Optical activity and evolution

J. theor. Biol. (1981) 90,191-198

Optical Activity and Evolution G. P. GLADYSHEV AND M. M. KHASANOV Institute of Chemical (Received

Physics, The U.S.S.R. Academy 117334 Moscow, U.S.S.R.

11 December

1979, and in revised form 1980)

of Sciences,

10 November

Dissymmetric combinations of weak natural fields exert a stereospecific effect, which may result under some conditions characteristic of outer space in the formation of appreciable amounts of optically active compounds. Synthesis of optically active matter could occur in the early stages of the formation of the Solar System. The optical activity of substances of biological origin is one of the salient features of life. Therefore those investigators who regard the emergence of life as a natural process seek to find a natural explanation of the formation of optically active substances (Calvin, 1969; Rutten, 1971; Fox & Dose, 1972; Thiemann, 1974; Broda, 1975). Many concepts of this phenomenon have been put forward (a sufficiently full exposition of them can be found in a number of works). All existing hypotheses and theories, however, give rise to some well-founded objections (Rutten, 1971; Fox & Dose, 1972; Thiemann, 1974). The formation of optically active substances involves the presence of some dissymetric agent. It is believed that such an “agent” in the formation of the early chiralic molecules may have been dissymetric combinations of various natural fields (electric, magnetic, gravitational) and radiations. The effect of such combinations of fields on the processes of chemical evolution could lead to the formation of a mixture of optical isomers containing in some excess (however small at first) isomers of some one chirality. The appearance of such excesses (“seeds”) of optically active material makes it possible for other mechanisms of dissymetric synthesis, more effective but incidental without the primary “seeds”, to come into play. Subsequent biological evolution violates once and for all the symmetry of the “right” and “left”. This is due to the fact that the appearance of living organisms, complicated “polycrystalline systems”, requires, as is well known (Broda, 1975; Wald, 1957), a stereospecific correspondence between molecules, supramolecular structures and their aggregates. 191

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It is thus appropriate to consider two stages in the analysis of the appearance of optical specificity characteristic of modern biological systems: (1) a stage characterized by the appearance of “seeds” of optically active material in sufficient amounts under the influence of dissymmetric combinations of natural fields early in chemical evolution; (2) a stage when there is a gradual increase, in the course of further chemical and biological evolutions, in the initial excess of enantiomers of the same sign. In the final steps of this stage a “phenomenal optical purity” of many substances of biological origin, and in particular an exclusive use of I -amino acids and D-sugars by most living systems, is attained. In the present paper we shall discuss only some of the problems concerning the first stage. We first of all consider the difficulties faced by the theories dealing with the question of the appearance of “seeds” under the impact of dissymmetric combinations of natural fields. These difficulties are basically due to the fact that natural fields are too weak to influence chemical processes at the molecular level in any essential way. Speaking about weak natural fields we imply in particular the fact that changes in the energy states of molecules induced by these fields are many orders smaller than the energy of the thermal motion of atoms and molecules. At normal temperatures (T = 300 K) the thermal motion of molecules is liable to blurr the effect of natural fields; the latter cannot influence appreciably the energy states of the molecules and the chemical reactions involved. However, at very low temperatures there may occur stereochemical effects in natural fields, due to a preferred orientation of the molecules of reagents in a certain direction. It is believed that the Earth’s magnetic field may have been one of the main factors the combined effects of which have led to the dissymmetry of living beings. A magnetic field has a plane of symmetry perpendicular to the magnetic lines of force. Therefore it exerts a dissymmetric effect only in conjunction with some physical phenomenon the effect of which is nonsymmetrical with respect to the plane of symmetry. To simplify the presentation of materiai that follows (it is rather difficult to consider some physical phenomena occurring together) an attempt could be made to estimate the contribution of the magnetic field to the appearance of such a dissymmetry. To this end one should first of all estimate the orienting effect of the magnetic field. The Earth’s magnetic field (its induction is B -0.6 G) can have an appreciable orienting effect only on non-rotating molecules. This is clear from a mere comparison of the rotational energy of molecules, usually -‘I 10P” erg for the lower rotational levels, with the orientation energy of a

OPTICAL

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paramagnetic molecule in the Earth’s magnetic field, which is W lo-” erg (k. being the Bohr magneton). We assume for simplicity that non-rotating molecules can be only in the following two ways: “fieldwise” [the concentration molecules will be denoted by n(l)] and “counterfieldwise” [the tration here is denoted by n (2)]. It is easy to show that n(l)-n(2) n0

“E’

