Review of the origin of asymmetry of biomolecules through weak interaction: latest developments

IR spectroscopy as a probe for C–H⋯X hydrogen bonded supramolecular synthons

We describe a five step IR spectroscopic method that identifies supramolecular synthons in weak hydrogen bonded dimer assemblies, bifurcated systems, and π-electron mediated synthons.

Enantioselective autocatalysis. IV. Implications for parity violation effects

Historically, parity violation at the contemporary biomolecular level (i.e., only L-amino acids in proteins and D-sugars in DNA and RNA) has been postulated to be the inevitable result of parity violations at the elementary particle level, involving either beta-decay electrons or parity violating energy differences (PVEDs) between enantiomers. These two chiral biases have in turn allegedly impressed a small but persistent chirality onto prebiotic chemistry which, after appropriate amplification, has culminated in our contemporary homochiral biopolymers. Experiments and controversies pertaining to the efficacy of these two chiral biases are reviewed briefly, with the conclusions that: a) there is no experimental evidence supporting the capability of beta-decay electrons or other spin-polarized chiral particles to generate chiral molecules, and b) only theoretical calculations, but no experimental evidence, support the allegation of a causal relation between PVEDs and biomolecular homochirality. We here attempt to examine the latter allegation experimentally. Spontaneous resolution under racemization conditions (SRURC) during the crystallization of the bromofluoro-1,4-benzodiazepinooxazole derivative I is capable of affording products of high enantiomeric purity. This process, which involves very efficient stereoselective autocatalysis, has now been examined statistically. If PVED effects are operative, the SRURC of racemic I should provide, either exclusively or with a strong and consistent bias, only one enantiomer of crystalline I. However, crystallization experiments of racemic I showed no bias in its SRURC, leading to the conclusion that PVED effects are ineffective in dictating a preferred chirality in this system. Several earlier experiments in the literature leading to a similar conclusion as to the inefficacy of PVED effects in promoting a preferred chirality are noted.

Asymmetries of nature and the origin of biomolecular handedness

New developments in the problem of the origin of the asymmetry of the living systems are reviewed. The time of the appearance of the asymmetry was very probably during the chemical evolution. The asymmetry of weak interaction is still considered small to be the cause of chiral purity. New sources of asymmetry should also be considered, like the one proposed by Gilat.

New estimates of asymmetric decomposition of racemic mixtures by natural beta-radiation sources

The Vester-Ulbricht hypothesis suggests that the chirality of biological molecules originates from the beta-radiolysis of prebiotic racemic mixtures. Despite the inconclusiveness of past investigations, recent calculations have shown that beta particles, because of their helicity, radiolyse L- and D-enantiomers at slightly different rates, the asymmetry, AR, being predicted to be 10(-11) (new experimental tests, give /AR/ < 2 x 10(-9)). Before this, the size of the radiolysis-induced chiral polarization, eta R (eta triple bond (nL - nD)/(nL + nD) where nL and nD are the numbers of L and D molecules present), was estimated for different values of AR; according to Keszthelyi et al., if /AR/ approximately 10(-11), /eta R/ can never exceed the chiral polarization, /eta F/, produced by statistical fluctuations, thus invalidating the V-U hypothesis. Here we re-examine the major assumptions on which these calculations were based and find that several overly restrictive conditions were imposed, which, when relaxed, allow the condition /eta R/ > /eta F/, in accordance with the V-U hypothesis.

Dissident remarks on the origin of optical activity. An intrabiological explanation

The attempts to explain the origin of natural optical activity through external, extrabiological, agents, typically ionizing rays, i.e. as a purely incidental effect without deeper significance, have failed and should be abandoned. An alternative, intrabiological, Darwinian, explanation seeks to explain optical activity as a necessary phenomenon in early evolution. This explanation is based on the idea that organisms with the existing stereospecificity initially happened, by chance, to be more efficient than organisms with the opposite stereospecificity on independent grounds that have nothing to do with their stereospecificity. If cells based on L aminoacids are called L cells, it is to be assumed that cosmic bodies carrying L cells are about equally frequent as those carrying D cells.