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Amino acids in the Murchison meteorite
Qeochimica et Cosmochimics Acts,1973,Vol. 37,pp.2207to 2212.Per#amon Press.Printedin Northern Ireland
Amino acids in the Murchison meteorite JAMES G. LAWLESS Ames Research Center, NASA, Moffett Field, Ca. 94035, U.S.A. (Received 20 June 1972;accepted in revised form 6 April 1973) Abstract-Continued investigation of the Murchison meteorite by gas chromatography combined with mass spectrometry has led to the characterization of at least 17 amino acids in addition to the 18 identified in earlier work. The total population consists of a wide variety of linear and cyclic difunctional and polyfunctional amino acids, of which the linear difunctional amino acids show a general decrease in concentration as the number of carbon atoms in the amino acid molecule increases. These results are consistent with the hypothesis that the amino acids are present as the result of an extraterrestrial, abiotic synthesis.
THE SEPARATION and identification
of each component in a complex mixture is a problem frequently encountered in the investigation of an organic geochemical sample. The solution to such a problem is generally of paramount importance if the role of carbon in geological environments is to be understood. To obtain such an understanding, scientists have for over 100 years analyzed for organic compounds in meteorites, but often their results could be accounted for by contamination from the terrestrial biosphere (HAYES, 1967). Recently, however, amino acids have been identified in the Murchison meteorite, a type II carbonaceous chondrite, where they are present, probably as the result of an extraterrestrial abiotic synthesis (KVENVOLDEN et al., 1970). Uncommon amino acids and amino acids with nearly equal amounts of both D and L isomers have been found. Further support, for this finding has been obtained from subsequent investigations of the Murchison and other meteorites (KVENVOLDEN et al., 1971; CRONIN and MOORE, 1971; ORO et al., 1971a; LAWLESS et al., 1971; ORO et al., 1971b; LAWLESS et al., 1972). Although the further identification by GC-MS of small quantities of amino acids in meteorites may not at first seem significant, the elucidation of their structure is important for at least two reasons. First, if the role of carbon and its compounds in the condensing solar nebula and on other planetary bodies is to be understood, a clear picture of the suite of organics present must be obtained. Second, certain specific compounds may well provide information about the source (biotic or abiotic) of organic compounds in meteorites. For example, isovaline is an cc-amino acid with an a,symmetric carbon atom, but without an a-hydrogen, thus, the commonly conceived mechanisms for racemization (Neuberger, 1948) are not applicable. This compound should, therefore, maintain the optical configuration it had when synthesized (D, L, or D-L pair). Recently developed analytical techniques (POLLOCK, 1972), if applied to isovaline, could determine its optical configuration and provide a criterion for determining its origin in an extraterrestrial sample. Because of this potential, similar compounds (u-amino acids with an asymmetric carbon atom, but without an u-hydrogen) should be identified if present. This note, therefore, reports on pre-
liminary attempts to further characterize the amino acids extracted from the Murchison meteorite. 2207
2208
JAMES G. LAWLESS EXPERIMENTAL
The procedures used in this experiment have been previously described (KVENVOLDEN et al., 1971). Briefly, a 10 g sample of the pulverized meteorite was refluxed with water for 20 hr. The water extract and washes were evaporated to dryness and refluxed with 6 N HCl for 20 hr. The hydrolyzate was evaporated to dryness, redissolved in water, and charged on a Dowex 50 (Hf) ion exchange column. The column was successively eluted with water and 2 N NH,OH. The NH,OH eluate was evaporated to dryness and the N-trifluoroacetyl-D-2-butyl ester dcrivatives of the amino acids were prepared. The gas chromatograph (Perkin-Elmer 900) was equipped with a flame ionization detector and a 150 ft x 0.02 in. wall coated (UCON 75-H90,000) capillary column. The gas chromatograph was programmed from 100’ to 150° at 1% per minute. The same column and gas chromatographic conditions were used on a Per&n-Elmer 881 gas chromatograph which was interfaced to a CEC 21-491 mass spectrometer through a membrane separator. As each compound emerged from the gas chromatograph and passed into the mass spectrometer, its mass spectrum was determined by taking successive scans at 4 set per decade. Additionally, the changing concentration of organic molecules was followed by the mass spectrometer beam monitor. The covariant mass spectral peaks were then plotted and the identity of the compound determined. This system has the capability of identifying less than 20 x lows g of N-trifluoroacetyl-D-2-butyl ester derivative of an amino acid injected onto the gas chromatographio column. RESULTS
