Preferential polymerization and adsorption of L-optical isomers of amino acids relative to D-optical isomers on kaolinite templates

Chemical Geology - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

PREFERENTIAL POLYMERIZATION AND ADSORPTION OF L-OPTICAL ISOMERS OF AMINO ACIDS RELATIVE TO D-OPTICAL ISOMERS ON KAOLINITE TEMPLATES TOGWELL A. JACKSON* Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Mass. (U.S.A.) ABSTRACT Jackson, T .A., 1971. Preferential polymerization and adsorption of L-optical isomers of amino acids relative to D-optical isomers on kaolinite templates. Chern. Geol., 7: 295-306. Experiments on the polymerization of the L- and D-optical isomers of aspartic acid and serine using kaolinite as a catalyst showed that the L-optical isomers were polymerized at a much higher rate than the D-optical isomers; racemic (DL-) mixtures were polymerized at an intermediate rate. The peptides formed from the L-monomers were preferentially adsorbed by the clay. In the absence of kaolinite, no significant or consistent difference in the behavior of the L- and D-optical isomers was observed. In experiments on the adsorption of L- and D-phenylalanine by kaolinite the L-optical isomer was preferentially adsorbed. Adsorption of L-phenylalanine was sensitive to pH, whereas adsorption of D-phenylalanine was not. The experimental results indicate that catalytically active faces of the kaolinite crystals acted as specific templates which preferentially adsorbed and polymerized molecules with the L-configuration. The observed discrimination between the L- and D-optical isomers of the amino acids evidently occurred on the enantiomorphous edge ( hk ) faces of the kaolinite. These findings may have significance for the pre-biotic origin of proteins. INTRODUCTION

A remarkable characteristic of life on the earth is that all proteins are composed of the L-optical isomers of amino acids to the complete exclusion of D-optical isomers. This can be explained in part by the requirements for stabilization of protein structures. If a polypeptide chain consisted of a random assortment of L- and D-optical isomers, steric hindrance involving the side-chains would impede formation of stable, orderly secondary structures such as the a-helix (Wald, 1957), and repulsion between eclipsed groups would probably decrease the stability and maximum attainable length of the primary chain itself (Miller et at, 1967; Gratzer and Cowburn, 1969); but in an isotactic polypeptide (composed entirely of L- or D-monomers), in which the side-chains are staggered in a regular fashion (Corey and Pauling, 1953), steric hindrance is avoided, intramolecular repulsive forces are minimized, and the molecule is able to attain the higher level of organization and complexity required for *Present address: Department of Geological Sciences, University of California at Santa Barbara, Santa Barbara, Calif. 93106, U.S.A. Chern. Geol., 7 (1971) 295-306

295

biological activity (cf. Coleman, 1958; Mislow, 1966, p.105). Thus, the dissymmetry of proteins may well be essential to life (Terent' ev and Klabunovskir, 1959). Several hypotheses have been advanced to explain the pre-biotic origin of protein dissymmetry (Byk, 1904; Bernal, 1951; Goldschmidt, 1952; Oparin, 1957; Wald, 1957; Klabunovskil, 1959; Terent'ev and Klabunovskil, 1959). In the present work, I tested the hypothesis that dissymmetric mineral surfaces such as the edge faces of dioctahedral clay crystals served as catalytic templates for stereospecific synthesis of dissymmetric polypeptides. The possible importance of adsorption and catalYSis on mineral surfaces in the pre-biotic evolution of macromolecules has been suggested on theoretical grounds (Bernal, 1951; Goldschmidt, 1952; Degens and Matheja, 1968) and demonstrated experimentally (Akabori, 1959; Degens and Matheja, 1968, 1970; Paecht-Horowitz et al., 1970). Degens and Matheja (1968, 1970) synthesized polypeptides with molecular weight~ exceeding 10,000 by polymerizing L-amino acids on clay-mineral catalysts, notably kaolinite, at temperatures well below the boiling point of water. PaechtHorowitz et al. (1870) synthesized polypeptides by reacting amino-acid adenylates with montmorillonite. In the present research, experiments were conducted to determine whether clay minerals can cata.lyze preferential polymerization of one optical isomer of an amino acid with respect to the other. Preferential adsorption of optical isomers was also investigated. My preliminary results were announced by Degens et al. (1970); the present paper reports the research in full.

