Why is chirality so important?

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<ul><li><p>Bio Systems, 20 (1987) 1--6 1 Elsevier Scientific Publishers Ireland Ltd. </p><p>PREFACE </p><p>WHY IS CHIRALITY SO IMPORTANT? </p><p>ANDREW S. GARAY Department of Biochemistry and Biophysics, Agricultural Experiment Station, Texas A &amp; M University, College Station, TX 77843-2128 (U.S.A.) </p><p>Introduction </p><p>At the last meeting of the "European Research group on Chemical Evolution, Early Biological Evolution and Exobiology" (Leiden, October 18--19, 1985), Dr. G. Spach and myself recommended the organization of an interdisciplinary meeting on "Chiral Sym- metry Breaking in Physics, Chemistry and Biology." While the majority of the partici- pants were not interested, about 30% very enthusiastically supported the suggestion. Only one person was between the two extremes: a well-known scholar who contributed much to our knowledge on the prebiological chem- istry of our planet. After the meeting, we met and he said, "I wish you would explain to me why chirality is ~so important." I was pressed by time, so I promised him I would reply later. This is my reply to him as well as to others interested iin the question. </p><p>I willingly admit that from the biological and chemical point of view the homochirality of life does not seem to be very important. Life has chosen in her great arbitrariness one enantiomer. But, she could have chosen the other one! Neither seems to have any advantage over the other. In chemical labora- tories, they are produced with equal prob- abilities. Life should work equally well with L- sugars and D- amino acids. There is hardly any argument against this view if we stay within the reabrL of biology and chemistry. If, however, we consider physics, chirality </p><p>is no longer a marginal problem but one of the most challenging and fundamental notions. I have to explain this in somewhat more detail in order to answer our question: "Why is chirality so important?" </p><p>A. Classic view of chirality </p><p>The existence of two oppositely chiral objects, such as a pair of hands, fascinated scientists even 300 years ago. The two hands are mirror images; they seem identical, yet they are not superimposable. The only differ- ence between them is that one can be described by +x, +y, +z coordinates, whereas the other by -x , -y , - z coordinates. Thus, they are space inverted forms. It was realized that nature prefers certain arrangements over their mirror images. Man has his heart on his left hand side, the majority of snails are right handedly helical. Leibniz considered this an arbitrary preference. He stated that laws of nature equally permit the existence of both mirror images. This was the first formulation of an important symmetry principle, the parity principle. </p><p>It was a great shock when Pasteur dis- covered that living beings can use only one form of mirror image crystals. His mold grew well on a nutrient solution made of L tartarate crystals, whereas it starved if the nutrient solution was made of the D crystals . Apparently, the crystals did not lose their handedness during dissolving and apparently </p><p>0303-2647/87/$03.50 1987 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland </p></li><li><p>Leibniz's symmetry principle did not apply to life. Pasteur believed in the unity of nature; he extrapolated the symmetry breaking to the universe and claimed that "life is a function of a cosmic asymmetry" (Dubos, 1960), We know today that his idea was basically correct, but it was wrong in details. </p><p>The handedness of liquids was puzzling because the solute molecules are randomly oriented and this should cancel asymmetries. Pasteur and Fresnel realized that solutions exhibit optical rotation; therefore, chiral molecules must have a helical structure. </p><p>In the coming century, Pasteur's discoveries were fully corroborated. Optical activity is now a chapter in each chemistry textbook. The optical rotation is a widely used physical constant. It was realized that homochirality is essential for the structure of polymers, consequently, for their biological action. His idea about the helical character of chiral molecules was further developed. It is now accepted that in chiral molecules the asym- metrically distributed static charges give rise to a helical potential field which distorts the molecular orbits (Rosenfeld, 1928). Thus, during photon absorption the rearrangement of the orbits has a helical character that gives rise to a parallel or antiparallel electric and magnetic transition moment (Kauzman, 1957). Therefore, chiral molecules absorb one handed circularly polarized light with higher prob- ability than that of opposite handed circularly polarized light. This explains circular dichroism which is very closely related to optical rotation (Caldwell and Eyring, 1971). In addition it defines chirality in physical terms: parallel or antiparallel combination of a polar and axial vector. </p><p>During these great discoveries Pasteur's idea about the asymmetry of the