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Hydrogen cyanide polymers: from laboratory to space
Planet. Space Sci., Vol. 43, Nos. lOill, pp. 1365-1370, 1995
Pergamon
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-0633/95
$9.50+0.00
0032-0633(95)00023-2
Hydrogen cyanide polymers : from laboratory to space Clifford N. Matthews
Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, U.S.A. Received 15 August 1994; revised and accepted 17 January 1995
Abstract. Hydrogen cyanide polymers-heterogeneous solids ranging in color from yellow to orange to red to black-may be among the organic macromolecules most readily formed within the solar system. The nonvolatile black crust of comet Halley, for example, may consist largely of such polymers. It seems likely, too, that HCN polymers are a major constituent of the dark, C=N bearing solids identified tentatively by IR spectra in the dust of some other comets, HCN polymerization could also account for some of the yelloworange-red coloration of’.Tupiter and Saturn, and perhaps for the orange haze high in Titan’s atmosphere. Studies of these polymers show that a yellow-brown powder can be extracted by water and further hydrolyzed to yield cl-amino acids. Several instrumental methods used for the separation and identification of these intriguing materials, including pyrolysis mass spectrometry, Fourier transform IR photoacoustic spectroscopy and supercritical fluid extraction chromatography, reveal fragmentation patterns and chemical functionalities consistent with the presence of polymeric peptide precursors-polyamidines-in HCN polymers. Implications for prebiotie chemistry are profound, Primitive Earth may have been covered by HCN polymers and other organic products through bolide bombardment or terrestrial synthesis, producing a proteinaceous matrix able to bring about the molecular interactions leading to the emergence of life. Cyanide polymerization could also be a preferred pathway beyondBarth and the solar system, on planetary bodies and satellites around other stars and in the dusty molecular clouds of spiral galaxies.
Hydrogen cyanide polymers : synthesis and structure
Liquid HCN (b.p. 25°C) polymerizes spontaneously to a dark brown or black solid at low temperatures in the presence of a base such as an amine or ammonia (Matthews and Moser, 1967). Polymerization also occurs
readily in non-aqueous solvents or in water (Vdlker, 1960 ; Matthews and Moser, 1966, 1967). Two types of structural units appear to be present in these solid materials. Most stable are thle ladder polymers (Vdlker, 1960), shown in Fig. 1, formally derived from the olefinic tetramer of HCN, diaminomaleonitrile (A), which is usually found among the products. It seems probable, however, that polymerization to the substituted polymethylene B proceeds by way of an HCN dimer (Volker, 1960 ; Kliss and Matthews, 1962), for which several structmes have been proposed [see Evans et al. (1991)]. Cyclization to C and D then leads to ladder structures possessing conjugated >C==N- bonds, as proposed by Viilker (1960) on the basis of extensive physical and chemical investigations further supplemented by the work of Umemoto et al. (1987). More controversial is the existence of the polyamidine structures shown in Fig. 2. Polyaminomalononitrile (F) can be considered an addition polymer of the reactive trimer aminomalononitrile (E), though again it is possible that polymerization occurs through an HCN dimer (Kliss and Matthews, 1962; Matthews and Moser, 1967; Yang et al., 1976). Cumulative reactions of HCN on the highly activated nitrile groups of F then yield the heteropolyamidines G, which are readily converted by water to heteropolypeptides (Matthews and Moser, 1966, 1967) with release of ammonia and CO,. Overall, this series of reactions constitutes a route for the direct synthesis of polypeptides without the intervening formation of ramino acids (Matthews and Moser, 1966, 1967; Matthews, 1992a, b) (Fig. 3). Several kinds of experiments have provided results consistent with this polyamidine model. In general, watersoluble, yellow-brown solids can be extracted from the products of each of the following types of reactions : 1. Base-catalyzed polymerization of liquid HCN, alone, in water or in non-aqueous solvents (Matthews and Moser, 1967). 2. Electric discharge experiments producing HCN from methane-ammonia mixtures (Matthews and Moser, 1966).
1366
DIAMINOMALEONITRILE
“OLYAMINOCYANOMETHYLENE
BLACK HCN POLYMERS (AZULMIC
ACID)
G
D
Fig. 1. HCN polymers formally derived from HCN tetramer (diaminomaleonitrile)
AMlNOMALONONlTRlLE
POLYAMINOMALONONITRILE
E
F
~E~ERO~L~AMl~l~~~ G
Fig. 2. HCN polymers formally derived from HCN trimer (aminomaiononitrile) 3. Alkaline hydrolysis of aminoacetonitrile (Moser and Matthews, 1968), aminomalononitrile (E ; HCN trimer) (Moser et al., 1968) and diaminomaleonitrile (A; HCN tetramer) (Moser et al., 1968), all of which are ready sources of HCN at high pH. 4. HCN modification of the reactive nitrile side chains of poly-a-cyanoglycine (see Fig. 3), a polyamide analog of polyaminomalononitrile (F) synthesized from the N-carboxyanhydride related to a-cyanoglycine (Warren et al., 1974; Minard et al., 1975). Acid hydrolysis of these yellow-brown polymers yields not just glycine, the major product, but other cc-amino acids as well, such as alanine, aspartic acid, glutamic acid, serine and threonine, together with some a-amino acids not found in proteins. Further GC-MS studies show that the glycine is perdeuterated when D,O-DC1 is used for hydrolysis instead of H,O-HCI (Matthews et al., 1977), an indication of the highly acidic nature of the methine carbons of F and of the great reactivity of its nitrile groups brought about by its delocalized structure.
