Pt on TiO2 in water under CH4 and NH3, or Pt-catalysed decomposition of H2O2 in water under CH4 and NH3, and hydrolysis of the reaction product of carbon vapour (from a carbon-arc discharge) with ammonia. The latter system creates glycine, alanine, β-alanine, N-methylglycine, serine, and aspartic acid, these last two amino-acids being formed with glycine and alanine in formaldehyde hydroxylamine reaction mixtures and arising from the reaction of the glycine with formaldehyde.

Contact glow-discharge electrolysis of amines in aqueous solutions gives mixtures of amino-acids.

Protein and Other Naturally Occurring Amino-acids. — The fermentative production and other biosynthetic routes to amino-acids continue to be well represented in the literature. Typical results cover the production of L-lysine by mutants of Bacillus licheniformis, of L-arginine by mutant organisms resistant to L-arginine analogues, and of L-aspartic acid by whole Escherichia coli cells immobilized on polyurethane, with ammonium fumarate as substrate. trans- Cinnamic acid gives L-phenylalanine in nutrient media containing L-phenylalanine ammonia-lyase. Familiar biosynthetic studies are represented in the establishment of L-pipecolic acid as an intermediate in the conversion of D-lysine into its enantiomer in Nicotiana glauca.

A route from L-glutamic acid to L-tyrosine (Scheme 1) involves a Diels-Alder addition to a vinyl sulphoxide as the key step and has been used in a total synthesis of pretyrosine (arogenic acid) and its epimer.

The first chiral total synthesis of anticapsin from L-tyrosine has been announced. Although this natural amino-acid (15) seems eminently approachable from L-tyrosine, its sensitive epoxycyclohexanone moiety rules out conventional protection strategies. The success of the present routine is due to the use of a new acid- and base-resistant N-protecting group that is inert to nucleophilic attack.

(Continues…)Excerpted from Amino-acids, Peptides, and Proteins Volume 14 by J. H. Jones. Copyright © 1983 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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