MATERIALS AND METHODS

The amino acids were obtained from the Mann Research Laboratories. Aspartic acid was used for most of the polymerization runs, because it has an especially strong tendency to polymerize on clay minerals. The clay mineral used in all of the experiments was kaolinite mined from Cretaceous sediments in Sandersville, Georgia, U.S.A. (kaolinite K-6, Fisher Scientific Co.,). X-ray diffraction analysis showed the clay to be mineralogically pure, and chemical micro-analysis revealed no organic matter except for traces (5-10 p.p.m.) of hydrocarbons (Degens et al., 1970). 5-ml aliquots of aqueous 0.01 M solutions of L-, D-, andDL-aspartic acid were incubated with 500-mg portions of the < 2 J1 fraction of the kaolinite at 90 o.C for varying lengths of time (19 hours to 32 days) in sealed Pyrex tubes. Aspartic acid solutions without clay served as blanks. After incubation, the samples were centx:ifuged; the supernates were withdrawn for analysis, and material adsorbed to the spun-down clay was extracted with 2 N NH3 and analyzed separately or in combination with the supernates. The extent of polymerization (irrespective of molecular weights) was determined by: (1) amino acid analyses before and after hydrolysis with 6 N HCI (Degens and Matheja, 1968); and (2) peptide analysis by the biuret method (Gornall et al., 1949) using glycylglycylglycine as a standard. The amino acid determinations were performed by means of an amino acid autoanalyzer employing ion-exchange chromatography; the precision of this method is better than 1% at the 10-8 molar level (Degens and Matheja, 1968). 296

Chern. Geo!., 7 (1971) 295-306

The percentage of aspartic acid molecules polymerized was computed as follows: % polymerized =

(total asp. acid) - (free asp. acid) x 100 (total asp. acid)

The concentrations of "tree" and "total" aspartic acid are the concentrations before and after hydrolysis, respectively. The significance of the differences between the L- and D- data was tested statistically at the 5% level by means of the Mann-Whitney U-test, the t-test, and the paired t-test; the tests were 1-sided for the experimental samples (HI: L > D) and 2-sided for the blanks (HI: L J D). Contamination of the samples by microbial growth is extremely unlikely, owing to the high temperature of incubation. Moreover, the chromatographic analyses showed no evidence of contamination arising from biological activity. Polarimetric analyses of sample solutions were performed using a Cary-60 spectropolarimeter. Standard aspartic acid solutions of approximately the same pH as the sample solutions were included for comparison. Optical rotary dispersion (ORD) was determined in the wavelength interval 200-300 mJ.1. Samples were also analyzed by infrared (IR) spectrophotometry. NH3 extracts, and clay samples which had been allowed to react with aspartic acid solutions at 90°C and at room temperature (and then washed X 3 with water), were taken to dryness at room temperature and analyzed in KBr pellets Pure kaolinite and crystalline aspartic acid were used as reference substances. A preliminary study of the polymerization of serine was undertaken. 0.01 M L-, D-, and DL-serine solutions were incubated at about 60°C for 6 days in the presence and absence of kaolinite. Standard solutions were used to compute a correction factor for serine lost during hydrolysis. Adsorption of L- and D-phenylalanine by kaolinite under different pH conditions was also investigated. 2.0000-gm portions of kaolinite were suspended in replicate 10-ml aliquots of 0.001 M L- and D-phenylalanine solutions. In one set of phenylalanine solutions, comprising 7 replicates per optical isomer, the solvent was water, and the initial pH 5.8; in another set, comprising 5 replicates per optical isomer, the solvent was 0.01 N HCI, and the initial pH 2.0. The blanks, which were run in triplicate, consisted of phenylalanine solutions without clay, and clay suspensions without phenylalanine. All samples and blanks were shaken continuously at room temperature for 24 hours in rubber-stoppered Pyrex flasks. The clay was then spun down, and the supernates withdrawn for analysis. All solutions were passed through 0.45 J.1 millipore fUters and analyzed by Beckman DU spectrophotometer at a wavelength of 260 mJ.1. Absorbance readings for the samples were corrected by subtracting the mean absorbance of the clay blanks. The quantities of phenylalanine adsorbed by the kaolinite were computed on the basis of the differences between the (corrected) absorbances of the sample solutions and the mean absorbances of the phenylalanine blanks. The significance of the differences between the sample sets was tested statistically at the 5% level by the t-test (unpaired) and the U-test.