Non-destructive analysis of the total solid product obtained from HCN, as well as of separate components, became possible with the advent of cross-polarization magic-angle spinning solid-state NMR spectroscopy (“C and “N). In particular, the unambiguous presence of secondary amide groups, as in peptides, has been established by double-cross-polarization studies on polymers synthesized from equimolar amounts of H13CN and HC”N (Schaefer et al., 1982; Matthews et al., 1984; McKay et al., 1984; Garbow et al., 1987). Several instrumental methods are now being used for the separation and identification of these intriguing polymeric materials, including photoacoustic Fourier transform IR spectroscopy, supercritical fluid extraction chromatography and pyrolysis mass spectrometry. Our integrated analytical approach has already revealed fragmentation patterns and chemical functionalities consistent with the presence of polymeric peptide precursors both in HCN polymers and in the Murchison meteorite (Liebman ei al., 1994). Taken together, these various analytical studies point to the presence of both peptide and ladder structures in
1367
C. N. Matthews : Hydrogen cyanide polymers H
I I C - C - N II NH
H
H HCN
I I C - C - N II NI
I c
H
I #
I
POLYAh4lNW4LONON~ll (HCN POLYMERS)
J-40 1
POLYnCYANOGLYClNE
W -co, I
l-Emu%1 H
40 H
I I c- - C - N
II IR 0 Fig. 3. Polypeplides
from polyamidines. Cumulative reactions of HCN on polyaminomalononitrile yield heteropolyamidines (with side chains R’), which are converted stepwise by water to heteropolypeptides (with side chains R)
HCN polymers exposed to water (see Table 1). Hybrid polymers-multimers or dendrimers-with peptides attached to ladders through their free amino groups may be major components. Hydrogen cyanide polymers in space
Comparable polymers are synthesized when methane and ammonia mixtures are converted to hydrogen cyanide by electric discharges (Matthews and Moser, 1967 ; Woeller and Ponnamperuma, 1969). The original presence on cometary nuclei of frozen volatiles such as methane, ammonia and water subjected to high energy sources (Whipple, 1974) thus makes them possible sites for the formation and condensed-phase polymerization of HCN (Matthews and Ludicky, 1986). Dust emanating from the nucleus, contributing to the coma and tail, would also arise partly from the polymer. Results from the recent Halley missions support this view. Following Vega observations of a dark surface largely masked by clouds (Sag-
deev et al., 1986), the Giotto multicolor camera showed the presence of a potato-shaped core (Z 15 x 10 km) with a very low albedo (Z&4%) (Reinhard, 1986; Keller et al., 1986). We propose that this non-volatile black crust is largely composed of hydrogen cyanide polymers and related compounds (Matthews and Ludicky, 1986). Continuing polymerization of HCN-a strongly exothermic process-may even be the cause of the dramatic cometary outbursts that have been noted by observers for many years (Rettig et al., 1992). The recognition that cyanide chemistry could be proceeding on Halley and other comets suggests that HCN polymerization is widespread within the solar system and beyond. Most significantly, through spectroscopic detection of the 2.2 pm overtone of the C=N stretching fnndamental mode, Cruikshank et al. (199 1) have tentatively identified molecules containing cyano groups in the very dark solids in the dust of the “new” comets, Panther and Bowell. The IR reflectance spectra of these bodies resemble the comparable spectra of HCN polymers in the same O-3 pm region. Therefore, carbonaceous chondrites from the asteroid
1368
C. h. htthews
Table 1. Summary of evidence reported 5y Marthews EI ~1, ‘0: :he presence ~1 peptrde-like and the Murchison meteorite Analytical mode
Previous studies [see Matthews (1992a, b)]
i. Non-destructive identification
Solid state (“C, “N) NMR spectroscopy : HCN polymer extracts (H,O) and insoluble residues contain polypeptide segments
2. Separation by fractionation
Liquid phase (Sephadex) column chromatography : HCN polymer extracts (H,O) contain heteropolypeptide segments
3. Anaiysis by fragmentation
Hydroiysis (GC-MS) : HCN polymers and the Murchison meteorite yield x-amino acids (also c(bydroxy acids)
~ beit may also possess HCN products. Studies of the Murchison meteorite have shown that free a-amino acids are present (Kvenvolden et al., 1973), together with acidiabile amino acid precursors of undefined structure (Cronin, 1976). Deuterolysis of these water-soluble yellow-brown solid extracts yields perdeuterated glycine, as would be expected from peptide segments in hybrid polymers derived from HCN (Matthews et al., 1977, 1980). Further evidence is being obtained by the integrated, organic analysis approach of Liebman et al. (1995) (see Table 1). Hydrogen cyanide polymerization could also account for much of the yellow-brown-orange coloration of Jupiter and Saturn (Matthews and Moser, 1966 ; Woeller and Ponnamperuma, 1969 ; Matthews, 1982), especially since HCN has been found in Jupiter’s reducing atmosphere and in the atmosphere of Titan, the largest moon of Saturn. Most intriguing is an orange haze high in Titan’s stratosphere that may consist of organic