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, Tc), 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 Tc ≈ 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.