193 = poB =

oriented of these concen-

w

where no is the concentration of molecules at the zero rotational level, k is the Boltzmann constant, and T is the excitation temperature of molecules. On denoting the total concentration of molecules by n, and the rotational statistical sum by 2 we obviously have no = n/Z. Assuming the rotation to be quasiclassical, the sum 2 can be replaced by the corresponding classical integral (Landau & Livshits, 1976). It is thus possible to arrive at the expression

(1) where A, B, C are the rotational constants of a molecule, h is Planck’s constant, and u’ is the number of physically indistinguishable orientations of a molecule. Further, we obtain the following formula for estimating the relative number of field-oriented molecules: n(l)-n(2) n By substituting

z ( !!)3’2( 43

1’2!yT-5/2,

(2)

in (2) T = 3OO”K, W = 10e2’ erg, A, B, C = 10” Hz we get n (1) - n (2) ~ 1o.-9 n

From (2) it is seen that the number of field-oriented molecules decreases rapidly as the temperature is increased, and at normal temperatures of about 300 K the orientation is quite chaotic indeed, and n (1) - n (2)/n is negligibly small. It is for this reason that the stereospecific effect of relatively weak natural fields does not manifest itself appreciably in the normal conditions of the Earth. One of the authors of this paper (Gladyshev, 1978~) has put forward the idea that the stereospecific synthesis of certain optically active substances could have taken place already in the early stages of the formative period of

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the Solar System, in the protoplanetary cloud or in the cloud that surrounded the Protoearth (Gladyshev, 1977, 19786: Budtov 8~ Gladyshcv. 1979). In super-rarefied cosmic objects the effect of force fields ~magnetic. electric, gravitational) on the orientation of reagent molecules can become greatly enhanced as compared with such an effect in the normal conditions ot the Earth. This is clearly demonstratod already by equation I 2 1. The orientation of molecules is stable and their rotation in a dense cloud is caused by collisions. In this case the temperature to appear in (2 1must be the kinetic temperature T = T,. But in the super-rarefied gases of outer space the collision frequencies are so small that the interaction of molecules with the cosmic radiation prevails over their collisional interactions. The controlling factor in dark clouds containing much dust which absorbs optical and ultraviolet radiation will be the interaction with the relict radiation having T,, = 2.7 K. A peculiar “cooling” of the clouds to 2.7 K takes piace relative to the rotational degrees of freedom of the molecules. At the same time the kinetic temperature of the molecules may be rather high, 100 to 200 K for example. When using formula (7) in estimating the number of field-oriented molecules in such clouds, we should put 7‘ ~=7;,. [Strictly speaking, the use of formula ( 1) at such low temperatures is not quite lawful. But for a rough estimate formula i 11, and therefore (2 1.may be quite eligible at T=2.7 K.] One must not ignore the fact that some of the phases in the evolution of interstellar clouds may lead to a sharp increase in magnetic induction in them as compared with 10ph G typical of such clouds. Thus it may be suggested that the compression of the Solar nebula and of the nebulae of the protoplanets may have been accompanied by an increase in magnetic intensity in some parts of the protoclouds of up to 1C-1 O5 G (Hoyle, 1960; Alfvin & Arrhenius, 1976). In the rarefied parts of dark clouds whose magnetic intensity has increased for one reason or other and become comparable with that of the Earth (B = 1 G) the orientation of molecules by the magnetic field must become rather noticeable. Indeed. from ( 1) we have. for T = 2.7 K and W = 1V” erg, n(l)-n(2) II

L=z10-j

In general it is possible to notice such a specific feature of chemical reactions in outer space as the much greater dependence of reaction parameters on the presence of force fields than that observed on Earth. And while the cause of the optical activity of substances was the stereospecificity of the reactions of their formation due to the effects of dissymmetric fields

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195

and radiations, the formation per se must have taken place in outer space rather than on Earth. For comprehensiveness, we should note that processes that are susceptible to weak fields under normal conditions are those connected with molecular aggregates of sufficiently large size. It may some day be possible to explain the formation of optically active substances under the action of natural fields even in the earth conditions, but this will have to wait for a supramolecular hypothesis. Let us note that the possibility of gravitational fields’ weak influence on stereospecific synthesis has recently been confirmed even in the conditions of the Earth (Dougherty, 1980; Edwards, Cooper & Dougherty, 1980). If these facts are verified, they may be regarded as one of the arguments in favour of the general idea that under certain conditions stereospecific reactions may in principle be sensitive to gravitational fields (Gladyshev, 1978b). We shall now estimate the particle concentration in clouds where one can neglect the collisional excitation of molecules. Assuming that the basic component of a cosmic cloud is molecular hydrogen, we shall take into account only collisions with HZ. In this case a molecule of mass Mparticipates in q = nH2ti collisions in unit time (Chapman & Kauling, 1960) where n& is the Hz concentration, g is the collision cross-section; 5 = w is the mean relative velocity, and p is the reduced mass of M and HZ molecules. A collision-excited molecule must spontaneously emit the excitation energy and return to its initial state long before the next collision takes place. To estimate nuz only the transitions between the most longlived, i.e. rotational, states should be considered. We thus arrive at the condition qf&, <
A UCP,,” .