polymers (Owen, 1982). These could be polycyanides formed directly from HCN, some of which would be converted by water to heteropolypeptides after settling on the frozen surface or the hydrocarbon oceans of the satellite (Matthews, 1982). Laboratory simulations of Jupiter and Titan chemistry-reactions of methane, ammonia (or nitrogen) and water subjected to high energy sources-have yielded “‘tholins” (Sagan and Khare, 1979 ; Khare et al., 1984 ; McDonald et al., 199 l), dark materials of undefined structure whose IR reflectance spectra are remarkably similar to the spectra of HCN polymers (Matthews and Ludicky, 1986 ; Cruikshank et al., 1991). Further analysis will show if polymers of hydrogen cyanide and of acetylene are indeed significant components of these “tholins”, which on hydrolysis also yield cc-amino acids. Since HCN is one of the more abundant interstellar molecules found outside the solar system (Irvine et al., 1987), the presence of HCN polymers on interstellar grains would be expected. Solid-phase cyano group molecules have indeed been detected by IR absorption spectra (4.6 pm) in the vicinity of protostars within molecular
‘dyuroge;i
qa;iiae po;yme;s
segments in hydrogen cyanide polymers
Current studies [see Liebman et ai. (1995)] --____.__-_ __Photoacoustic Fourier transform IR spectroscopy (FTIR-PAS) : Aqueous extracts of HCN polymer and the Murchison meteorite possess polypeptide segments Supercritical fluid extraction chromatography @FE-Transcap FTIR-Mic) : Aqueous extracts of HCN polymer and the Murchison meteorite possess polypeptide segments Analytical pyrolysis (DIP-MS) : Hydrocarbons, fatty acids and polypeptides are present in Murchison meteorite. HCN polymers yield polypeptides
clouds of the Milky Way (Lacy et sl., l984). Further detections by Tegler et al. (1995) suggest that these X(CN) molecules may result from chemical processing of dust grains dominated by non-polar icy mantles in the local environments of pre-main-sequence stars.
Hydrogen cyanide Which came first, amino acids or their Folymers’! The ubiquitous presence of HCN in reducing environments invites the re-examination and possible reinterpretation of almost all previous research concerned with the origin of a-amino acids, including simulations of the chemistry of primitive atmospheres (Miller and Orgel, 1974), studies of aqueous cyanide chemistry (Or6 and Lazcano-Araujo, 1980; Ferris and Hagan, 1984) and meteorite analysis (Kerridge, 1991). In my view, these investigations of reactions ostensibly yielding x-amino acids actually supply evidence for the abundant prebiotic and extraterrestrial existence of polymeric protein ancestors-heteropolypeptides synthesized directly from hydrogen cyanide and water (Matthews and Moser, 1966, 1967 ; Matthews, 1984, 1985). The detection of amino acid polymers in some of these experiments (Lowe et al., 1963 ; Matthews and Moser, 1966, 1967; Woeller and Ponnamperuma, 1969; Su et al., 1989; Khare et al., 1989; McDonald et al., 1991) adds to the plausibility of this conclusion. Implications for prebiotic chemistry are profound. Primitive Earth may have been covered by HCN polymers, either through cometary deposition or by photochemical reactions in a reducing atmosphere. As polyamidines settled onto land and sea together with other organic products, a proteinaceous matrix developed able to take part in and promote interactions leading to the emergence of life. These polyamidines could have been the original condensing agents directing the synthesis of nucleosides and nucleotides from available sugars, phos-
1369
C. N. Matthews : Hydrogen cyanide polymers phates and nitrogen bases. Most significant would have been the parallel synthesis of polypeptides and polynucleotides arising from the dehydrating action of polyamidines on nucleotides. On our dynamic planet this polypeptide-polynucleotide symbiosis mediated by polyamidines may have set the pattern for the evolution of protein-nucleic acid systems controlled by enzymes, the mode characteristic of life today (Matthews, 1986, 1988, 1990, 1992a, b). Beyond Earth and the solar system, on planetary bodies around other stars and satellites, and in between, the existence of this preferred pathway-HCN polymerization-could surely give rise to circumstellar habitable zones rich in the molecular precursors of life.
phase CO and cyano group molecules toward compact infrared sources. Astrophys. J. 276, 533-543, 1984. Liebman, S. A., Pesce-Rodriquez, R. A. and Matthews, C. N.,
Organic analysis of hydrogen cyanide polymers : prebiotic and extraterrestrial chemistry. Adv. Space Res. 15, 3(71)3(80), 1995. Lowe, C. U., Rees, M. W. and Markham, R., Synthesis of complex organic compounds from simple precursors : formation of amino acids, amino acid polymers, fatty acids and purines from ammonium cyanide. Nature 199,219-222, 1963. Matthews, C. N., Heteropolypeptides on Titan? Origins of Life 12,281-283, 1982. Matthews, C. N., Chemical evolution : protons to proteins. Proc. R. Inst. Gt Britain 55, 199-206, 1984. Matthews, C. N., Universal protein ancestors from hydrogen cyanide and water, in The Search for Extraterrestrial Lzfe : Recent Developments (edited by Papagiannis), pp. 151-156.