AND
DISCUSSION
18 amino acids have been identified in Murchison meteorite extracts et al., 1971). Six of these (valine, alanine, glycine, proline, aspartic acid, and glutamic acid-see Table 1) are commonly found in proteins; whereas the remaining 12 (isovaline, a-aminoisobutyric acid, N-methylalanine, a-amino-n-butyric acid, N-methylglycine, N-ethylglycine, norvaline, b-amino-isobutyric acid, ,!Caminon-butyric acid, pipecolic acid, p-alanine, and y-amino-n-butyric acid) are structures which are not common to proteins. In addition, at least 17 other amino acids can be partially characterized as members of various classes (i.e. difunctional linear aliphatic, polyfunctional linear aliphatic, and difunctional cyclic), and, further, as members of subgroups within a class (i.e. with 4, 5, or 6 carbon atoms within the difunctional To
date,
(KVENVOLDEN
Table
1. Amino
1 Isovaline 2 3 4 6 6 7 8 9 10 11 12 13 14 16 16 17 18
a-Aminoisobutyria o-Valine Difunctional linear L-V&m N-Methylalanine n-cc-Amino-n-butyrio millsnine Difunctional linear r,-cc-Amino-n-butyric r,-Alanine Difunctional linear NH,* Difunctional linear Difunctional linear N-Methylglycine N-Ethylglycine D-Norvaline
acid aliphatic
(C,)
acid aliphetic acid
(CJ
aliphatic
(C,)
aliphetic aliphatic
(C,) (C,)
* Present in blank. 7 Peaks labeled ‘unknown’ reeolution mass spectrometry.
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
acids in the Murchison
meteorite
L-Norvaline Difunctional linear aliphatic (C,) Difunctional linear aliphatic (C,) D-j-Aminoisobutyrio acid L-p-Aminoisobutyric acid /?.Amino-n-butyrio acid Unknownf D-Pipecolic acid L-Pipecolic acid Glycine Difunctional cyclic Difunctional cyclic B-Ala&m Difunctional cyclic Polyfunctional linear aliphatic n-Proline 5Proline
do not appear to be amino
acids.
Their
36 37 38 39 40 41 42 43 44 45 46 47 48 49 60 61 52
Difunctional linear aliphatic (C,) Unknownt Unknownt Difunotionel linear aliphatic (C,) Unknownt y-Aminobutyric acid D-Aspartic acid L-Aspartic acid Polyfunctional linear aliphatic Polyfunctional linear aliphatic Polyfunctional linear aliphatic Polyfunctional linear aliphatic Polyfunctional linear aliphetic Unknownt n-Glutamic acid z.-Glutamic acid Unknownt
identification
awaits
the
use of high
2209
Amino acids in the Murchison meteorite Table 2. Types of amino acids in the ~urohison meteo~te
Polyfunetional
Difunctional
~_-~
Number of carbon atoms
-
2 3 4 5 ii
-t I -L I i-
Aliphatic
Number previously identified* 1 3 7 3
Cyclic
5(6)t 3 6
2
+
Number this paper
+
5
1
1
i-
6
1
2
* KVENVOLDEN et al. (1971). t At least five are present, possibly six if peaks 9 and 12
in Fig. 1 are not a D, L pair.
linear aliphatic class-see Table 2). These chara~te~~ations are based solely on mass spectral fragmentation patterns. The mass spectrum of the compound represented by peak number 36 in Fig. 1 is shown in Fig. 2 as a typical example. Consideration of the mass spectral behavior of ammo acids (BIEMANN et a$., 1961; GELPI et al., 1969; LAWLESS and CHADHA, 1971) leads to the charac~rization of this compound as a difunctional linear aliphatic amino acid with 5 carbon atoms. Other compounds present in the meteorite extract can be similarly characterized by mass spectrometry. The results of these characterizations show that six po~yfunctional linear aliphatic ammo acids (Fig. 1, peaks 33, 44, 45, 46, 47, 48) appear to be present in addition to those previously identitied (KVENVOLDEN et al., 1971). They appear to be dicarboxylic amino acids which are isomeric with glutamic acid and higher homologs. Also, three additional difunctional cyclic amino acids (isomers of proline and pipecolic acid) are present (Fig. f, peaks 29, 30, 32). As has been previously pointed out (KVENVOLDEN et al., 1971), all of the linear aliphatic amino acids with two carbon atoms (glycine) and three carbon atoms (alanine and isomers) are present, and seven of the nine possible isomers with four carbon atoms ~aminobut~c acids and isomers) are also present. One C, isomer, N-methyl-p-alanine, is known not to be present at a
MURCHISON METEORITE (ATTENUATION
X 8)
ifi i5 & 2
t 0
I
I 8
I
I 16
i
I
24
L
I
1
40 48 32 TIME, min
i
’
56
1
’
64
8
’
72
’
’
80
Fig. 1. The gas chromatogram of the N-trifluoroacetyl-D-2-butyl esters of amino acids in the acid hydrolyzed extract of the Murchison meteorite. The identifying numbers correspond to those in Table 1.