Chern. Geol., 7 (1971) 295-306

297

RESULTS AND DISCUSSION

In the presence of the kaolinite, the L-optical isomers of aspartic acid and serine were always more highly polymerized than the D-optical isomers, the racemic DL-mixtures being intermediate (Tables I and II; Fig.I-3). Polymerization proceeded logarithmically in both the L- and the D-aspartic acid samples, but the L-isomer polymerized at a much higher rate than the D{Fig. 1). According to the statistical tests, these differences are Significant at the 2 %level or less (U = 3; t (paired) = 2.72, with 6 degrees of freedom; t (unpaired) = 2.64, with 12 degrees of freedom). L 20


0 Z

30

015 CD

...

25

0

;:

... ...0 10 ... Q. Q.

20·

•


Cl W N Cl::

..J

w

..

15

::;:

0 :2

:It

D

•

5

~

.

o~

~

0

0

A, 0

10

o a.

..J

I

2

3

4

TIME (DAYS)

6

7

8

9

5

oL-----_ D

DL

OPTICAL

L ISOMER

Fig.!. Plot showing rates of peptide bond formation in solutions of Land D-aspartic acid heated in the presence of kaolinite (data from biuret analyses). Circles: L-asp. Triangles: D-asp. Fig.2. Relative amounts of aspartic acid isomers polymerized on kaolinite after 32 days (from analyses of supernates). In contrast, the blanks showed no consistent or significant relationship between chirality (" handedness") and "percent polymerized" (Table I). The statistical tests indicated that the difference between the L- and Dblanks was inSignificant, being 55-80 %probable under the null hypothesis (U = 13; t (paired) = 0.268, with 4 degrees of freedom; t (unpaired) = 0.618, with 9 degrees of freedom). In some of the blanks, polymerization did not occur to any appreciable extent, but in a number of blanks (both L- and D- ) small quantities of polymeric material were formed, possibly owing to catalysiS by certain sites on the walls of the sample tubes. The data for blanks show no consistent trend with respect to time of incubation. The polymers formed in the presence of kaolinite were largely retained on the clay by adsorption, as shown by the following evidence: (I) most of the polymeric material (chiefly poly-L-aspartic acid) appeared 298

Chern. Geol., 7 (1971) 295-306

TABLE I Amino acid analyses for the polymerization experiment Sample

Incubation time (days)

Optical isomer

Initial pH of supernate

Clay present (+) or absent

Amino acid concentration (J.L -moles/ml)

% Polymerized

free

total

8.568 9.788 8.592 9.867 8.667 10.07 8.336 9.545 8.564 9.719 8.724 9.222 8.593 9.440 8.839 9.806 8.891 9.275 8.510 9.388 8.537 1.634 8.545 9.361 0.7244 1.178 0.8899 1.220 0.8825 n.d.

9.161 10.12 8.918 9.. 771 9.135 10.14 8.953 9.606 8.613 10.14 9.083 9.266 9.135 10.05 8.944 10.17 9.153 9.492 8.648 9.336 8.617 1.686 8.643 9.658 0.9740 1.125 0.9124 n.d. 1.026 n.d.

6.47 3.28 3.65 0 5.12 0.690 6.90 0.635 0.569 4.16 3.95 0.475 5.93 6.07 1.17 3.58 2.86 2.29 1.60 0 0.928 3.08 1.13 3.08 25.6 0 2.47 n.d. 14.0 n.d.

0.1483 0.1731 0.1655

0.1771 0.1747 0.1777

16.3 0.916 6.87

(-)

L

3.06

+

D

3.05

+

DL

3.07

+

L

3.06

+

D

3.05

+

DL

3.07

+

L

3.06

+

D

3.06

+

DL

3.06

+

L

3.05

+

D

3.05

+

DL

3.05

+

L

3.50

+

D

3.43

+

DL

3.44

+

Aspartic 21 acid (NH3 extract)

L D DL

3.05 3.05 3.05

+ + +

Serine (supernate)

L

5.48

+

D

5.48

+

DL

5.53

+

Aspartic acid (supernate)

3

7

14

21

32

6

9.608 10.65 9.856 10.43 9.779 10.51

10.26* 11.08* 10.02* 10.97* 10.09* 11.15*

6.36 3.88 1.64 4.92 3.08 5.74

*Corrected values computed on basis of 14.1% lost during hydrolysis.

