Enantiomeric excesses in meteoritic amino acids.

Gas chromatographic-mass spectral analyses of the four stereoisomers of 2-amino-2,3-dimethylpentanoic acid (dl-alpha-methylisoleucine and dl-alpha-methylalloisoleucine) obtained from the Murchison meteorite show that the L enantiomer occurs in excess (7.0 and 9.1%, respectively) in both of the enantiomeric pairs. Similar results were obtained for two other alpha-methyl amino acids, isovaline and alpha-methylnorvaline, although the alpha hydrogen analogs of these amino acids, alpha-amino-n-butyric acid and norvaline, were found to be racemates. With the exception of alpha-amino-n-butyric acid, these amino acids are either unknown or of limited occurrence in the biosphere. Because carbonaceous chondrites formed 4.5 billion years ago, the results are indicative of an asymmetric influence on organic chemical evolution before the origin of life.

Amino acids in an antarctic carbonaceous chondrite.

Amino acids have been found in aqueous extracts of a C2 carbonaceous chondrite recovered from Antarctica. The composition of the amino acids strongly suggests that they have a meteoritic origin. Comparison of these results with those obtained with other C2 chondrites supports the view that Antarctic meteorites have not been significantly altered by terrestrial processes since their fall.

Amino Acid analyses of the murchison, murray, and allende carbonaceous chondrites.

Three carbonaceous chondrites were examined for water-extractable amino acids. The Murchison Murray specimens were found to be of similar amino acid composition. This similarity suggests that these amino acids in are indigenous to type II carbonaceous chondrites. The Allende (type III) carbonaceous chondrite was found to be essentially devoid of amino acids on the basis of on identical analysis.

13C-NMR spectroscopy of acetyltyrosyl-guanidinated horse heart cytochrome c.

The tyrosine residues of guanidinated horse heart cytochrome c have been specifically acetylated by reaction with N-[1-13C]acetylimidazole (90 atom%). Acetylation was monitored by 13C-NMR spectroscopy. The tyrosine residues were found to show widely varying reactivities ranging from one that is completely and exclusively acetylated at low reagent concentration (residue 67) to one that is acetylated only when the protein is unfolded (residue 97). Homogeneous derivatives were prepared containing one (either residue 67 or 97), three 48, 67 and 74), or four (residues 48, 67, 74 and 97) O-[1-13C]acetyl groups. 13C-NMR spectra of selected derivatives were obtained at pH 5.8, in the presence of cyanide ion, in the ferrous and ferric oxidation states, and after denaturation with 6M guanidine hydrochloride. The O-[1-13C]acetyltyrosyl resonances gave chemical shift values ranging from 171.8 to 176.0 ppm. These resonances were assigned to specific groups based on the known order of reactivity of the tyrosyl side chains toward N-acetylimidazole. The chemical shift of O-[1-13C]acetyltyrosyl 67 was found to be particularly sensitive to changes in protein structure. The proximity of this group to the heme makes it subject to distance-dependent paramagnetic and ring current effects. Acetylation of tyrosyl 74 gives rise to a pH-dependent equilibrium between conformers in the ferric state and a conformation change in the ferrous state. Acetylation of this residue also leads to an absorbance decrease at 695 nm that can be related to the 13C-NMR-detected conformational equilibrium. Addition of cyanide ion abolished this equilibrium.

Ionization of tyrosine residues in horse-heart ferricytochrome c and its guanidinated and acetylated-guanidinated derivatives.

Spectrophotometric titration curves were obtained at 242 nm for native and fully guanidinated horse-heart ferricytochrome c. The cytochrome c data were fit over the pH range 9-12 (I = 0.35) by a theoretical curve with pK' values of 10.35 and 11.70. The slope of the experimental data increases sharply above pH 12.5 suggesting that two tyrosine residues with pK' values greater than 12.5 are exposed by conformation change. The guanidinated cytochrome c data after correction for the alkaline spin-state transition were fit over the entire pH range 9-13.6 (I = 0.35) by a theoretical curve with pK' values 10.37, 10.78, 11.50, and 13.60. These results along with viscosity measurements indicate that the unfolding transition occurs at higher pH in the guanidinated derivative. N-Acetylimidazole was used to acetylate specific tyrosyl groups of guanidinated cytochrome c. Assignments of acetylated tyrosine residues were confirmed by peptide mapping of 14C-labelled derivatives. Spectrophotometric titrations with rapid data acquisition of two monoacetylated derivatives allowed assignments of pK'1 (10.37) to Tyr-67 and pK'4 (13.60) to Tyr-97. The basis for the large differences in acidity and chemical reactivity of these two residues is not obvious from the crystallographic structure and may arise from differences in solvent access due to motions of the polypeptide chain.