JAMES G. L.~WLESS
2210
MURCHISON METEORITE UNKNOWN
I I’96
I
SO
I
80
100
120
140
m/e
160
180
200
I
220
Fig. 2. Mass spectrum of N-trifluoroacety&D-2-butyl ester of compound (&b&d
36 in Fig. 1) from the acid hydrolyzed extract of the Murchison m&so&e. concentration level greater than 20 x 10Tg g/g. The presence of the other isomer, N,N-dimethylglycine, is uncertain at this time. Of the amino acids with five carbon atoms (valine and its isomers), three have previously been identified (KVENVOLDEN et al., 1971) and there appear to be at least six other isomers present (Fig. 1, peaks 9, 12, 14, 20, 36, 39). Two of these (Fig. 1, peaks 9 and 12) may represent D and L isomers of the same compound. Although previous work from this laboratory has not mentioned the amino acids with six carbon atoms (leucine and its isomers), ORO et al., (1971a) have suggested their presence. At least three of these amino acids appear to be present (Fig. 1, peaks 4, 15, 21). The explicit identification of the characterized compounds can only be made a.fter comparison of their gas chromatographi~ retention times and mass spectra with those of known standards. Since the standards are not readily available, the mass spectral data can be used to predict the compounds to be synthesized. Thus, the spectrum shown in Fig. 2 suggests the N-trifluoroacetyl-n-2-butyl ester derivative of either N-methyl-y-aminobutyric acid or y-aminovalerie acid as possible structures for this compound. The synthesis of these two compounds, and of others suggested by the meteorite mass spectral data, will be undertaken in an effort to elucidate the true nature of the compounds extracted. The results of this work should focus attention on the complex suite of amino acids present in a meteorite extract. The extract contains at least 35 amino acids, and it seems reasonable to assume that given enough sample and a gas chromat,ographic column with sufficient resolution, mass spectrometry would probabiy reveal the presence of most of the 103 possible (Fig. 3) difunctional linear aliphatic amino acid isomers with two through six carbon atoms. Because of the potential complexity of such an abiotic mixture, a single retention value (GC, ion exchange, paper chromatographic, etc.) is insufficient for the identification of an individual component. This fact should be considered in the design of experiments for future space missions ; and consideration should be given to a combined GC-MS if amino acids are to be sought. These results also show (Fig. 3) that, similar to laboratory prebiotic experiments but unlike terrestrial life, the concentration of the difunctional linear aliphatic amino acids in the Murchison meteorite appears to decrease as the number of carbon atoms
2211
Amino acids in the Murchison meteorite
NUMBEROF THEORETICAL
,SOMERSFO”ND
NUMBER
OF ISOMERS
NUMBER OF CARBON ATOMS
Fig. 3. Approximate
3
w
MURCHISON METEORITE
m
SPARK DISCHARGE*
m
E.COLI (BACTERIUMI’*
31
Ezl 3
3
amino acid distribution
from biotic and abiotic sources.