Chern. Geol., 7 (1971) 295-306

299

TABLE II Biuret data representing peptide bonds formed by polymerization of L- and D-aspartic acid in the presence of kaolinite* Incubation time (days)

Optical isomer

Total JJ. -moles of peptide bonds

0.79

L D L D L D

5.5 0.0 10 4.5 20 7.5

2.71 7.54

*The data are based on analyses of NH3 extracts+supernates. Neither the supernates alone, nor the blanks, contained detectable quantities of peptides, according to the biuret analyses. 20

o ~ a:: w

::;:

15

10

~

o

n.

5

o -'----'-----"--'--

o

DL

OPTICAL

L

ISOMER

Fig.3. Relative amounts of aspartic acid isomers polymerized on kaolinite after 21 days. White bars: Data from supernates. Black bars: Data from NH3 extracts of the clay. in the NH3 extracts rather than in the supernates (Fig.3; Tables I and II); and (2) in the supernates of the 3- to 21-day samples, the concentrations of polymeric material deqyeased progressively with time (Table I), suggesting continuous uptake by the clay, whereas the total amount of polymer produced increased at a logarithmic rate (Fig.l). As illustrated in Fig.3, the kaolinite preferentially adsorbed the polymerized L-aspartic acid; after 21 days, the percentage of polymerized L-aspartic acid was much higher in the adsorbed phase than in the corresponding aqueous phase, whereas the percentage of polymerized D-aspartic acid was equally low in the adsorbed phase and the aqueous phase, DL-aspartic acid falling about midway between L- and D-. These data demonstrate a relationship between stereos elective adsorption and stereoselective catalysis, and indicate specific epitaxial interaction between tile L-optical isomer and the clay surface. (The slight increase in the absolute quantity of polymerizE;!d aspartate in the L- and DL- supernates after 32 days suggests spontaneous desorption from the clay, possibly owing to increase in pH. However, the large, abrupt increase in the relative amount of polymerized aspartate ("% polymerized ") in the supernates of the L-, and to a lesser extent the DL-, samples after 32 days may be due to 300

Chern. Geol., 7 (1971) 295-306

thermal decomposition of the free (unpolymerized, unadsorbed) aspartic acid accompanied by little or no decomposition of the polymerized aspartate; this seems likely, because the total quantities of aspartic acid in the 32-day supernates are an order of magnitude smaller than those of the samples incubated for shorter periods of time, whereas the absolute quantity of polymerized aspartate is of the same order of magnitude as that found in the other samples). The data for the NH3 extracts show that the discrimination between L- and D-optical isomers by the kaolinite was far more pronounced than is indicated by the data for the supernates (with the exception of the 32-day supernates). For all practical purposes, the discrimination in favor of the L-optical isomer was nearly absolute. Moreover, the fact that the NH3 extracts and the supernates both showed an excess of polymerized L-aspartate over polymerized D-aspartate supports the interpretation that discrimination by the clay involved preferential polymerization of the L-optical isomer. This tends to rule out the possibility that the differences observed in the data for the supernates might be an apparent effect due merely to selective adsorption or selective decomposition instead of selective polymerization. Furthermore, the fact that the rate of polymerization of DL-aspartic acid was approximately midway between the polymerization rates of the L- and D-optical isomers strongly suggests that in the racemic mixtures the molecules polymerized were chiefly those of the Lconfiguration. I infer, tentatively, that the clay can preferentially catalyze polymerization of the L-amino acid molecules in an initially racemic mixture, thereby producing isotactic L-peptides in much greater abundance than D- or DL-peptides. Polarimetric analysis of the supernates and NH3 extracts of I-week DL- samples failed to yield definitive evidence for the presence of optically active material, but the results are inconclusive, owing to: (I) possible racemization of L-peptides following desorption from the clay; (2) possible interference by suspended clay particles; and (3) the possibility that the concentration of optically active peptides, if any, was too small to elicit a response from the instrument. The problem is still under investigation. Polarimetry indicated that the free L- and D-aspartic acid in the supernates and blanks were largely racemized during the first week of incubation. Since the curves showing the polymerization rates of L- and D-aspartic acid (Fig. 1 ) diverge continuously despite racemization of free aspartic acid in the aqueous phase, the observed discrimination between L- and D-optical isomers by the clay must have occurred by: (I) very rapid adsorption at the outset, before appreciable racemization had occurred; followed by (2) slower polymerization of the adsorbed molecules. Adsorption of the molecules to the clay evidently protected them from racemization. Evidently polymerization of the L-optical isomer was sterically favored, and therefore occurred much more rapidly than polymerization of the Doptical isomer. Possibly the small amounts of polymeric material detected in the D-samples were synthesized by polymerization of L-molecules formed by racemization of the D-aspartic acid. Infrared spectrophotometry confirmed the conclusion that peptide

