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CHAPTER 12 EXOTIC BIOCHEMISTRY IN EXOBIOLOGY* G. C. PIMENTEL, K. C. ATWOOD, HANS GAFFRON, H. K. HARTLINE, T. H. JUKES, E. C. POLLARD, and CARL SAGAN Possibly the most interesting and important exobiological discovery that could be made would be a life-form based upon chemistry radically different from that on Earth. It would be as great an error to omit consideration of non-Earth-like biochemical possibilities as it would be to fail to look for DNA. Of course, possibilities must be assessed within the known chemical and physical principles and speculations must stay within bounds of reason- ableness defined by the available knowledge about a given environment. Yet a significant argument can be made that a program of exploration for extraterrestrial life must include specific experiments directed toward what we shall call exotic biochemistry. ARGUMENTS FOR EXOTIC BIOCHEMISTRY, OR, HAZARDS OF "DOWN-TO-EARTH" THINKING If there is one principle upon which we might confidently build models of biogenic developments on other planets, it is that life-forms tend to evolve and persist that are amenable to their environment. Taken at face value, this principle makes it unlikely that any Martian biochemistry would be Earth-like. There are at least three highly probable features of 'Report prepared by Professor Pimentel as chairman of a study group on this subject. 243

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244 SOME EXTRAPOLATIONS AND SPECULATIONS the Martian environment that would imply that an Earth-like biogeny would have to evolve in opposition to its environment rather than in har- mony with it. 1. In the Martian environment, free water is in extremely short supply; Earth organisms usually contain more than 75 per cent water (except in certain resting states, such as spores and seeds). 2. The Martian atmosphere apparently provides little protection from solar ultraviolet light in the 1700-3000 A range. This spectral region is, in general, lethal to Earth organisms. 3. The average surface temperature on Mars probably ranges from 180°K to 300°K (—93°C to + 27°C); thus chemical reactions that pro- ceed with reasonable reaction rates in Earth organisms, namely, with a reaction half-time that is a small fraction of a diurnal period, would require many, possibly hundreds or thousands of diurnal periods in this low tem- perature regime. With these forbidding challenges to its existence, will the Martian life hunt for a capillary (where there is water) under a rock (where there is shade) in a hot spot (it is hunting for our famous room temperature!) and then proceed to evolve into chlorella? Yes, that is a possibility. But there are also the reasonable possibilities that water will find quite a dif- ferent role in the development of a Martian biota; that Martian evolution will take advantage of the greater range of photosynthetic reactions fur- nished by ultraviolet radiation; and that the Martian biochemistry will sort out and put to use reactions of much lower activation energy, so that a day's work can be done in a day. The importance of these considerations about the Martian environment is amplified manyfold when we remember that Mars is the solar planet that most closely resembles Earth, though the search for extraterrestrial life must include the other planets, as well as the more remote regions of the Universe. The chemical contrasts, for example, between Jupiter and Earth, are far more dramatic than those cited above. With this background, perhaps we should look again at a few rather familiar reference points many of us use to justify "down-to-Earth" think- ing about extraterrestrial life. Elemental Abundance One is led intuitively to expect a certain universality in biochemistry because of the evidence that there is a reasonably uniform cosmic abun- dance of the elements. However, what must matter is the elemental abundance at the time of biogeny. On Earth, life evolved in the presence of