* RING et al., 1972; WOLMAN et al., 1972. ** LURIA, 1960. increases. Living organisms show greater selectivity. Escherichia coli, chosen here as an example, contains no four-carbon amino acid, and selectively synthesizes only one or two isomers of the five- and six-carbon amino acids in relatively high concentration. The distributional patterns of these amino acids might well provide insight into the synthetic and/or diagenetic processes responsible for their presence, and lend further support to the hypothesis that they are present as the result of an extraterrestrial, abiotic synthesis. Aclcnowled~ementsI thank Drs. KEITH A. KVENVOLDEN and DENNIS H. SMITH, and Miss ETTA PETERSON for their assistance. Thanks are also due to Dr. CARLETON MOORE and the Center for Meteorite Studies, Arizona State University for the sample used in this work. REFERENCES BIEMANN K., SEIBL J. and GAPP F. (1961) Mass spectra of organic molecules. I. Ethyl esters of amino acids. J. Amer. Chem. Sot. 83, 3795-3804. CRONIN J. R. and MOORE C. B. (1971) Amino acid analyses of the Murchison, Murray, and Allende carbonaceous chrondrites. Science 172,1327-1329. GXLPI E., KOENIG IV. A., GIBERT J. and ORO J. (1969) Combined gas chromatography-mass spectrometry of amino acid derivatives. J. Chromatogr. Sci. 7, 604-613 HA.Y-ES J. M. (1967) Organic constituents of meteorites-a review. Geochim. Cosmochim. Actn
31,1395-1440. KVENVOLDEN K. 9., LAWLESS J., PERING K., PETERSON E., FLORES J., PONNAMPERUNA C., KAPLAN I. R. and MOORE C. (1970) Evidence for extraterrestrial amino acids and hydrocarbons in the Murchison meteorites. Nature 228, 923-926. KVENVOLDEN K. A., LAWLESS J. G. and PONNAMPERUMAC. (1971) Nonprotein amino acids in the Murchison meteorite. I’roc. h’at. Acad. Sci. 68, 486-490. LAWLESS J. G., KVENVOLDEN K. A., PETERSON E., PONNAXPERUMA C. and MOORE C. (1971) Amino acids indigenous to the Murray meteorite. Science 173, 6266627. LAWLESS J. G. and CHADHA M. S. (1971) Mass spectral analysis of C, and C, aliphatic amino acid derivatives. Anal. Riochem. 44, 473-485.
2212
JAMES G. LAU’LESS
LAWLESS J. G., KVENVOLDEN K. A., PETERSON E., PONNAMPERUIVIAC. and JAROSEWICH E. (1972) Evidence for amino acids of extraterrestrial origin in the Orgueil meteorite. Nature
266, 66-67. LURIA S. E. (1960) The bacterial protoplasm: composition and organization. In The BacteriaA Treatise on Structure and &n&on, 1, (editors I. C. Gunsalus and R. Y. Stanier), pp. l-34. Academic Press. IVEWERGER A. (1948) Stereochemistry of amino acids. In Aclvances in I’rotein Chemistry, Vol. IV, (editors M. L. Anson and J. T. Edsall), pp. 298-383. Academic Press. ORO J., GIBERT J., LICHTENSTEIN H., WIKSTROM S. and FLORY D. A. (1971a) Amino acids, aliphatic and aromatic hydrocarbons in the Murchison meteorite. Nature 230, 105-106. ORO J., NAKAPARKSIN S., LICHTENSTEIN H. and GIL-AV E. (1971b) Configurations of amino acids in carbonaceous chondrites and a Precambrian chert. Nature 230, 107-105. POLLOCK G. E. (1972) Resolution by gas-liquid chromatography of diastereomers of five nonprotein amino acids known to occur in the Murchison meteorite. Anal. Chem. 44, 2368-2372. RING D., WOLMAN Y., FRIEDMANN N. and MILLER S. (1972) Prebiotic synthesis of hydrophobic and protein amino acids. Proc. Nat. Acad. SC?:. 69, 765-768. WOLMAN Y., HA~ERLAPI’DW. J. and MILLER 8. (1972) Sonprotein amino acids from spark discharges and their comparison with the Murchison meteorite amino acids. I’roc. Nut. Acnd. b”ci. 69, 809-11.
Amino acids in the Murchison meteorite JAMES G. LAWLESS Ames Research Center, NASA, Moffett Field, Ca. 94035, U.S.A. (Received 20 June 1972;accepted in revised form 6 April 1973) Abstract-Continued investigation of the Murchison meteorite by gas chromatography combined with mass spectrometry has led to the characterization of at least 17 amino acids in addition to the 18 identified in earlier work. The total population consists of a wide variety of linear and cyclic difunctional and polyfunctional amino acids, of which the linear difunctional amino acids show a general decrease in concentration as the number of carbon atoms in the amino acid molecule increases. These results are consistent with the hypothesis that the amino acids are present as the result of an extraterrestrial, abiotic synthesis.