universe was forgotten. The ones who remembered considered it an "alchemist dream" (Dubos, 1960). The existence of the helical potential field was not considered to be important in biological phenomena. The choice of L amino acids and D sugars by life seemed to be arbitrary. Chemists and biologists dis- regarded as irrelevant that quantum mech- </p><p>anics does not permit the existence of chiral molecules. As Born and Jordan stated: "The Hamiltonian is always invariant with respect to space inversion. Consequently, there could not exist any optically active molecules, which contradicts experience" (Born and Jordan, 1930). Even today the answer to this problem is not known, although several recommend- ations have been made (Barron, 1986). </p><p>B. The violation of chiral symmetry at the elementary particle level </p><p>The above definition of chirality permits all elementary particles to be chiral if they have a momentum (polar vector) and spin (axial vector). Clearly, a moving electron and its mirror image cannot be superimposed; their timetracks are oppositely helical, wbich is a special case of parity, is violated by weak in 1956 predicted that space inversion, which is a special~ casel of parity, is violated by weak interactions. It was soon proven that in decay, which is caused by the weak force, the atomic nuclei emit preferentially left handed g- particles. Their spin and momentum are antiparallel. Thus similar to life, one handedness is preferred over the other by the short range weak force. The preference is nearly total when the emitted electrons move almost at the velocity of light, at lower velocity it reduces to the v/c ratio. Thus chirality is related to relativity. The differ- ence between the broken symmetry of life and ~ decay is essential. The unnatural D amino acids can be produced, but right handed ~- particles with relativistic velocity were never observed. This difference is expressed by the distinction of spontaneous symmetry breaking versus symmetry violation. In the case of symmetry breaking, the two degenerate states, e.g. left and right handed enantiomers are permitted by laws of nature. The obse~Jed preference of one" is accidental. In the case of symmetry violation, the existence of one chiral form is not permitted by laws of nature. In space inversion, which is a special case of parity violation, Leibniz's symmetry principle does not hold. There is even speculation that </p></li><li><p>this is the reason why enantiomers exist at all (Barron, 1986). </p><p>Symmetry principles are invaluable in formulating physical laws; quantum theory would be much poorer without them. Thus, when Leibniz's pality principle was violated, a more general symmetry was sought after and found to replace it. When atomic nuclei emit ~+ anti,electrons, they are preferentially right handed. One must conclude that for weak interactions, space and charge are in- separable. The ~ymmetry violation in space inversion is counterbalanced by the symmetry violation of matter-antimatter conjugation. This combined charge-parity (CP) symmetry holds for weak interactions. The solution, however, did not last long. </p><p>C. The violation of the combined CP sym- metry </p><p>In 1964, Cronin and Fitch (see Christen- son, 1964) observed that the decay of KL meson violates the combined CP symmetry. It produces left handed electrons at a slower rate than right handed positrons (Bennet et al., 1967). </p><p>R(K L -'-* r~-e*d~'l) = 1.003 + 0.0004 (1)- </p><p>R(KL --'* n+ef~0!) </p><p>As before, physicists searched for a still more general .symmetry principle. So far there is only one recommendation: all laws of physics should be invariant under the combined space inversion, charge conjugation and time reversal. This is the CPT theorem. In other words, the combined CP violation must be count~r-balanced by time reversal violation. Time goes differently forward and backward. The meaning of backward moving time is enigmatic. If we believe that time direction is dictated by the second law of thermodynamics, then backward moving time means a time direction towards increased complexity (Blum, 1968). But this in turn may be ',relevant to the t ime direction of biological evolution. Important conclusions follow from thiL~. (1) The internal timing of </p><p>KL meson toward increasing entropy is perturbed by a backward moving time if left handed matter is produced. Thus, time direction enters in a quantum event as an ingredient. This is a surprise, because in any other case quantum theory denies all meaning to the concept of before and after. (2) It seems that matter, space and time is interlocked differently in our world from the interlocking in the antiworld. It is a challenging question how this interlocking is related to Einstein's relativity theory according to which matter, space and time are inseparable. (3) Dirac argued that the combination of CPT theorem and relativity theory will lead to a modified Weyl (1952) theory of space in which vectors lengthen when they are shifted into the future and they contract when they are shifted