Dilip K. Kondepudi

Chiral Symmetry Breaking in Sodium Chlorate Crystallizaton

Sodium chlorate (NaClO3) crystals are optically active although the molecules of the compound are not chiral. When crystallized from an aqueous solution while the solution is not stirred, statistically equal numbers of levo (L) and dextro (D) NaClO3 crystals were found. When the solution was stirred, however, almost all of the NaClO3 crystals (99.7 percent) in a particular sample had the same chirality, either levo or dextro. This result represents an experimental demonstration of chiral symmetry breaking or total spontaneous resolution on a macroscopic level brought about by autocatalysis and competition between L- and D-crystals.

WEAK NEUTRAL CURRENTS AND THE ORIGIN OF BIOMOLECULAR CHIRALITY

It has long been known that Earths biochemistry is overwhelmingly dissymmetric or chiral1–4. In model chemical systems5–7 that spontaneously evolve to a state dominated by either the L or the D enantiomer, parity violation in β-decay and that attributable to weak neutral currents (WNC) in molecules8,9 is thought to be too small to have any significant influence on the emergent chirality10,11. Other conceivable systematic chiral influences are generally even weaker12–14. We show here that there is a simple and extremely sensitive mechanism by which a minute but systematic chiral interaction, no stronger than the WNC interaction in amino acids, can, over a period of ∼15,000 yr, determine which enantiomer will dominate. Such a mechanism is especially interesting when considering the origins of terrestrial biochemistry, particularly in view of the work by Mason and Tranter15, who found that it is the terrestrially dominant L amino acids that are favoured by the WNC interaction.

Chiral-symmetry-breaking states and their sensitivity in nonequilibrium chemical systems

Nonequilibrium symmetry breaking and sensitivity are discussed in the context of the chemistry of chiral molecules. It is shown that systems that break chiral symmetry are so sensitive to small chiral interaction that an interaction energy Eint > 10−15 -10−17kT is sufficient to have strong chiral selectivity. Such energies are within the range of parity violating weak-neutral-current interactions. Chiral interaction energies due to a combination of electric, magnetic, gravitational and centrifugal fields, even when high field strengths are considered, are found to be less than 10−19kT. Plausible chemical systems that involve rhodium catalysts in which weak-neutral-current energies should be high are also briefly discussed.

Sensitivity of nonequilibrium systems

Simple mechanisms through which nonequilibrium structures can be influenced by external fields are discussed. It is shown that a very weak gravitational or electric field can have a large influence on selection or creation of structures. In the absence of cooperativity, the influence of a weak field, to the leading order, is characterized by the ratio (Eint/kT), where Eint is the energy of interaction; however, when there is far-from-equilibrium cooperativity, it is shown that the influence of the field is characterized by (EintkT)13.

Selection of molecular chirality by extremely weak chiral interactions under far-from-equilibrium conditions

A general formalism for chiral symmetry breaking in far-from-equilibrium chemical systems is presented. It is pointed out that slow passage through the transition point makes the system sensitive to very weak, but systematic, chiral influences. The general implications of this process for the origin of biomolecular chirality is discussed.

Secondary nucleation that leads to chiral symmetry breaking in stirred crystallization

Abstract We have recently reported the breaking of chiral symmetry in stirred crystallization. One of the most important aspect of the mechanism that leads to the symmetry breaking is the phenomenon of secondary nucleation. The empirical expressions currently used for the rates of secondary nucleation are not based on a clear physical model. We present here some experimental observations on NaClO3 crystallization that gives us a physical model to develop an expression for the rates. Such expressions are important for a quantitative and statistical understanding of the phenomenon of chiral symmetry breaking in stirred crystallization.

Homochirality as the signature of life: the SETH Cigar

A characteristic hallmark of life is its homochirality: all biomolecules are usually of one hand, e.g. on Earth life uses only L-amino acids for protein synthesis and not their D mirror images. It is therefore suggested that a search for extra-terrestrial life can be approached as a Search for Extra-Terrestrial Homochirality (SETH). A novel miniaturized space polarimeter, called the SETH Cigar, is described which could he used to detect optical rotation as the homochiral signature of life on other planets. Moving parts are avoided by replacing the normal rotating polarizer by multiple fixed polarizers at different angles as in the eye of the bee. It is believed that homochirality will be found in the subsurface layers on Mars as a relic of extinct life.

Sensitivity of branch selection in nonequilibrium systems

Abstract A study of the branch selection process for the case of the general cubic in the presence of both additive and multiplicative noise is studied. If the bifurcation parameter passes slowly through the bifurcation point the system shows almost the same sensitivity as a system with no quadratic term.

Chiral symmetry breaking in nonequilibrium chemical systems: Time scales for chiral selection

Abstract The process through which an extremely weak parity violating interaction, of energy Δ E , can become the selector of molecular chirality over a sufficiently long time is discussed. For a model system it is shown that Δ E ≈ 5 × 10 -15 kT can give a 90% selectivity in about 300 h - smaller Δ E being relevant to prebiotic evolution.

Mechanism of chiral asymmetry generation by chiral autocatalysis in the preparation of chiral octahedral cobalt complex

Chiral asymmetry generation, the predominant production of one enantiomer in a non-chiral environment, could occur in the production of the chiral complex cis-[CoBr(NH3)(en)2]Br2 by the reaction of [Co(H2O)2{(OH)2Co(en)2}2](SO4)2 with ammonium bromide in an aqueous medium. The main kinetic steps in the reaction system have been determined. During the reaction, the product crystallizes at an early stage. When a very small amount of crystalline enantiomer was added to the reaction system at an early stage, the same enantiomer was produced preferentially; in addition, the enantiomeric excess of the product increased with increasing the stirring rate. Thus, it seems that each enantiomer generates chiral crystals that could self-replicate through secondary nucleation when the solution is stirred; these crystals in turn enhance the production of the same enantiomer. With a computer code that simulates such a kinetic mechanism, it is shown that enantiomeric excess observed in the experiments could be reproduced. Chirality 10:343–348, 1998.