Reidei, Dordrecht, 11985. Matthews, C. N., Simultaneous synthesis of polypeptides and References Cronin, J. R., Acid-labile amino acid precursors in the Murchison meteorite. Origin of Ltfe 7, 331-342, 343-348, 1976. Cruikshank, D. P., Hartmann, W. K., Tholen, D. J., Allamandola, L. J., Brown, R. H., Matthews, C. N. and Bell, J. F., Solid C=N bearing material on outer solar system bodies, Icarus
94,345-353,199l. Evans, R. A., Lorencak, P., Ha, T.-K. and Wentrup, C., HCN dimers : iminoacetonitrile and N-cyanomethanimine. J. Am. them. Sot. 113,7261-7276, 1991. Ferris, J. P. and Hagan, W. J., HCN and chemical evolution :
the possible role of cyano compounds in prebiotic synthesis. Tetrahedron 40, 1093-l 120, 1984. Garhow, J. R., Schaefer, J., Ludicky, R. A. and Matthews, C.
N., Detection of secondary amides in HCN polymers by dipolar rotational spin-echo 15N NMR. Macromolecules 20, 305-309, 1987. Greenberg, J. M., Interstellar dust: an overview of physical and chemical evolution, in Evolution of Interstellar Dust and Related Topics (edited by A. Bonetti, J. M. Greenberg and S. Aeillo), pp. 7-52. North Holland, Amsterdam, 1989. Irvine, W. M., Goldsmith, P. F. and Hjabnarson, A., Chemical abundances in molecular clouds, in Interstellar Processes (edited by D. J. Hollenbach and H. A. Thronson, Jr.), pp. 561-609. Reidel, Dordrecht, 1987. Keller, H. U., Arpigny, C., Barbieri, C., Bonnet, R. M., Cazes, S., Coradini, M., Cosmovici, C. B., Delamere, W. A., Huebner, W. F., Hughes, D. W., Jamar, C., Malaise, I)., Reitsema, H. J., Schmidt, W. K. H., Seige, P., Whipple, F. L. and Wilhelm, K., First Halley multicolour camera imaging results from Giotto. Nature 321, 320-326, 1986. Kerridge, J. F., A note on the prebiotic synthesis of organic acids in carbonaceous meteorites. Origins ofLzfe 21, 19-29, 1991. Khare, B. N., Sagan, C., Thompson, W. R., Arakawa, E. T., Suits, F., Calcott, T. A., Williams, M. W., Shrader, S., Ogino, H., Willingham, T. 0. and Nagy, B., The organic aerosols of Titan. Adv. Space Res. 4, 59-68, 1984. Khare, B. N., Sagan, C., Thompson, W. R., Flynn, L. and Morrison, M. A., Amino acids and their polymers in the lower clouds of Jupiter? Preliminary findings. Origins of Life 19,495496, 1989. Kliss, R. M. and Matthews, C. N., Hydrogen cyanide dimer and chemical evolution. Proc. natn. Acad. Sci. U.S.A. 48, 1300-
1306, 1962. Kvenvolden, K. A., Lawless, J. G. and Folsome, C. E., Organic matter in meteorites. Scient. Am. 227, June, 38-54, 1973. Lacy, J. H., Baas, F., Allamandola, L. J., Persson, S. E., McGregor, P. J., Lonsdale, C. J., Geballe, T. R. and Van DeBult, C. E. P., 4.6 micron absorption features due to solid
polynucleotides? Hydrogen cyanide polymers as prebiotic condensing agents. (Origins of Ltfe 16, 500, 1986. Matthews, C. N., Cosmic metabolism: the origin of macromolecules, in Bioastronomy-The Next Steps (edited by G. Marx), pp. 167-178. Kluwer, Dordrecht, 1988. Matthews, C. N., Simultaneous synthesis of polypeptides and polynucleotides. Hydrogen cyanide polymers as prebiotic condensing agents, in Prebiological Self Organization of Matter (edited by C. Ponnamperuma and F. R. Eirich), pp. 171178. Deepak, Hampton, VA, 1990. Matthews, C. N., Dark matter in the soiar system: hydrogen cyanide polymers. Origins of Life 2L421-434, 1992a. Matthews, C. N., Origin of life: polymers before monomers? in Environmental Evolution (edited by L. Margulis and L. Olendzenski), pp. 29-38. MIT Press, Cambridge, MA, 1992b. Matthews, C. N. and Ludicky, R. A., The dark nucleus of comet Halley: hydrogen cyanide polymers, in 20th ESLAB Symposium on the Exploration of Halley’s Comet (edited by B. Battrick, E. J. Rolfe and R. Reinhard), Vol. 1, pp. 2733277. ESA, Noordwijk, 1986. Matthews, C. N. and Moser, R. E., Prebiological protein synthesis. Proc. natn. Acad. Sci. U.S.A. 56, 1087-1094, 1966. Matthews, C. N. and Moser, R. E., Peptide synthesis from hydrogen cyanide and water. Nature 215, 1230-1234, 1967. Matthews, C. N., Nelson, J., Varma, P. and Minard, R. D.,