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Exotic Biochemistry in Exobiology 245 very large amounts of silicon, large amounts of aluminum and quite small concentrations of phosphorus. A comparison of the relative biochemical importance of these elements places the abundance argument in perspective. Geochemistry provides such significant fractionation and biogeny decides its elemental constituents with such selectivity that cosmic abundances furnish, at best, dubious guideposts. Water, the Special Solvent Because water plays such a key role in terrestrial biochemistry, it is not surprising that many persons have concluded that some of the distinctive properties of water are essential to the development of life. Firsoff [1963], for example, considers at length the possibilities of life developing in such solvents as NH3, H2S, SO2, HCN, HF, F2O, etc., each selected and evalu- ated by contrasting it to H2O. A rather long list of properties of water is considered to identify its special value: its acid-base chemistry, high di- electric constant, long liquid range, high boiling point, large heat of fusion, large heat of vaporization, hydrogen bonding capabilities, large specific heat, ionizing solvent action, density inversion on freezing, high cosmic abundance (see, for example, Henderson [1958]). Only three or four of these properties need be considered to see that these properties are impor- tant, if at all, because of historical contingency only. The fact that ice floats seems a great convenience to a fish that must spend the winter in a pond that freezes. On another planet, ponds may never freeze or, if they do, the biota may be well acclimated to hibernation, encased in solid, as are known life-forms on Earth. The high dielectric properties of water account for its ability to dissolve salts to give electrolyte solutions. It is not at all clear, however, that electrolytic conduction plays an essential role in the development of life. While conduction and ion adsorption participate in the chemical processes that occur in cells, it does not seem that there are functions involved that cannot be otherwise discharged. The acid-base properties of water are certainly important in the chem- istry of aqueous solutions. There are, of course, many other solvents that possess analogous acid-base behavior. Ammonia is an obvious example of such a substance that might play a solvent role on Jupiter [Franklin, 1912; Sisler, 1961]. More important, however, is the question whether acid-base properties are indispensable to biochemical solvents. Hydro- carbon solvents, for example, are used in the laboratory in a multitude of organic synthetic reactions for which the chemist has at his disposal any solvent he wishes. Furthermore, the loss of solvent acid-base properties

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246 SOME EXTRAPOLATIONS AND SPECULATIONS does not delete acid-base reactions (in the generalized definitions of acid and base) from the possible synthetic routes; it merely implies that the solvent itself is not a reactant or product. The hydrogen bonding capabilities of water seem to be of crucial impor- tance. They are not necessarily essential, however. It must be remem- bered that absence of water does not imply the loss of other hydrogen- bonding functional groups that could serve similar structural functions, as in terrestrial biochemistry. Secondly, the solvent action of water is aug- mented by its hydrogen bonding but this action is specifically effective for solutes that are themselves hydrophilic. In a non-aqueous biochemistry, hydrophobic substances might predominate, and in such a case the hydro- gen bonding of water might be a liability rather than an asset. The liquid range of water (100 C°) can be contrasted unfavorably to the rather short liquid ranges of other pure substances such as hydrogen sulfide, 21 C°, and methane, 22 C°. Such contrasts neglect, however, the possibility presented by solutions. For example, a hydrocarbon solution of isopentane in 3-methyl pentane boils above room temperature and it remains fluid at temperatures as low as 120°K (—153°C). On a hydro- carbon-rich planet, such as Jupiter, a variety of hydrocarbons would be expected, hence the boiling ranges of pure hydrocarbons are hardly relevant. Careful reflection about each of the properties of water reveals that no matter how they are woven into the Earth-pattern of life, none of them seems to involve a functional uniqueness that would preclude life without that property (cf,, Blum [1951]). The Special Role of Carbon The idea that some other element or combination of elements might replace carbon in biochemistry is usually assigned to science fiction, though the arguments are mostly quite contingent upon the assumption that every biogeny must parallel very closely our own. This point will be discussed at length later in this section but, for the moment, consider some specific arguments sometimes cited to eliminate silicon as a possible carbon substi- tute. First, it is sometimes noted that silicon is from a row of the periodic table in which double bonds are not important. Yet phosphorus, adjacent to silicon in the periodic table, has a busy role in our own biochemistry. It is sometimes remarked that the stability of the silicon-oxygen bond precludes the possibility that silicon-silicon bonds could be important. Putting aside the obvious thought that silicon-oxygen chains might them- selves suffice as skeletal substitutes for hydrocarbon or amide chains, we must note that the prevalence of nitrogen in our biochemistry occurs in