Chern. Geol., 7 (1971) 295-306

301

bonds were formed on the kaolinite. The IR pattern for the NH3 extracts of the 14- and 21-day L-aspartic acid samples has a peak at 1620-1630 cm- 1 corresponding to thE' bonded -NH- group of peptides. The IR pattern of L-aspartic acid adsorbed on kaolinite at room temperature closely matches the IR pattern of pure crystalline L-aspartic acid, but material adsorbed to kaolinite after incubation with L-aspartic acid at 90°C for 7 days gave a peak at 1611.4 cm- 1 which is best interpreted as representing a peptide bonCl. Serine behaved in the same way as aspartic acid, showing that preferential polymerization of L-optical isomers on kaolinite is not confined to the acidic amino aCids, nor to the particular pH conditions prevailing in the aspartic acid solutions (Table I). Preferential polymerization of L-optical isomers of amino acids on kaolinite can be attributed to the inherent enantiomorphism of the edges of the octahedral layer of kaolinite (Fig.4), and to the fact that in a wellcrystallized kaolin mineral (with the exception of dickite) the crystals are either entirely right-handed or entirely left-handed (Bailey, 1963). In the present case, all or nearly all of the kaolinite crystals evidently have the same chirality. The enantiomorphism of the edge faces of kaolinite arises from the arrangement of O-atoms, OH-groups, AI-atoms, and octahedral vacancies (trioctahedral minerals are not enantiomorphous). A

o o

0

•

AI

OH

•

Si

. Fig.4. S.c?ematic representation of the edge of a kaolinite crystal of ldeal composltion (A), and its mirror image (B) viewed along the a-axis. 302

Chern. Geol., 7 (1971) 295-306

The phenylalanine data (Table III) support the hypothesis that discrimination between the L- and D-optical isomers of amino acids occurs (In the edge faces· of kaolinite crystals. L-phenylalanine was more highly adsorbed than D-phenylalanine at pH 5.8. According to the t-test and U-test (both 1-tailed), the difference is Significant at the 4% level (t = 2.05; U = 10). At pH 2.0, the percentage of D-phenylalanine adsorbed was not significantly different than at pH 5.8 (t = 0.115, and U = 17, signifying a probability of about 50% under the null hypothesis in a 1-tailed test); but the quantity of L-phenylalanine adsorbed at pH 2.0 was about half the quantity adsorbed at pH 5.8, the difference being significant below the 1% level (t = 4.27, and U = 1, in 1-tailed tests). The difference between the quantities of L- and D-phenylalanine adsorbed at pH 2.0 is insignificant or of questionable significance at the 5% level (t= 1.86, and U= 7, in 1-tailed tests). These results are interpreted as follows. The edge faces of the kaolinite crystals preferentially adsorbed the L-optical isomer of phenylalanine in the absence of HCl. This effect was detectable at pH 5.8, though partially masked owing to non-discriminatory adsorption of the L- and D-isomers on the 001 faces, which comprise a much greater area than the edge faces. At pH 2.0 most of the phenylalanine molecules were cationic, and the crystal edges had a net positive charge (Marshall, 1964), resulting in mutual repulsion rather than adsorption. This would account for the drop in the percentage of L-phenylalanine adsorbed at the lower pH. Possibly suppression of the ionization of the carboxyl group of phenylalanine by the HCI also contributed to the inhibition of adsorption on the edge faces of the mineral. The fact that lowering the ambient pH had no appreciable effect on the adsorption' of D-phenylalanine is readily explained if the D-molecules were almost exclusively adsorbed to the 001 faces, which, in contrast to the edge faces, were presumably insensitive to the pH change (Marshall, 1964), Evidently the edge faces discriminated against the D-optical isomer on the basis of configuration, even under pH conditions favorable for adsorption. TABLE III