Deuterolysis of amino acid precursors : evidence for hydrogen cyanide polymers as protein ancestors. Science 198,622624, 1977. Matthews, C. N., Nelson, J. E. and Minard, R. D., Deuterolysis of amino acid precursors from the Murchison meteorite. Abstracts 6th Int. Conf. on the Origins of Life, Jerusalem, 2227 June 1980, p. 100. Matthews, C. N., Ludlicky, R. A., Schaefer, J., Stejskal, E. 0. and McKay, R. A., Heteropolypeptides from hydrogen cyanide and water : solid state i5N NMR investigations. Origins of Lzfe 14,2433249, 1984. McDonald, G. D., Khare, B. N., Thompson, W. R. and Sagan,
C., CH4/NH,/H20 spark tholin : chemical analysis and interaction with Jovian aqueous clouds. Icarus 94,354367, 1991. McKay, R. A., Schaefer, J., Stejskal, E. O., Ludicky, R. A. and Matthews, C. N., Double-cross-polarization detection of labelled chemical bonds in HCN polymerization. Macromolecules 17, 11241130, 1984. Miller, S. L. and Orgel, L. E., The Origins of Life on the Earth.
Prentice-Hall, Englewood Cliffs, NJ, 1974. Minard, R. D., Yang, W,, Varma, P., Nelson, J. and Matthews,
C. N., Heteropolypeptides from poly-a-cyanoglycine and hydrogen cyanide : a model for the origin of proteins. Science 190,387-389,197s. Moser, R. E. and Matthews, C. N., Hydrolysis of amino-
1370 acetonitrile : peptide formation. Experientlu 24, 658439, i 968. iMoser, R. E., Claggett, A. R. and Matthews, C. N., Peptide formation from diaminomaleonitrile (HCN tetramer) and from aminomalononitrile (HCN trimer). Tetrahedron Lett. 1599-1608, 1605-1608, 1968. Ore, J. and Lazcano-Araujo, A., The role of HCN and its derivatives in prebiotic evolution, in Cyanide in Biology (edited by B. Vennesland, E. E. Corm, C. J. Knowles, J. Westley and F. Wissing), pp. 517-541. Academic Press, New York, 1980. Owen, T., Titan. Scient. Am. 246, February, 98-109, 1982. Reinhard, R., The Giotto encounter with comet Halley. N a t u r e 321,313-318, 1986. Rettig, T. W., Tegler, S. C., Pasto, D. J. and Mumma, M. J., Comet outbursts and polymers of HCN. Astrophys. J. 398,
293-298, 1992. Sagan, C. and Khare, B.
stellar grains and gas.
N., Tholins : organic chemistry of interNature 277, 102-107, 1979.
Sagdeev, R. Z., Blamont, J., Galeev, A. A., Moroz, V. I., Shapiro, V. D., Shevchenko, V. I. and SzegS, K., Vega spacecraft encounters with comet Halley. Nature 321, 259-262,
1986. Schaefer, J., Stejskal, E. O., Jacob, G. S. and McKay, R. M.,
Natural abundance N-15 NMR of the solids from the reac-
tion of HCN and ammonia. Appl. Spectrosc. 1982.
36, 179-182,
Search of peptide-like materials in electric discharge experiments.
So, Y.-L., Honda, Y., Hare, P. E. and Ponuamperuma, C.,
Origins of Life 19,237-238, 1989. Tegler, S. C., Weintraub, D. A., Rettig, T. O., Pendlleton, Y. J., Whittet, C. B. and Kulesa, C. A., Evidence for chemical
processing of pre-cometary icy grains in circumstellar environments of pre-main-sequence stars. Astrophys. J. 439, 2799287, 1995. TJmemoto, K., Takahashi, M. and Yokota, K., Studies on the structure of HCN oligomers. Origins of Life 17, 283-293, 1987. Viilker, T., Polymeric hydrogen cyanide. Angew. Chem. 72,379384, 1960. Warren, C. B., Minard, R. D. and Matthews, C. N., Synthesis of x-cyanoglycine N-carboxyanhydride and a-cyanoglycine. J. org. Chern. 39, 3375-3378, 1974. Whipple, F. L., The nature ofcomets. S c i e n t . Am. 230, February, 48-57, 1974. Woeller, F. and Ponnamperuma, C., Organic synthesis in a simulated Jovian atmosphere. Icarus 1 0 , 3 8 6 - 3 9 2 , 1 9 6 9 . Yang, W., Minard, R. D. and Matthews, C. N., Azacyclopropenylidenimine: a low energy dimer of hydrogen cyanide. J. theor. Biol. 56, 11 l-123, 1976.
