Evidence for a Grotthuss-like mechanism in the formation of the rhamnose alkoxy radical based on periodic DFT calculations

Abstract

Molecular modeling adopting a periodic approach based on density functional theory (DFT) indicates that a Grotthuss-like mechanism is active in the formation of the radiation-induced alkoxy radical in α-L-rhamnose. Starting from an oxidized crystal structure, a hydroxyl proton is transferred along an infinite hydrogen bond chain pervading the entire crystal. The result of this proton shuttling mechanism is a stable radical species dubbed RHop. Only after several reorientations of crystal waters and hydroxyl groups, the more stable radical form RO4 is obtained, which differs in structure from the former by the absence of only one hydrogen bond. Calculations of the energetics associated with the mechanism as well as simulated spectroscopic properties reveal that different variants of the rhamnose alkoxy radical can be observed depending on the temperature of irradiation and consecutive EPR measurement. Cluster calculations on both radical variants provide hyperfine coupling and g tensors that are in good agreement with two independent experimental measurements at different temperatures. © 2008 by Radiation Research Society.