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Exotic Biochemistry in Exobiology 247 spite of the extreme stability of elemental nitrogen, N2 (bond energy 225 kcal). We are reminded by this example that on Earth, nature has found innumerable ways to circumvent obstacles posed by thermodynamic instability. EXOTIC BIOCHEMISTRIES: SOME SUGGESTIVE POSSIBILITIES To explore the possibility that exobiology might furnish us some exam- ples of unfamiliar biochemistry, we must make a preliminary examination of the biochemical functions that seem important to life. Only then can we support a contention that an exotic biochemistry is likely or unlikely. From a biological point of view, the most crucial manifestations of life seem to be the capability for information storage and for information transfer. The second of these implies replication and the two together imply mutability, and hence natural selection. We might also find useful guidance in the chemical restraints that seem important: 1. A functioning biochemistry must include provision for energy storage and transfer through molecular rearrangements. 2. There must be reasonable synthetic routes toward biochemically important molecules, beginning with available starting materials. 3. There must be aperiodic, but informationally significant, molecular elements. None of the constraints, nor their assembly, leads uniquely to terrestrial biochemistry. To illustrate this, consider the elements of an informationally significant polymer. First, there must be aperiodic but non-random func- tional appendages. Any polymeric skeleton that could be interrupted aperiodically will suffice. Symbolizing the functional appendages as R (which might be carbonaceous) some possibilities are illustrated below: .). R (S)- R (S), R \ / v \ A/ \ A s s s s s VS), R (S). R (S).. R (S) s Sx S S S S S S \/\/\ S R: S R:, S

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248 SOME EXTRAPOLATIONS AND SPECULATIONS H R,. H R, V/ w X X x H R, H R:t \H H K9 H H H H ( V V V V N./ N-/ N /\ A X H R, H H H Rs H II \ N—N N—N N—N N—N N R, H Rs In the first example, information is contained in the chain lengths, and their order Si, S~, &., etc. In the second and subsequent examples, the information is contained in the nature of the functional groups, Rj, R2, and R3, and their order. With this barest of introductions to the possibilities, we might consider the likelihood of chemical synthesis. The ease of rearrangement and re- assembly of sulfur chains is well known. Silicate structural elements are already prevalent in great variety around us. They could furnish skeletal stability in a biogeny that must adapt to higher ambient temperatures where hydrocarbons might be unstable. The conjugated carbon chain is included merely as a reminder that very many polymer possibilities are already known that could be utilized in a biogeny that evolved in an environment rich in hydrocarbons and deficient in water. It must be remembered that most hydrocarbons are not soluble in water and that most organic labora- tory syntheses are carried out in non-aqueous media. The conjugated carbon chain shown has another aspect of some impor- tance. This is a chain type in which stereospecific control is now possible on a commercial basis. The laboratory development, within the last ten years, of stereospecific polymerization catalysts demonstrates that stereo- specificity should not be considered to be uniquely connected with the examples we find in terrestrial biochemistry. Finally, there are listed two chain types that will have quite low stability relative to amide chains. This would be the type of chain we might expect and look for in an environment in which an organism must carry out its

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Exotic Biochemistry in Exobiology 249 metabolic processes on a diurnal time scale comparable to that of Earth, but at a temperature 50° lower (viz., like Mars?). In view of the unfamiliarity of the nitrogen chain shown in the last example, it warrants more discussion. Many nitrogen-rich compounds are well-known and well-characterized; they are not, however, widely known. It may surprise many readers, for example, to learn that the compound glyoxal Wj-guanyl hydrazone, with a skeletal chain consisting of four carbon and six nitrogen atoms, is available commercially at a price below $2.00 per gram. The atomic ratio, N/C, in this compound is 2. HN=C—NH—N=CH—CH=N—NH—C=NH ! | NH2 NH2 glyoxal 6«-guanyl hydrazone As a second example, the compound diimine, N2H2, has just been prepared and spectroscopically identified by Rosengren and Pimentel [1964] through low temperature techniques. The indications are that this com- pound, which is the nitrogen counterpart of ethylene, / H diimine has a well moderated and rich chemistry at temperatures well below room temperature, though the compound is extremely reactive under our ambient conditions. The likelihood that nitrogen-rich polymers could have biologi- cal importance on a low temperature planet cannot really be assessed (and it cannot be rejected) until we have much more laboratory knowledge of their properties. In any event, reactivity and instability can be used as a basis for rejection of a structural possibility only if due attention is paid also to the ambient temperature on the planet of interest. Of the environmental factors, temperature has dramatic importance through its control of reaction rates. In evaluating this factor, it must be remembered that enzymatic reactions, like all chemical reactions, are characterized by activation energies that control their reaction rates. A metabolic reaction with a half-time of one minute at 27°C (300°K) might have an activation energy of 20 kcal. The same reaction would have a half-time of 25 hours at —27°C (246°K), possibly destroying its useful- ness if the diurnal period is comparable to ours. On the other hand, con- sider a polymer that decomposes with a half-time of seven days at 27 °C