Origin of organic compounds in carbonaceous chondrites.

Carbonaceous chondrites, a class of primitive meteorite, have long been known to contain their complement of carbon largely in the form of organic, i.e., hydrocarbon-related, matter. Both discrete organic compounds and an insoluble, macromolecular material are present. Several characteristics of these materials provide evidence for their abiotic origin. The principal formation hypotheses have invoked chemistry occurring either in the solar nebula or on the parent body. However, recent stable isotope analyses of the meteorite carboxylic acids and amino acids indicate that they may be related to interstellar cloud compounds. These results suggest a formation scheme in which interstellar compounds were incorporated into the parent body and subsequently converted to the present suite of meteorite organics by the hydrothermal process believed to have formed the clay minerals of the meteorite matrix.

Amino acids in meteorites.

Carbonaceous chondrites carry a record of chemical evolution that is unparalleled among presently accessible natural materials. Within the complex suite of organic compounds that characterize these meteorites, amino acids occur at a total concentration that may reach 0.6 micromole g-1 meteorite (approximately 60 ppm). Both free amino acids and acid-labile amino acid derivatives have been found in hot-water extracts of a CI1 and seven CM2 chondrites. Although the amino acid composition of all CM2 chondrites is not the same, differences may be largely explicable on the basis of spontaneous and biologically-caused decomposition occurring during their terrestrial residence. The amino acids of the Murchison meteorite (CM2) have been extensively analyzed and 52 amino acids have been positively identified. Thirty three of these amino acids are unknown in natural materials other than carbonaceous chondrites. Thus the Murchison meteorite has recently been the major source of new naturally-occurring amino acids. The Murchison amino acids comprise a mixture of C2 through C8 cyclic and acyclic monoamino alkanoic and alkandioic acids of nearly complete structural diversity. Within the acyclic monoamino alkanoic acid series, primary alpha-amino alpha-branched amino acids are predominant. The concentrations of individual amino acids decline exponentially with increasing carbon number within homologous series. Amino acid enantiomers are found in approximately equal amounts. Eight of the terrestrial protein amino acids have been found.

Characteristics and formation of amino acids and hydroxy acids of the Murchison meteorite.

Eight characteristics of the unique suite of amino acids and hydroxy acids found in the Murchison meteorite can be recognized on the basis of detailed molecular and isotopic analyses. The marked structural correspondence between the alpha-amino acids and alpha-hydroxy acids and the high deuterium/hydrogen ratio argue persuasively for their formation by aqueous phase Strecker reactions in the meteorite parent body from presolar, i.e., interstellar, aldehydes, ketones, ammonia, and hydrogen cyanide. The characteristics of the meteoritic suite of amino acids and hydroxy acids are briefly enumerated and discussed with regard to their consonance with this interstellar-parent body formation hypothesis. The hypothesis has interesting implications for the organic composition of both the primitive parent body and the presolar nebula.

Acid-labile amino acid precursors in the Murchison meteorite. 1: Chromatographic fractionation.

The amino acid content of a hot water extract of the Murchison meteorite can be increased by over 100 per cent by subjecting the extract to acid hydrolysis. The acid-labile compounds in the extract that account for this increase were fractionated by column chromatography on a cation exchange resin. Seventy mole per cent behaved as neutral or acidic compounds and were eluted from the column with an initial water wash. The remaining 30 mole per cent (basic precursors) were retained on the column and were eluted with the free amino acids by aqueous NH4OH. The acid-labile amino acid precursors in the water eluate could be retained and further fractionated on an anion exchange column, indicating that they are acidic compounds.

Acid-labile amino acid precursors in the Murchison meteorite. II: A search for peptides and amino acyl amides.

