Large electronic circular dichroism effects measured in chiral amino acids

Abstract:

Life is homochiral, and all life uses almost exclusively left-handed (L-) amino acids and right handed (D-) sugars in their biochemistry. However, it is unknown why life should choose one chirality over the seemingly equally probably enantiomer. One suggestion is that biological homochirality may have begun in space or on the surface of early Earth due to polarized radiation formation of enantiomeric excesses (ee). Circularly polarized UV light (CPL) is often invoked to explain the initial symmetry breaking between enantiomers, and this initial imbalance can then be enriched during subsequent aqueous alteration events. However, ee created by CPL are small and require the destruction of large proportions of the initial molecular population. A largely unexplored mechanism for the formation of ee in chiral molecules exposed to ionizing radiation is stereoselective interactions with spin-polarized electrons (SPE), termed electronic circular dichroism (ECD). This paper will present recent results from the Advanced Photon Source synchrotron showing, for the first time, measured ECD in an amino acid. Approximately monolayer thick films of enantiopure L-histidine were vapor deposited onto a bare, magnetized Co substrate and exposed to 695 eV x-rays. Due to unequally filled electron valence levels in the magnetized substrate, low energy secondary electrons which escape the surface leave with a preferred spin-polarization direction. The ejected SPE subsequently interacted with the adsorbed histidine causing molecular bond breaking and desorption, and the chemical state and abudance of the molecules was monitored using x-ray photoelectron spectroscopy (XPS). Fitting the decay curves for the various molecular components identified in the N-1s transition allowed motif-specific damage cross-sections to be determined. Furthermore, by reversing the magnetization direction of the substrate in situ and comparing the growth/decay rates for the two conditions, the differential cross-sections due to SPE helicity were determined. The results indicate cross-section differences as large as 20% for the reversed SPE helicities, and suggest that ECD could significantly contribute to the formation of ee in both airless bodies and on planetary surfaces.