THE SEPARATION and identification
of each component in a complex mixture is a problem frequently encountered in the investigation of an organic geochemical sample. The solution to such a problem is generally of paramount importance if the role of carbon in geological environments is to be understood. To obtain such an understanding, scientists have for over 100 years analyzed for organic compounds in meteorites, but often their results could be accounted for by contamination from the terrestrial biosphere (HAYES, 1967). Recently, however, amino acids have been identified in the Murchison meteorite, a type II carbonaceous chondrite, where they are present, probably as the result of an extraterrestrial abiotic synthesis (KVENVOLDEN et al., 1970). Uncommon amino acids and amino acids with nearly equal amounts of both D and L isomers have been found. Further support, for this finding has been obtained from subsequent investigations of the Murchison and other meteorites (KVENVOLDEN et al., 1971; CRONIN and MOORE, 1971; ORO et al., 1971a; LAWLESS et al., 1971; ORO et al., 1971b; LAWLESS et al., 1972). Although the further identification by GC-MS of small quantities of amino acids in meteorites may not at first seem significant, the elucidation of their structure is important for at least two reasons. First, if the role of carbon and its compounds in the condensing solar nebula and on other planetary bodies is to be understood, a clear picture of the suite of organics present must be obtained. Second, certain specific compounds may well provide information about the source (biotic or abiotic) of organic compounds in meteorites. For example, isovaline is an cc-amino acid with an a,symmetric carbon atom, but without an a-hydrogen, thus, the commonly conceived mechanisms for racemization (Neuberger, 1948) are not applicable. This compound should, therefore, maintain the optical configuration it had when synthesized (D, L, or D-L pair). Recently developed analytical techniques (POLLOCK, 1972), if applied to isovaline, could determine its optical configuration and provide a criterion for determining its origin in an extraterrestrial sample. Because of this potential, similar compounds (u-amino acids with an asymmetric carbon atom, but without an u-hydrogen) should be identified if present. This note, therefore, reports on pre-
liminary attempts to further characterize the amino acids extracted from the Murchison meteorite. 2207
2208
JAMES G. LAWLESS EXPERIMENTAL
The procedures used in this experiment have been previously described (KVENVOLDEN et al., 1971). Briefly, a 10 g sample of the pulverized meteorite was refluxed with water for 20 hr. The water extract and washes were evaporated to dryness and refluxed with 6 N HCl for 20 hr. The hydrolyzate was evaporated to dryness, redissolved in water, and charged on a Dowex 50 (Hf) ion exchange column. The column was successively eluted with water and 2 N NH,OH. The NH,OH eluate was evaporated to dryness and the N-trifluoroacetyl-D-2-butyl ester dcrivatives of the amino acids were prepared. The gas chromatograph (Perkin-Elmer 900) was equipped with a flame ionization detector and a 150 ft x 0.02 in. wall coated (UCON 75-H90,000) capillary column. The gas chromatograph was programmed from 100’ to 150° at 1% per minute. The same column and gas chromatographic conditions were used on a Per&n-Elmer 881 gas chromatograph which was interfaced to a CEC 21-491 mass spectrometer through a membrane separator. As each compound emerged from the gas chromatograph and passed into the mass spectrometer, its mass spectrum was determined by taking successive scans at 4 set per decade. Additionally, the changing concentration of organic molecules was followed by the mass spectrometer beam monitor. The covariant mass spectral peaks were then plotted and the identity of the compound determined. This system has the capability of identifying less than 20 x lows g of N-trifluoroacetyl-D-2-butyl ester derivative of an amino acid injected onto the gas chromatographio column. RESULTS
AND
DISCUSSION
18 amino acids have been identified in Murchison meteorite extracts et al., 1971). Six of these (valine, alanine, glycine, proline, aspartic acid, and glutamic acid-see Table 1) are commonly found in proteins; whereas the remaining 12 (isovaline, a-aminoisobutyric acid, N-methylalanine, a-amino-n-butyric acid, N-methylglycine, N-ethylglycine, norvaline, b-amino-isobutyric acid, ,!Caminon-butyric acid, pipecolic acid, p-alanine, and y-amino-n-butyric acid) are structures which are not common to proteins. In addition, at least 17 other amino acids can be partially characterized as members of various classes (i.e. difunctional linear aliphatic, polyfunctional linear aliphatic, and difunctional cyclic), and, further, as members of subgroups within a class (i.e. with 4, 5, or 6 carbon atoms within the difunctional To
date,
(KVENVOLDEN
Table
1. Amino
1 Isovaline 2 3 4 6 6 7 8 9 10 11 12 13 14 16 16 17 18
a-Aminoisobutyria o-Valine Difunctional linear L-V&m N-Methylalanine n-cc-Amino-n-butyrio millsnine Difunctional linear r,-cc-Amino-n-butyric r,-Alanine Difunctional linear NH,* Difunctional linear Difunctional linear N-Methylglycine N-Ethylglycine D-Norvaline
acid aliphatic
(C,)
acid aliphetic acid
(CJ
aliphatic
(C,)
aliphetic aliphatic
(C,) (C,)
* Present in blank. 7 Peaks labeled ‘unknown’ reeolution mass spectrometry.