The basic fraction of the acid-labile amino acid precursors found in hot water extracts of the Murchison meteorite was treated with ninhydrin under conditions that give destruction of free amino acids but allow nearly complete recovery to peptides and amino acylamides. After this treatment and a subsequent acid hydrolysis, the amount of amino acids found corresponded to less than 30 per cent of the precursor content of the basic fraction. Since only 30 percent of the total extract precursors are found in the basic fraction, these results indicate that such compounds, if present, account for no more than 9 mole per cent of the total acid-labile amino acid precursors of the meteorite hot water extract.

Aliphatic hydrocarbons of the Murchison meteorite.

The indigenous organic compounds of carbonaceous chondrites have been difficult to characterize because of problems arising from terrestrial contamination. The fall of the Murchison meteorite (CM2) provided pristine samples which allowed the resolution of some prior ambiguities as, for example, in the case of the amino acids. However, the nature of the aliphatic hydrocarbons has remained unclear. Shortly after the Murchison fall, one laboratory found them to be mainly cycloalkanes; another found, in order of abundance, branched alkanes, olefins, and cycloalkanes; while a third reported predominantly n-alkanes followed by methyl alkanes and olefins. We have reinvestigated this question using benzene-methanol as the extraction solvent, silica-gel chromatography for fractionation of the extract, and GC-MS, and IR and NMR spectroscopic techniques for the analyses. When interior samples were obtained and the analyses carried out under conditions that minimized environmental contaminants, we have found the principal aliphatic components of the Murchison meteorite to be a structurally diverse suite of C15 to C30 branched alkyl-substituted mono-, di-, and tricyclic alkanes. Comparative analyses were carried out on the Murray (CM2), Allende (CV3), and New Concord (L6) chondrites that illustrate the nature of the contamination problem encountered with carbonaceous chondrites.

Non-racemic amino acids in the Murray and Murchison meteorites.

Small (1.0-9.2%) L-enantiomer excesses were found in six alpha-methyl-alpha-amino alkanoic acids from the Murchison (2.8-9.2%) and Murray (1.0-6.0%) carbonaceous chondrites by gas chromatography-mass spectroscopy of their N-trifluoroacetyl or N-pentafluoropropyl isopropyl esters. These amino acids [2-amino-2,3-dimethylpentanoic acid (both diastereomers), isovaline, alpha-methyl norvaline, alpha-methyl valine, and alpha-methyl norleucine] are either unknown or rare in the terrestrial biosphere. Enantiomeric excesses were either not observed in the four alpha-H-alpha-amino alkanoic acids analyzed (alpha-amino-n-butyric acid, norvaline, alanine, and valine) or were attributed to terrestrial contamination. The substantial excess of L-alanine reported by others was not found in the alanine in fractionated extracts of either meteorite. The enantiomeric excesses reported for the alpha-methyl amino acids may be the result of partial photoresolution of racemic mixtures caused by ultraviolet circularly polarized light in the presolar cloud. The alpha-methyl-alpha-amino alkanoic acids could have been significant in the origin of terrestrial homochirality given their resistance to racemization and the possibility for amplification of their enantiomeric excesses suggested by the strong tendency of their polymers to form chiral secondary structure.

Amino acids of the Murchison meteorite. III. Seven carbon acyclic primary alpha-amino alkanoic acids.

All of the eighteen possible seven-carbon acyclic primary alpha-amino alkanoic acids have been positively identified in a hot-water extract of the Murchison meteorite by the combined use of gas chromatography-mass spectrometry, ion exchange chromatography and reversed-phase chromatography. None of these amino acids has previously been found in meteorites or in any other natural material. They range in concentration from < or = 0.5 to 5.3 nmol g-1. Configuration assignments were made for 2-amino-3,4-dimethylpentanoic acid and allo-2-amino-3,4-dimethylpenatonoic acid and the diasteromer ratio was determined. Fifty-five amino acids have now been positively identified in the Murchison meteorite, 36 of which are unknown in terrestrial materials. This unique suite of amino acids is characterized by the occurrence of all structural isomers within the two major classes of amino acids represented, by the predominance of branched chain isomers, and by an exponential decline in amount with increasing carbon chain length within homologous series. These characteristics of the Murchison amino acids are suggestive of synthesis before incorporation into a parent body.