Pergamon
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-0633/95
$9.50+0.00
0032-0633(95)00023-2
Hydrogen cyanide polymers : from laboratory to space Clifford N. Matthews
Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, U.S.A. Received 15 August 1994; revised and accepted 17 January 1995
Abstract. Hydrogen cyanide polymers-heterogeneous solids ranging in color from yellow to orange to red to black-may be among the organic macromolecules most readily formed within the solar system. The nonvolatile black crust of comet Halley, for example, may consist largely of such polymers. It seems likely, too, that HCN polymers are a major constituent of the dark, C=N bearing solids identified tentatively by IR spectra in the dust of some other comets, HCN polymerization could also account for some of the yelloworange-red coloration of’.Tupiter and Saturn, and perhaps for the orange haze high in Titan’s atmosphere. Studies of these polymers show that a yellow-brown powder can be extracted by water and further hydrolyzed to yield cl-amino acids. Several instrumental methods used for the separation and identification of these intriguing materials, including pyrolysis mass spectrometry, Fourier transform IR photoacoustic spectroscopy and supercritical fluid extraction chromatography, reveal fragmentation patterns and chemical functionalities consistent with the presence of polymeric peptide precursors-polyamidines-in HCN polymers. Implications for prebiotie chemistry are profound, Primitive Earth may have been covered by HCN polymers and other organic products through bolide bombardment or terrestrial synthesis, producing a proteinaceous matrix able to bring about the molecular interactions leading to the emergence of life. Cyanide polymerization could also be a preferred pathway beyondBarth and the solar system, on planetary bodies and satellites around other stars and in the dusty molecular clouds of spiral galaxies.
Hydrogen cyanide polymers : synthesis and structure
Liquid HCN (b.p. 25°C) polymerizes spontaneously to a dark brown or black solid at low temperatures in the presence of a base such as an amine or ammonia (Matthews and Moser, 1967). Polymerization also occurs
readily in non-aqueous solvents or in water (Vdlker, 1960 ; Matthews and Moser, 1966, 1967). Two types of structural units appear to be present in these solid materials. Most stable are thle ladder polymers (Vdlker, 1960), shown in Fig. 1, formally derived from the olefinic tetramer of HCN, diaminomaleonitrile (A), which is usually found among the products. It seems probable, however, that polymerization to the substituted polymethylene B proceeds by way of an HCN dimer (Volker, 1960 ; Kliss and Matthews, 1962), for which several structmes have been proposed [see Evans et al. (1991)]. Cyclization to C and D then leads to ladder structures possessing conjugated >C==N- bonds, as proposed by Viilker (1960) on the basis of extensive physical and chemical investigations further supplemented by the work of Umemoto et al. (1987). More controversial is the existence of the polyamidine structures shown in Fig. 2. Polyaminomalononitrile (F) can be considered an addition polymer of the reactive trimer aminomalononitrile (E), though again it is possible that polymerization occurs through an HCN dimer (Kliss and Matthews, 1962; Matthews and Moser, 1967; Yang et al., 1976). Cumulative reactions of HCN on the highly activated nitrile groups of F then yield the heteropolyamidines G, which are readily converted by water to heteropolypeptides (Matthews and Moser, 1966, 1967) with release of ammonia and CO,. Overall, this series of reactions constitutes a route for the direct synthesis of polypeptides without the intervening formation of ramino acids (Matthews and Moser, 1966, 1967; Matthews, 1992a, b) (Fig. 3). Several kinds of experiments have provided results consistent with this polyamidine model. In general, watersoluble, yellow-brown solids can be extracted from the products of each of the following types of reactions : 1. Base-catalyzed polymerization of liquid HCN, alone, in water or in non-aqueous solvents (Matthews and Moser, 1967). 2. Electric discharge experiments producing HCN from methane-ammonia mixtures (Matthews and Moser, 1966).