mn

(3)

Of all the molecules so far discovered in the interstellar clouds only three, viz. HCN, CH&N and CH$HCN, are used here for comparison. Taking Tk = 100 K, (+ = 10-l’ cm*, we have (Khasanov & Gladyshev, 1980) %X2<< 10’ cmP3 for the J = 0- 1 transition of the HCN molecule, n&<< lo4 crnm3 for the J = 0- 1, K= 0 transition of the CH3CN molecule, and nH2<< lo3 cmM3 for the transition llO-OOo of the CHzCHCN molecule. Thus we should have n& 2: lo* cme3. This implies that spontaneously emitted photons escape the cloud without being absorbed. With concentrations nH, 5 lo2 cmP3 this holds for clouds up to 10’ a.u. wide.

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We suggest that some of the optically active substances of the Solar System may have formed already during its formation as a result of the stereospecific effect of the magnetic and some other fields in the course ot chemical processes in a pre-planetary cloud. It is clear from the foregoing that for this to have happened: (1) there must have been zones with low concentrations t 100 cm ’ or less) in a pre-planetary cloud; (2) those zones must have been protected by thicker layers of the cloud against optical and ultraviolet emission and thus interacted only with the relict radiation whose temperature according to the Hot Universe theory may have been about 5 K, i.e. a little higher than the modern value: (3) the magnetic intensity of a pre-planetary cloud must have been at least comparable with that of the Earth (B = 1 G). Note that a number of investigators recognizing a possible role of magnetic fields in the Solar System cosmogony have come to the conclusion that induction must have been high enough. But was it possible, for example, for any distant areas of the protoplanetary cloud to have as low a concentration as 10’ cm- ’ or even less? Yes, if one assumes (Alfvtn & Arrhenius, 1976; Gladyshev, 1977, 1978aJ 1that the primary matter of the planets was being introduced into the Solar System gradually, over a very long period of time, say, of the order of lo’-lo9 years. Some authors have suggested (Gladyshev, 1978a,6; Budtov & Gladyshev. 1979) that the substance of the Protosun may have been gradually diffusing into the initially rarefied protoplanetary cloud. It is not ruled out either that stereospecific synthesis may have taken place in some zones of the cloud that remained after the formation of the Protoearth under the effect of its magnetic field. The amount of active substances which could form in such a rarefied space would be small of course. But the importance of the proposed mechanism of dissymmetric synthesis lies in the fact that it produces an excess (however small) of one of the enantiomers, the “seed” of optically active substances. Does the proposed idea imply that organic matter of extraterrestrial origin in the Solar System will exhibit appreciable optical activity? Not necessarily. For a primary “seed” of optically active substances to be maintained or all the more increased, as already noted, the more effective mechanisms of dissymmetric synthesis must “come into play” at later stages of chemical evolution or stages of pre-biological and biological evolutions. This phenomenon has taken place on the Earth, but it is not clear if it has anywhere else in the Solar System. In conclusion we should like to draw attention once again to the already mentioned mechanism of enhancing the ‘*seeding” effect of natural fields

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which requires no extraordinary cosmic conditions for manifestation. As already noted, aggregates of sufficiently large size, supramolecular formations (structures or domains), are sensitive to weak fields. (A quantitative estimate of the orienting effect of weak fields on such structures can be readily obtained by classical methods.) Biological evolution may lead to the selection of one of the dissymmetric types of such structures for the construction of structures of higher orders, which must further a shift in the corresponding quasi-equilibria (Gladyshev, 1978~) and in the long run the selection of stereoisomers of one form or another (either L- or o-molecules). The authors are very grateful to G. A. Tolstikov

and N. S. Zefirov for valuable

discussions and support.

Addendum

Recently C. A. Mead and A. Mascowitz (J. Am. 7301) have theoretically examined the possibility synthesis from achiral reactants contained in (Dougherty, 1980). It is concluded that the attempts

Chem. Sot. (1980) 102, to achieve dissymmetric rapidly rotating vessel to achieve this must fail.

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