Linear and cyclic aliphatic carboxamides of the Murchison meteorite: hydrolyzable derivatives of amino acids and other carboxylic acids.

Analyses of fractionated aqueous extracts of the Murchison meteorite by gas chromatography-mass spectrometry after silylation with N-methyl-N (tert-butyldimethylsilyl) trifluoroacetamide have revealed an extensive series of linear and cyclic aliphatic amides. These include monocarboxylic acid amides, dicarboxylic acid monoamides, hydroxy acid amides, lactams, carboxy lactams, lactims, N-acetyl amino acids, and substituted hydantoins. Numerous isomers and homologues through at least C8 were observed in all cases, except for the N-acetyl amino acids and hydantoins. Carboxy lactams, lactams, hydantoins, and N-acetyl amino acids are converted to amino acids by acid hydrolysis, thus, these compounds qualitatively account for the earlier observation of acid-labile amino acid precursors in meteoritic extracts. Laboratory studies of the spontaneous decomposition of N-carbamyl-alpha-amino acids and their dehydration products, the 5-substituted hydantoins, have led to the recognition of a series of aqueous phase reactions by which amino acids and cyanic acid/cyanate ion in the primitive parent body might have given rise to several of the observed classes of amides, as well as to monocarboxylic acids, dicarboxylic acids, and hydroxy acids. A previously undescribed reaction of 5-substituted hydantoins with cyanic acid/cyanate ion to give carboxamides of the 5-substituent groups was observed in the course of these studies. The presence of an extensive suite of amides in a CM chondrite appears to be consistent with the interstellar-parent body formation hypothesis for the organic compounds of these meteorites. The presence of carboxy lactams and lactams along with free amino acids suggests the possibility of further chemical evolution of meteorite amino acids by thermal polymerization. The cyclic amides, given their potential for hydrogen-bonded pair formation, might be considered candidate bases for a primitive sequence coding system.

Oxidation induced by phthalocyanine dyes causes rapid calcium release from sarcoplasmic reticulum vesicles.

The copper containing phthalocyanine dyes, alcian blue, copper phthalocyanine tetrasulfonic acid, and Luxol fast blue MBSN are found to induce rapid calcium efflux from actively loaded sarcoplasmic reticulum (SR) vesicles. Alcian blue (5 microM), with 1 mM free Mg2+ triggered Ca2+ efflux at rates greater than 20 nmol/mg of SR/s. As in the case of Ca2+ efflux induced by calcium, heavy metals, or SH oxidation with Cu2+/cysteine, efflux induced by phthalocyanines is also stimulated by adenine containing nucleotides and inhibited by millimolar Mg2+ and submicromolar ruthenium red (RR). In addition, analogs of RR, such as hexamminecobalt(III) chloride or hexammineruthenium(III) chloride also inhibit Ca2+ efflux but are effective at somewhat higher concentrations (approximately 50 microM). Calcium release stimulated by phthalocyanines is specific for SR derived from the terminal cisternae region rather than longitudinal SR. Preincubation of alcian blue with the reducing agents, sodium dithionite, dithiothreitol, or cysteine causes complete loss of Ca2+ release activity from SR vesicles. Reoxidation of the alcian blue leads to return of the Ca2+ release activity of the phthalocyanine dye. The copper containing phthalocyanine dyes appear to cause rapid Ca2+ release from SR vesicles by oxidizing sulfhydryl groups associated with the calcium release channel. Moreover, phthalocyanines appear to act by oxidizing a pair of neighboring sulfhydryls to a disulfide because subsequent additions of the reducing agent dithiothreitol promote the closure of the Ca2+ channel and calcium re-uptake.

Insoluble organic material of the Orgueil carbonaceous chondrite and the unidentified infrared bands.

