Enantioselective autocatalysis .4. 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.