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250 SOME EXTRAPOLATIONS AND SPECULATIONS and a presumed activation energy of 25 kcal. This reaction would have a half-time of 175 years at —27 °C. This change shows that a polymer that cannot even be stored at room temperature could be quite stable enough for biological utilization in a Martian climate. As a specific example, the Si-Si bond energy in Si2H6 is about 50 kcal/mole, in contrast to the 83 kcal/mole bond energy of the C-C bond in C2H6. This difference guarantees that silane chains would be unsuited to an Earth-like climate. On the other hand, the reaction rate considerations just given indicate that a low temperature environment might be quite favorable for biological utilization of such chemical linkages. IMPLICATIONS IN THE EXOBIOLOGICAL PROGRAM The possibility of exotic biochemistry is important insofar as it may affect experimental investigations and the interpretation of results. Thus the search for evidence of a biota leads to a search for characteristic forms. The significance of considering the exotic aspect of biochemistry, in this context, is that it may imply a revision in the manner in which the morpho- logical investigation is conducted. A morphologically characteristic struc- ture that is stable at an ambient temperature of —27 °C might be destroyed merely by warming it to +27 °C for the purpose of the experiment. More generally, the considerations raised in this section lead to the assignment of high priority to experiments that disturb the sample as little as possible and that test the response of the sample to change of environ- mental conditions (e.g., the diurnal cycle, water vapor, pressure, tempera- ture, organic solvents). Finally, the possibilities of exotic biochemistries lead to the design of experiments that are less presumptive and more fundamental in intent (e.g., elemental and functional group analyses, passive calorimetry). Some of the most sensitive specific tests one might make for Earth-like biochem- istry (e.g., a response to a particular enzyme) are of the least value for non-Earth-like biochemistries. The very specificity that makes the experi- ment significant if the test is positive, reduces its informational content almost to zero if the test is negative. CONCLUSION No matter how exciting the prospects of discovering an exotic biochem- istry, the search for life on Mars will, no doubt, begin with an emphasis on the search for evidence of terrestrial biochemistry. Even so, the present

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Exotic Biochemistry in Exobiology 251 discussion suggests two guiding principles that should be rigidly followed in this quest. First, the conclusion that life does not exist in a given environment cannot be established by experiments directed solely toward the biochemistry we know on Earth. Second, every set of exobiological experiments should yield some definite results that will serve to improve the next set of experiments, even if all direct tests for evidence of life are negative. This second guide, wisely applied, would slowly and surely lead us to the discovery of quite unexpected biological phenomena, if any exist. REFERENCES Blum, H. F. (1951), Time's Arrow and Evolution, Princeton Univ. Press. Firsoff, V. A. (1963), Life Beyond the Earth, Basic Books, Inc., N. Y., p. 108-46. Franklin, E. C. (1912), The Ammonia System of Acids, Bases, and Salts. Am. Chem. J., 47, 285. Henderson, L. J. (1958), The Fitness of the Environment, Beacon Press, Boston. Rosengren, K., and Pimentel, G. C. (1964), Unpublished. Sisler, H. H. (1961), Chemistry in Non-Aqueous Solvents, Reinhold Publ. Corp., N. Y.