Search for Electroweak Interactions in Amino Acid Crystals. II. The Salam Hypothesis

Abstract

In 1991, A. Salam proposed that the electroweak interaction might promote tunneling through the potential barrier existing between chiral molecules (i.e., a phase transition at a critical temperature, T c ), which could change the structure of D-amino acids into the reputedly more stable L-form of the enantiomer. A recent report by Wang et al. has presented experimental evidence for such a transition at T c ≈ 270 ( 1 K in enantiomorphs of L-and D-alanine and -valine crystals using differential scanning calorimetry (DSC), magnetic susceptibility, and Raman spectroscopy. Experimental verification of the Salam prediction has great implications for the origins of specific homochirality in biomolecules, that is, the exclusive use of L-amino acids in proteins. In this contribution, we reexamine these measurements, as well as present measurements of the temperature dependence of X-ray diffraction and C-13 solid-state NMR for alanine. While the DSC measurements show interesting features similar to those reported by Wang et al. at ∼270 K, there are significant differences, including the observation that the small feature observed in the DSC experiments becomes even smaller upon recrystallization (i.e., purification). Also, the small change in specific heat at ∼270 K is found to be absent in (natural) L-valine. We find no unusual behavior in our additional X-ray diffraction or NMR experiments in this temperature range. We also present arguments against the Salam hypothesis for the molecules under study.

Mirror Symmetry Breaking in Helical Polysilanes: Preference between Left and Right of Chemical and Physical Origin

“…Based on the Salam's hypothesis describing that homochirality is the consequence of the cooperative second-order phase transition of an electron-neutron pair [26], Wang et al examined a pair of D-and L-alanine single crystals by means of Raman, NMR, neutron scattering, supersonic wave, magnetic susceptivity, and calorimeter [63]. She claimed the detection of subtle differences in physical properties between the D-and L-crystals below 270 K. Although these results were verified by other independent research groups, Compton and Schwerdtfeger [64], and Wilson et al [65] did not agree with her claim due to a lack of reproducibility. They ascribed her results to certain impurities incorporated in non-naturally occurring alanine, crystal imperfection, and restricted motions in the solid crystals.…”

Supramolecular Chirality: Solvent Chirality Transfer in Molecular Chemistry and Polymer Chemistry

“…However, regardless of a pair of enantiopure (S)-and (R)-alcohols, we are aware that CD profiles (wavelength extrema and absolute magnitude) are exactly in non-mirror-image relation, although we did not mention this subtle difference in the original paper [121]. This non-mirror-image relation may be ascribed to some impurities, as claimed in several papers [68,71,73,117]. It may also infer other unknown reasons, such as an inherent mirror symmetry breaking at a global level upward, our universe scale [123,124] and macroscopic MPV effects [21,37,43,46,51,67,72,74].…”

“…Although it is already established that left-right symmetry is definitively broken at the elemental particle, subatomic and atomic levels [52][53][54][55][56][57][58][59][60][61][62][63][64][65][66], the detection of dissymmetry at the molecular level, so-called molecular parity violation (MPV), remains an unsolved issue. It is much debated whether the MPV hypothesis is valid and, even if it is true, whether it is detectable spectroscopically or by other acceptable methods [67][68][69][70][71][72][73][74][75][76].…”

An Introduction to Chirality at the Nanoscale

“…However, it is still not clear whether or not the PVED preferentially stabilizes the naturally occurring L-amino acids and D-sugars. Measurable differences reported in the physical properties of crystals of D-and L-amino acids and claimed to be due to parity violation have not been corroborated [77]: they have been shown instead to arise from traces of different impurities in the enantiomorphous crystals [78]. So far there is no convincing evidence that the PVED itself has any enantioselective influence on the crystallization of sodium chlorate (vide supra) or on that of any other system [30,59].…”