Data on the adsorption of L- and D-phenylalanine under different pH conditions Initial pH

Optical isomer

Mean % adsorbed

± standard error

Standard deviation

(to. 95 s /,JN) 5.8

L

D 2.0

L

D

19.0 ± 15.0 ± 9.77 ± 15.3 ±

3.58 3.16 4.30 7.13

3.87 3.42 3.44 5.70

The mechanism whereby polymerization of amino acids is catalyzed by kaolinite is unknown, but itprobably involves coordination of the a-carboxyl and 0' -amino groups of the amino acids to the octahedral sites of the mineral by means of Coulomb forces, ion exchange, and hydrogen bonding. Since the edge of a kaolinite crystal possesses an array of periodically alternating positively and negatively charged (basic and acidic) sites, and sites suitable for hydrogen bonding, it is not difficult to see how a series of amino acid zwitierions could line up along the crystal edges with their Chern. Geol., 7 (1971) 295-306

303

carboxyl and amino groups in juxtaposition, and hence in positions favorable for the formation of peptide linkages (cf. Degens and Matheja, 1968). Epitaxial alignment of amino acids on the edge of the octahedral layer is sterically feasible, since the mean distance between the O-atoms of the a -carboxyl group and the N-atom of the a-amino group (about 2.65 A, according to computations based on the data of Marsh and Donahue (1967)) is similar to the 0-0 distance in the octahedral layer of kaolinite (Brown, 1961). The results of this investigation have a bearing on the pre-biotic origin of proteins. The data demonstrate that when an enantiomorphous mineral catalyzes the polymerization of amino acids, the chirality of the mi.neral may pre-determine the chirality of the resulting peptide molecule. Thus, a racemic mixture of free amino acids may be resolved into isotactic L- or D-peptide molecules by epitaxial interaction with a dissymmetric mineral template such as a kaolinite crystal. An epitaxial mechanism of this sort could well have been responsible for the synthesis of optically active polypeptides on the pre-biotic Earth at temperatures below the boiling point of water. TQ.e great abundance, large surface area, adsorptive and catalytic activity, chemical diversity, alternating positively and negatively charged sites, and enantiomorphism of dioctahedral clay minerals suggest that this class of minerals may have played an important role in the bulk synthesis of primitive proteins (cf. Bernal, 1951; Goldschmidt, 1952; Degens and Matheja, 1968). There remains, however, a fundamental question: Why do the amino acids of contemporary proteins universally possess the L- instead of the D - configuration? In view of the complex and delicate web of ecological interactions between different. organisms, there may be strong selective pressure for all or most organisms to synthesize proteins of the same chirality (Wald, 1957), but it is not clear why the L- configuration was "chosen." If we may assume, tentatively, that the pre-biotic amino acids were initially racemiC, there are two possible explanations: (1) L- and D-polypeptides could have evolved separately on separate right- and left-handed crystals, which may have been equally abundant in the earth's crust; the mineral templates of opposite chirality could have been mixed together at random or concentrated in separate deposits, depending on provenance and dispersal. On this hypothesis, primitive organisms of opposite chirality evolved, and a caprice of Darwinian evolution caused the "L" organisms to win the" struggle for existence" (Wald, 1957). (2) Alternatively, it is possible that in nature there is, or was, an absolute preponderance of "L-fixing" minerals, with the result that all proteins were pre-destined to have the "L" configuration. The fact that the kaolinite used in this investigation is overwhelmingly, if not entirely, "L-fixing" implies that this hypothesis merits further investigation. Additional research is now in progress to test these hypotheses.

ACKNOW LEDGMENTS

I am indebted toDr. Egon T. Degens (Department of Chemistry, Woods Hole Oceanographic Institution, U.S.A.) for much valuable advice and 304

Chern. Geol., 7 (1971) 295-306

assistance. I thank Drs. Johann Matheja (Department of Physical Chemistry, Kernforschungslage, West Germany), G.E. Hutchinson (Department of Biology, Yale University, U.S.A.), and Preston Cloud (Department of Geological Sciences, University of California, Santa Barbara, U.S.A.) for critical comments, and Dr. G.W. Brindley (Department of Geochemistry and Mineralogy, Pennsylvania State University, U.S.A.) for technical suggestions regarding amino acid adsorption. The polarimeter was kindly provided by the Department of Chemistry, Yale University. The research was supported by post-doctoral fellowships awarded to me by the Department of Chemistry, Woods Hole Oceanographic Institution and the School of Forestry, Yale University (and funded by the U.S. National Science Foundation and the Westvaco Corp., respectively), and by grant NSR 22-014-001, awarded to Dr. Egon T. Degens by the National Aeronautics and Space Administration, U.S.A.