1366
DIAMINOMALEONITRILE
“OLYAMINOCYANOMETHYLENE
BLACK HCN POLYMERS (AZULMIC
ACID)
G
D
Fig. 1. HCN polymers formally derived from HCN tetramer (diaminomaleonitrile)
AMlNOMALONONlTRlLE
POLYAMINOMALONONITRILE
E
F
~E~ERO~L~AMl~l~~~ G
Fig. 2. HCN polymers formally derived from HCN trimer (aminomaiononitrile) 3. Alkaline hydrolysis of aminoacetonitrile (Moser and Matthews, 1968), aminomalononitrile (E ; HCN trimer) (Moser et al., 1968) and diaminomaleonitrile (A; HCN tetramer) (Moser et al., 1968), all of which are ready sources of HCN at high pH. 4. HCN modification of the reactive nitrile side chains of poly-a-cyanoglycine (see Fig. 3), a polyamide analog of polyaminomalononitrile (F) synthesized from the N-carboxyanhydride related to a-cyanoglycine (Warren et al., 1974; Minard et al., 1975). Acid hydrolysis of these yellow-brown polymers yields not just glycine, the major product, but other cc-amino acids as well, such as alanine, aspartic acid, glutamic acid, serine and threonine, together with some a-amino acids not found in proteins. Further GC-MS studies show that the glycine is perdeuterated when D,O-DC1 is used for hydrolysis instead of H,O-HCI (Matthews et al., 1977), an indication of the highly acidic nature of the methine carbons of F and of the great reactivity of its nitrile groups brought about by its delocalized structure.
Non-destructive analysis of the total solid product obtained from HCN, as well as of separate components, became possible with the advent of cross-polarization magic-angle spinning solid-state NMR spectroscopy (“C and “N). In particular, the unambiguous presence of secondary amide groups, as in peptides, has been established by double-cross-polarization studies on polymers synthesized from equimolar amounts of H13CN and HC”N (Schaefer et al., 1982; Matthews et al., 1984; McKay et al., 1984; Garbow et al., 1987). Several instrumental methods are now being used for the separation and identification of these intriguing polymeric materials, including photoacoustic Fourier transform IR spectroscopy, supercritical fluid extraction chromatography and pyrolysis mass spectrometry. Our integrated analytical approach has already revealed fragmentation patterns and chemical functionalities consistent with the presence of polymeric peptide precursors both in HCN polymers and in the Murchison meteorite (Liebman ei al., 1994). Taken together, these various analytical studies point to the presence of both peptide and ladder structures in
1367
C. N. Matthews : Hydrogen cyanide polymers H
I I C - C - N II NH
H
H HCN
I I C - C - N II NI
I c
H
I #
I
POLYAh4lNW4LONON~ll (HCN POLYMERS)
J-40 1
POLYnCYANOGLYClNE
W -co, I
l-Emu%1 H
40 H
I I c- - C - N
II IR 0 Fig. 3. Polypeplides
from polyamidines. Cumulative reactions of HCN on polyaminomalononitrile yield heteropolyamidines (with side chains R’), which are converted stepwise by water to heteropolypeptides (with side chains R)
HCN polymers exposed to water (see Table 1). Hybrid polymers-multimers or dendrimers-with peptides attached to ladders through their free amino groups may be major components. Hydrogen cyanide polymers in space
Comparable polymers are synthesized when methane and ammonia mixtures are converted to hydrogen cyanide by electric discharges (Matthews and Moser, 1967 ; Woeller and Ponnamperuma, 1969). The original presence on cometary nuclei of frozen volatiles such as methane, ammonia and water subjected to high energy sources (Whipple, 1974) thus makes them possible sites for the formation and condensed-phase polymerization of HCN (Matthews and Ludicky, 1986). Dust emanating from the nucleus, contributing to the coma and tail, would also arise partly from the polymer. Results from the recent Halley missions support this view. Following Vega observations of a dark surface largely masked by clouds (Sag-
deev et al., 1986), the Giotto multicolor camera showed the presence of a potato-shaped core (Z 15 x 10 km) with a very low albedo (Z&4%) (Reinhard, 1986; Keller et al., 1986). We propose that this non-volatile black crust is largely composed of hydrogen cyanide polymers and related compounds (Matthews and Ludicky, 1986). Continuing polymerization of HCN-a strongly exothermic process-may even be the cause of the dramatic cometary outbursts that have been noted by observers for many years (Rettig et al., 1992). The recognition that cyanide chemistry could be proceeding on Halley and other comets suggests that HCN polymerization is widespread within the solar system and beyond. Most significantly, through spectroscopic detection of the 2.2 pm overtone of the C=N stretching fnndamental mode, Cruikshank et al. (199 1) have tentatively identified molecules containing cyano groups in the very dark solids in the dust of the “new” comets, Panther and Bowell. The IR reflectance spectra of these bodies resemble the comparable spectra of HCN polymers in the same O-3 pm region. Therefore, carbonaceous chondrites from the asteroid
1368
C. h. htthews
Table 1. Summary of evidence reported 5y Marthews EI ~1, ‘0: :he presence ~1 peptrde-like and the Murchison meteorite Analytical mode
Previous studies [see Matthews (1992a, b)]
i. Non-destructive identification
Solid state (“C, “N) NMR spectroscopy : HCN polymer extracts (H,O) and insoluble residues contain polypeptide segments
2. Separation by fractionation
Liquid phase (Sephadex) column chromatography : HCN polymer extracts (H,O) contain heteropolypeptide segments
3. Anaiysis by fragmentation