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
acids in the Murchison
meteorite
L-Norvaline Difunctional linear aliphatic (C,) Difunctional linear aliphatic (C,) D-j-Aminoisobutyrio acid L-p-Aminoisobutyric acid /?.Amino-n-butyrio acid Unknownf D-Pipecolic acid L-Pipecolic acid Glycine Difunctional cyclic Difunctional cyclic B-Ala&m Difunctional cyclic Polyfunctional linear aliphatic n-Proline 5Proline
do not appear to be amino
acids.
Their
36 37 38 39 40 41 42 43 44 45 46 47 48 49 60 61 52
Difunctional linear aliphatic (C,) Unknownt Unknownt Difunotionel linear aliphatic (C,) Unknownt y-Aminobutyric acid D-Aspartic acid L-Aspartic acid Polyfunctional linear aliphatic Polyfunctional linear aliphatic Polyfunctional linear aliphatic Polyfunctional linear aliphatic Polyfunctional linear aliphetic Unknownt n-Glutamic acid z.-Glutamic acid Unknownt
identification
awaits
the
use of high
2209
Amino acids in the Murchison meteorite Table 2. Types of amino acids in the ~urohison meteo~te
Polyfunetional
Difunctional
~_-~
Number of carbon atoms
-
2 3 4 5 ii
-t I -L I i-
Aliphatic
Number previously identified* 1 3 7 3
Cyclic
5(6)t 3 6
2
+
Number this paper
+
5
1
1
i-
6
1
2
* KVENVOLDEN et al. (1971). t At least five are present, possibly six if peaks 9 and 12
in Fig. 1 are not a D, L pair.
linear aliphatic class-see Table 2). These chara~te~~ations are based solely on mass spectral fragmentation patterns. The mass spectrum of the compound represented by peak number 36 in Fig. 1 is shown in Fig. 2 as a typical example. Consideration of the mass spectral behavior of ammo acids (BIEMANN et a$., 1961; GELPI et al., 1969; LAWLESS and CHADHA, 1971) leads to the charac~rization of this compound as a difunctional linear aliphatic amino acid with 5 carbon atoms. Other compounds present in the meteorite extract can be similarly characterized by mass spectrometry. The results of these characterizations show that six po~yfunctional linear aliphatic ammo acids (Fig. 1, peaks 33, 44, 45, 46, 47, 48) appear to be present in addition to those previously identitied (KVENVOLDEN et al., 1971). They appear to be dicarboxylic amino acids which are isomeric with glutamic acid and higher homologs. Also, three additional difunctional cyclic amino acids (isomers of proline and pipecolic acid) are present (Fig. f, peaks 29, 30, 32). As has been previously pointed out (KVENVOLDEN et al., 1971), all of the linear aliphatic amino acids with two carbon atoms (glycine) and three carbon atoms (alanine and isomers) are present, and seven of the nine possible isomers with four carbon atoms ~aminobut~c acids and isomers) are also present. One C, isomer, N-methyl-p-alanine, is known not to be present at a
MURCHISON METEORITE (ATTENUATION
X 8)
ifi i5 & 2
t 0
I
I 8
I
I 16
i
I
24
L
I
1
40 48 32 TIME, min
i
’
56
1
’
64
8
’
72
’
’
80
Fig. 1. The gas chromatogram of the N-trifluoroacetyl-D-2-butyl esters of amino acids in the acid hydrolyzed extract of the Murchison meteorite. The identifying numbers correspond to those in Table 1.