Deuterium abundance measurements and the 13C NMR spectrum of the HF/HCl insoluble residue of the Orgueil carbonaceous chondrite indicate the presence of an extensive component of polycyclic aromatic hydrocarbons (PAH) that is of possible interstellar origin or is similar to PAH hypothesized to exist in interstellar space. Infrared spectra have been obtained using an FTIR spectrometer of the acid insoluble residue, the residue after heating in vacuum, and condensate. Bulk acid insoluble residue was pressed into KBr pellets and also heated under high vacuum to sublimate a volatile component onto KBr disks. The remaining non-volatile organic component of the Orgueil meteorite from such evaporations pressed into KBr pellets exhibits a spectral signature similar to that observed in emission from the Orion Nebula and found in Raman spectra of interplanetary dust particles (IDPs). In addition it has an 11.3 microns (885 cm-1) band indicating PAH having single hydrogens per peripheral aromatic ring. We conclude the nonvolatile material is similar to interstellar PAH because the observed 11.3 microns (885 cm-1) unidentified infrared (UIR) emission suggests interstellar PAH have single hydrogens per peripheral aromatic ring. The volatile fraction of the Orgueil shows an aliphatic CH stretch feature and its spectrum in the 2-25 microns range is similar to that of the bulk residue.

13C NMR spectroscopy of the insoluble carbon of carbonaceous chondrites.

13C NMR spectra have been obtained of the insoluble carbon residues resulting from HF-digestion of three carbonaceous chondrites, Orgueil (C1), Murchison (CM2), and Allende (CV3). Spectra obtained using the cross polarization magic-angle spinning technique show two major features attributable respectively to carbon in aliphatic/olefinic structures. The spectrum obtained from the Allende sample was weak, presumably as a consequence of its low hydrogen content. Single pulse excitation spectra, which do not depend on 1H-13C polarization transfer for signal enhancement were also obtained. These spectra, which may be more representative of the total carbon in the meteorite samples, indicate a greater content of carbon in aromatic/olefinic structures. These results suggest that extensive polycyclic aromatic sheets are important structural features of the insoluble carbon of all three meteorites. The Orgueil and Murchison materials contain additional hydrogenated aromatic/olefinic and aliphatic groups.

Amino acids of the Murchison meteorite: II. Five carbon acyclic primary beta-, gamma-, and delta-amino alkanoic acids.

All ten of the possible five-carbon acyclic primary beta-, gamma-, and delta-amino alkanoic acids (amino position isomers of the valines) have been positively identified in hot-water extracts of the Murchison meteorite using combined gas chromatography-mass spectrometry (GC-MS) and ion exchange chromatography. With the exception of delta-aminovaleric acid, none of these amino acids has been previously reported to occur in meteorites or in any other natural material. The gamma-amino acids (4-aminopentanoic acid, 4-amino-2-methlybutanoic acid, and 4-amino-3-methylbutanoic acid) are present at higher concentrations (about 5 nmol g-1)than are the beta-amino isomers (3-aminopentanoic acid, 3-amino-2-methylbutanoic acid, allo-3-amino-2-methylbutanoic acid, 3-amino-3-methylbutanoic acid, 3-amino-2-ethylpropanoic acid, and 3-amino-2,2-dimethylpropanoic acid) which are present at concentrations of 1-2 nmol g-1. These amino acids are less abundant in the meteorite than either the corresponding alpha-amino acids or the four-carbon homologues. Thirty-six amino acids have now been positively identified in the Murchison meteorite, 17 of which are apparently unique to carbonaceous chondrites. The fact that the meteorite contains all possible five-carbon acyclic primary alpha-, beta-, gamma-, and delta-amino alkanoic acids is consistent with a synthetic process involving random combination of single-carbon precursors.

Alkyl phosphonic acids and sulfonic acids in the Murchison meteorite.

Homologous series of alkyl phosphonic acids and alkyl sulfonic acids, along with inorganic orthophosphate and sulfate, have been identified in water extracts of the Murchison meteorite after conversion to their t-butyl dimethylsilyl (tBDMS) derivatives. The methyl, ethyl, propyl, and butyl compounds have been observed in both series. Five of the eight possible alkyl phosphonic acids and seven of the eight possible alkyl sulfonic acids through C4 have been identified. Abundances decrease with increasing carbon number as observed of other homologous series indigenous to Murchison. Concentrations range downward from approximately 380 nmol/gram in alkyl sulfonic acid series, and from 9 nmol/gram in the alkyl phosphonic acid series. Inorganic phosphate is present at about 25 micromoles/gram.