Magnesium Silicates and Possible Chiral Interactions between Organic Molecules and Chiral Surfaces 49 views

Life’s origin remains an enigma, including the peculiar characteristic of biomolecular homochirality ‒ specifically, the predominant use of left-handed amino acids and right-handed sugars in biology. The same bias shows up in carbonaceous meteorites, especially aqueously-altered carbonaceous chondrites. It is likely then that life’s enantiomeric preference has a cosmic origin. However, the enantiomeric excess of left-handed amino acids and right-handed sugars is strange as a normal chemical reaction yields a racemic mixture. The mechanism leading to this symmetry splitting within these biomolecules remains a mystery. Several hypotheses have been tested but none have given a fully satisfactory explanation. Chiral mineral surfaces offer one explanation to show how symmetry in the minerals themselves can be broken. I focus in on demonstrating that symmetry breaking is possible in chiral enstatite-type molecules. A total of eight three-dimensionally chiral hydrogenated molecular structures were identified utilizing the HSE06/aug-cc-pVQZ level of theory by hydrogenating a two-dimensionally chiral enstatite-type dimer with either neutral hydrogen, a proton, or molecular hydrogen. Unexpectedly, most of the hydrogenated magnesium silicates display large absorption features well below 10 μm in the regions usually attributed to polycyclic aromatic hydrocarbons and other organic compounds. TD-DFT calculations predicted small g factors (g ≲ 0.007), comparable to gas-phase amino acids, which suggest asymmetry in the absorption of circularly polarized light. Similar computational methods were then utilized on two gas-phase chiral magnesium silicate structures synthesized in the laboratory. Having experimental spectra allowed the testing of the computational methodology used for the hydrogenated magnesium silicates. It was found that the functional utilized in the hydrogenated structures, HSE06, was able to perform well in predicting the spectra of small molecular magnesium silicates. However, PBE0 was found to be more computationally efficient while providing the same level of accuracy (<5% MAE). TD-DFT calculations predicted small anisotropy effects for the two studied molecules that were only slightly smaller than those predicted for the hydrogenated chiral structures (~0.001). Additional work concerning (1) DFT, MP2, and CCSD(T) analyses of 7-carbon α-amino acids identified in the Murchison meteorite; and (2) surface interactions between glutamic acid enantiomers and tyrosine enantiomers on (110) quartz by X-ray photoelectron spectroscopy (XPS) will be briefly presented. I will conclude with a prediction that a cosmic origin of homochirality can provide a guiding principle to finding the correct origin of life (OoL) hypothesis since OoL hypotheses that lose the chiral bias information in intermediate achiral molecules are unlikely to be correct.

PHD (Doctor of Philosophy)

magnesium silicates; astrochemistry; planetary science ; homochirality; interstellar dust; origin of life; circular dichroism; X-ray photoelectron spectroscopy; density functional theory; quantum computational chemistry

English

2025-07-31