REFERENCES Akabori, S., 1959. On the origin of the fore-protein. Proc. Intern. Symp. Origin Life Earth, 1st Moscow, 1957, Intern. Union Biochem. Symp. Ser., 1: 189-196. Bailey, S.W., 1963. Polymorphism of the kaolin minerals. Am. Mineralogist, 48: 1196-1209. Bernal, J.D., 1951. The Physical Basis of Life. Routledge and Kegan Paul, London, 80 pp. Brown, G. (Editor), 1961. The X-ray Identification and Crystal Structures of Clay Minerals. Mineralogical Soc., London, 544 pp. Byk, A., 1904. Zur Frage der Spaltbarkeit von Racemverbindungen durch zirkularpolarisiertes Licht, ein Beitrag zur primaren Entstehung optisch-aktiver Substanz. Z. Phys. Chem., 49: 641-687. Coleman, B.D., 1958. On the properties of polymers with random stereo-sequences. J. Polymer Sci., 31: 155-164. Corey, R.B. and Pauling, L., 1953. Fundamental dimensions of polypeptide chains. Proc. Roy. Soc. London, Ser. B, 141: 10-20. Degens, E.T. and Matheja, J., 1968. Origin, development, and diagenesis of biogeochemical compounds. J. Brit. Interplanet. Soc., 21: 52-82. Degens, E.T. and Matheja, J., 1970. Formation of organic polymers on inorganic templates. In: Or6, J. and Kimball, K. (Editors), Structure, Function, and Origin of Nucleic Acids and Proteins. North-Holland, Amsterdam. Degens, E.T., Matheja, J. and Jackson, T.A., 1970. Template catalysis: Asymmetric polymerization of amino acids on clay minerals. Nature, 227: 492-493. Goldschmidt, V.M., 1952. Geochemical aspects of the origin of complex organic molecules on the earth, as precursors to organic life. New BioI., 12: 97-105. Gornall, A.G., Bardawill, C.J. and David, M.M., 1949. Determination of serum proteins by means of the biuret reaction. J. BioI. Chem., 177: 751-766. Gratzer, W.B. and Cowburn, D.A., 1969. Optical activity of biopolymers. Nature, 222: 426-431. Klabunovskif, E.!., 1959. Absolute asymmetric synthesis and asymmetric catalysis. Proc. Intern. Symp. Origin Life Earth, 1st. Moscow, 1957, Intern. Union Biochem. Symp. Ser., 1: 158-168. Marsh, R.E. and Donohue, J., 1967. Crystal structure studies of amino acids and peptides. Adv. Protein Chem., 22: 235-255. Marshall, C.E., 1964. The Physical Chemistry and Mineralogy of Soils, vol. 1. Wiley New York, N.Y., 388 pp. Miller, W.G., Brant, D.A. and Flory, P.J., 1967. Random coil configurations of polypeptide copolymers. J. Molec. BioI., 23: 67- 80. Chem. Geol., 7 (1971) 295-306

305

Mislow, K., 1966. Introduction to Stereochemistry. Benjamin, New York, N.Y., 193 pp. Oparin, A.!., 1957. The Origin of Life on the Earth. Academic Press, New York, N.Y., 495 pp. Paecht-Horowitz, M., Berger, J. and Katchalsky, A., 1970. Prebiotic synthesis of polypeptides by heterogeneous polycondensation of amino-acid adenylates. Nature, 228: 636-639. Terent'ev, A.P. and KlabunovskH, E.!., 1959. The role of dissymmetry in the origin of living material. Proc. Intern. Symp. Origin Life Earth 1st, Moscow, 1957, Intern. Union Biochem. Symp. Ser., 1: 95-105. Wald, G., 1957. The origin of optical activity. Ann. New York Acad. Sci., ti9: 352-3ti8.

306

Chern. Geol., 7 (1971) 295-306