JAMES G. L.~WLESS
2210
MURCHISON METEORITE UNKNOWN
I I’96
I
SO
I
80
100
120
140
m/e
160
180
200
I
220
Fig. 2. Mass spectrum of N-trifluoroacety&D-2-butyl ester of compound (&b&d
36 in Fig. 1) from the acid hydrolyzed extract of the Murchison m&so&e. concentration level greater than 20 x 10Tg g/g. The presence of the other isomer, N,N-dimethylglycine, is uncertain at this time. Of the amino acids with five carbon atoms (valine and its isomers), three have previously been identified (KVENVOLDEN et al., 1971) and there appear to be at least six other isomers present (Fig. 1, peaks 9, 12, 14, 20, 36, 39). Two of these (Fig. 1, peaks 9 and 12) may represent D and L isomers of the same compound. Although previous work from this laboratory has not mentioned the amino acids with six carbon atoms (leucine and its isomers), ORO et al., (1971a) have suggested their presence. At least three of these amino acids appear to be present (Fig. 1, peaks 4, 15, 21). The explicit identification of the characterized compounds can only be made a.fter comparison of their gas chromatographi~ retention times and mass spectra with those of known standards. Since the standards are not readily available, the mass spectral data can be used to predict the compounds to be synthesized. Thus, the spectrum shown in Fig. 2 suggests the N-trifluoroacetyl-n-2-butyl ester derivative of either N-methyl-y-aminobutyric acid or y-aminovalerie acid as possible structures for this compound. The synthesis of these two compounds, and of others suggested by the meteorite mass spectral data, will be undertaken in an effort to elucidate the true nature of the compounds extracted. The results of this work should focus attention on the complex suite of amino acids present in a meteorite extract. The extract contains at least 35 amino acids, and it seems reasonable to assume that given enough sample and a gas chromat,ographic column with sufficient resolution, mass spectrometry would probabiy reveal the presence of most of the 103 possible (Fig. 3) difunctional linear aliphatic amino acid isomers with two through six carbon atoms. Because of the potential complexity of such an abiotic mixture, a single retention value (GC, ion exchange, paper chromatographic, etc.) is insufficient for the identification of an individual component. This fact should be considered in the design of experiments for future space missions ; and consideration should be given to a combined GC-MS if amino acids are to be sought. These results also show (Fig. 3) that, similar to laboratory prebiotic experiments but unlike terrestrial life, the concentration of the difunctional linear aliphatic amino acids in the Murchison meteorite appears to decrease as the number of carbon atoms
2211
Amino acids in the Murchison meteorite
NUMBEROF THEORETICAL
,SOMERSFO”ND
NUMBER
OF ISOMERS
NUMBER OF CARBON ATOMS
Fig. 3. Approximate
3
w
MURCHISON METEORITE
m
SPARK DISCHARGE*
m
E.COLI (BACTERIUMI’*
31
Ezl 3
3
amino acid distribution
from biotic and abiotic sources.
* RING et al., 1972; WOLMAN et al., 1972. ** LURIA, 1960. increases. Living organisms show greater selectivity. Escherichia coli, chosen here as an example, contains no four-carbon amino acid, and selectively synthesizes only one or two isomers of the five- and six-carbon amino acids in relatively high concentration. The distributional patterns of these amino acids might well provide insight into the synthetic and/or diagenetic processes responsible for their presence, and lend further support to the hypothesis that they are present as the result of an extraterrestrial, abiotic synthesis. Aclcnowled~ementsI thank Drs. KEITH A. KVENVOLDEN and DENNIS H. SMITH, and Miss ETTA PETERSON for their assistance. Thanks are also due to Dr. CARLETON MOORE and the Center for Meteorite Studies, Arizona State University for the sample used in this work. REFERENCES BIEMANN K., SEIBL J. and GAPP F. (1961) Mass spectra of organic molecules. I. Ethyl esters of amino acids. J. Amer. Chem. Sot. 83, 3795-3804. CRONIN J. R. and MOORE C. B. (1971) Amino acid analyses of the Murchison, Murray, and Allende carbonaceous chrondrites. Science 172,1327-1329. GXLPI E., KOENIG IV. A., GIBERT J. and ORO J. (1969) Combined gas chromatography-mass spectrometry of amino acid derivatives. J. Chromatogr. Sci. 7, 604-613 HA.Y-ES J. M. (1967) Organic constituents of meteorites-a review. Geochim. Cosmochim. Actn
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