Hydroiysis (GC-MS) : HCN polymers and the Murchison meteorite yield x-amino acids (also c(bydroxy acids)
~ beit may also possess HCN products. Studies of the Murchison meteorite have shown that free a-amino acids are present (Kvenvolden et al., 1973), together with acidiabile amino acid precursors of undefined structure (Cronin, 1976). Deuterolysis of these water-soluble yellow-brown solid extracts yields perdeuterated glycine, as would be expected from peptide segments in hybrid polymers derived from HCN (Matthews et al., 1977, 1980). Further evidence is being obtained by the integrated, organic analysis approach of Liebman et al. (1995) (see Table 1). Hydrogen cyanide polymerization could also account for much of the yellow-brown-orange coloration of Jupiter and Saturn (Matthews and Moser, 1966 ; Woeller and Ponnamperuma, 1969 ; Matthews, 1982), especially since HCN has been found in Jupiter’s reducing atmosphere and in the atmosphere of Titan, the largest moon of Saturn. Most intriguing is an orange haze high in Titan’s stratosphere that may consist of organic polymers (Owen, 1982). These could be polycyanides formed directly from HCN, some of which would be converted by water to heteropolypeptides after settling on the frozen surface or the hydrocarbon oceans of the satellite (Matthews, 1982). Laboratory simulations of Jupiter and Titan chemistry-reactions of methane, ammonia (or nitrogen) and water subjected to high energy sources-have yielded “‘tholins” (Sagan and Khare, 1979 ; Khare et al., 1984 ; McDonald et al., 199 l), dark materials of undefined structure whose IR reflectance spectra are remarkably similar to the spectra of HCN polymers (Matthews and Ludicky, 1986 ; Cruikshank et al., 1991). Further analysis will show if polymers of hydrogen cyanide and of acetylene are indeed significant components of these “tholins”, which on hydrolysis also yield cc-amino acids. Since HCN is one of the more abundant interstellar molecules found outside the solar system (Irvine et al., 1987), the presence of HCN polymers on interstellar grains would be expected. Solid-phase cyano group molecules have indeed been detected by IR absorption spectra (4.6 pm) in the vicinity of protostars within molecular
‘dyuroge;i
qa;iiae po;yme;s
segments in hydrogen cyanide polymers
Current studies [see Liebman et ai. (1995)] --____.__-_ __Photoacoustic Fourier transform IR spectroscopy (FTIR-PAS) : Aqueous extracts of HCN polymer and the Murchison meteorite possess polypeptide segments Supercritical fluid extraction chromatography @FE-Transcap FTIR-Mic) : Aqueous extracts of HCN polymer and the Murchison meteorite possess polypeptide segments Analytical pyrolysis (DIP-MS) : Hydrocarbons, fatty acids and polypeptides are present in Murchison meteorite. HCN polymers yield polypeptides
clouds of the Milky Way (Lacy et sl., l984). Further detections by Tegler et al. (1995) suggest that these X(CN) molecules may result from chemical processing of dust grains dominated by non-polar icy mantles in the local environments of pre-main-sequence stars.
Hydrogen cyanide Which came first, amino acids or their Folymers’! The ubiquitous presence of HCN in reducing environments invites the re-examination and possible reinterpretation of almost all previous research concerned with the origin of a-amino acids, including simulations of the chemistry of primitive atmospheres (Miller and Orgel, 1974), studies of aqueous cyanide chemistry (Or6 and Lazcano-Araujo, 1980; Ferris and Hagan, 1984) and meteorite analysis (Kerridge, 1991). In my view, these investigations of reactions ostensibly yielding x-amino acids actually supply evidence for the abundant prebiotic and extraterrestrial existence of polymeric protein ancestors-heteropolypeptides synthesized directly from hydrogen cyanide and water (Matthews and Moser, 1966, 1967 ; Matthews, 1984, 1985). The detection of amino acid polymers in some of these experiments (Lowe et al., 1963 ; Matthews and Moser, 1966, 1967; Woeller and Ponnamperuma, 1969; Su et al., 1989; Khare et al., 1989; McDonald et al., 1991) adds to the plausibility of this conclusion. Implications for prebiotic chemistry are profound. Primitive Earth may have been covered by HCN polymers, either through cometary deposition or by photochemical reactions in a reducing atmosphere. As polyamidines settled onto land and sea together with other organic products, a proteinaceous matrix developed able to take part in and promote interactions leading to the emergence of life. These polyamidines could have been the original condensing agents directing the synthesis of nucleosides and nucleotides from available sugars, phos-
1369
C. N. Matthews : Hydrogen cyanide polymers phates and nitrogen bases. Most significant would have been the parallel synthesis of polypeptides and polynucleotides arising from the dehydrating action of polyamidines on nucleotides. On our dynamic planet this polypeptide-polynucleotide symbiosis mediated by polyamidines may have set the pattern for the evolution of protein-nucleic acid systems controlled by enzymes, the mode characteristic of life today (Matthews, 1986, 1988, 1990, 1992a, b). Beyond Earth and the solar system, on planetary bodies around other stars and satellites, and in between, the existence of this preferred pathway-HCN polymerization-could surely give rise to circumstellar habitable zones rich in the molecular precursors of life.
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