into the past. Dirac (1982) argued that this model gives rise to time reversal violation that involves a long range force in contrast to the recent theory which attributes the time reversal violation to a short range force. Let us consider the +x, +y, +z axis as vec- tors. Then the left handed system is an expanding one, whereas the right handed -x , -y , - z system is contracting. In this model it is easy to relate left-handedness to forward moving time, right-handedness to backward moving time, especially if we accept that the second law of thermody- namics is a consequence of the expansion of the universe. Namely the expansion re- presents a huge thermodynamic sink in which photons escape irreversibly. (Gal,)r, 1972). One can also understand how charge asymmetry counterbalances parity violation. In a contracting frame of reference, the strength of attraction and repulsion should be changed from their values as compared to that of the expanding frame if CPT theorem holds. </p><p>D. Approach to connect symmetry violations to biological homochirality </p><p>Our concept of chirality has been changed </p></li><li><p>dramatically in the past three decades. Chiral- ity is not a marginal problem but one of the most challenging and fundamental notions. Details are uncertain and some are seriously questioned. Not everyone shares the opinion that the existence of rest mass can be traced back to chiral symmetry break- ing (Cicogna et al., 1972). It is, however, clear that chirality is an essential part of quark theory and axiomatic field theory. It represents an unsolved problem in quantum theory, and is related to relativity and cos- mology. Consequently, it enriched our under- standing of matter, space and time. The notion of backward moving time may be related to the time direction of biological evolution, because the mixed parity state of enantiomers enables the CP and con- sequently the T violating superweak inter- actions to make an impact on the internal timing of chiral molecules. Any future theory and experiments with respect to the origin and significance of homochixality of life must take into consideration the full mean- ing of chirality with all of its ramifications. </p><p>But how can this be done? There seems to be a tremendous rift between chirality at the molecular level and at the elementary par- ticle level. Life works with electromagnetic forces and is not sensitive to short range forces. Life is not a relativistic phenomenon. The decay of KL meson, which possesses simultaneously its antiparticle, is a very isolated case. Life has nothing to do with antimatter. The symmetry violations are extremely small. In short, the fascinating aspects of chirality at the elementary particle level seem irrelevant to biochemistry. Chance, selection by the pressure of environment and the theory of dissipative systems ex- plain evolution, there is no need to involve time reversal asymmetry to explain it. </p><p>These statements are not quite true. Molecules are composed of elementary particles. The electromagnetic and short range forces (weak and superweak) have been united; wherever one is active, the other contributes to it (Weinberg, 1974). </p><p>The electron spin is a relativistic pheno- menon and it is important in biology. It is generally accepted that the time sym- metry violation is common but masked in many events. Wigner (1965) related the KL meson to racemic mixtures. Even if life has nothing to do with antimatter, a spin direction which is characteristic to anti- matter in weak interaction may be pre- ferred in biochemical processes. Not every- one is satisfied with the recent "explan- ation" of evolution (Moorhead and Kaplan, 1967). There are a large number of examples that very small perturbations cause dramatic effects in life. </p><p>Thus it is justified to try new avenues even if some of them are bound to be cul- de-sacs; a very challenging task which has been taken by many. In spite of many at- tempts, there has been no break-throughs during the past 30 years. The field is saturated with speculations, theoretical calculations with practically no solid experimental fact to support them. A few of us had given up, but the number of scientists who react positively to the challenge is increasing. The meeting in Rouen was the fourth in a series, and the enthusiasm of previous meet- ings* had not diminished. The opposite is true, because if homochirality of life is conceived as part of the general asymmetries of nature, then chirality is one of the few problems which challenges our recent under- standing both of inanimate matter and life. This is why chirality is so important. </p><p>*The previous conferences: (1) International Sym- posium on Generation and Amplification of Asym- metry in Chemical Systems. Julich, FRG 1974. Organizers: A.S. Garay, C. Ponnamperuma, W. Thiemann and K. Wagener; (2) International Con- ference on Origin of Optical Activity in Nature. Vancouver, Canada. 1979 Organizer: D. Walker; (3) International Symposium on Generation and Amplifi- cation of Chirality in Ch...</p></li></ul>