Abstract: We investigated the mechanism of optical isomerism of valine catalyzed by hydroxyl water clusters of two conformations of valine molecules and the damage induced by hydroxy radicals in water environment by using MP2/6-311++G(2df,pd)//B3LYP/6-31+G(d,p) doubletheory and SMD model method for self consistent reaction field (SCRF) theory. The results show that the optical isomerism of valine can be  achieved in two channels a and b. In channel a, the substrate is formed by hydrogen bonds of hydroxyl water clusters with α-H and amino groups, hydroxyl abstracts α-H, and then α-C abstracts the hydrogen of water molecular at the other side. And in channel b, the substrate is formed by hydrogen bonds of hydroxyl water clusters with α-H and carbonyl groups, hydroxylabstracts α-H, and then α-C abstracts the hydrogen of water molecular at the other side. Water molecular assists abstraction of αH by hydroxy radicals can induce valine damage in channel b. The optical isomerism energy barriers of ratedetermining step of conformation Val1 (mono-hydrogen bond between aminoand carboxylgroup) and Val2 (dualhydrogen bond between aminoand carboxylgroup) are 6057, 6524 kJ/mol in channel a, while the energy barriers are 5676,64.11 kJ/mol in channel b. So the damage of valine by hydroxy radicalwater clusters of them in channel b was moderate exothermic reaction.

Key words:  , optical isomerism, valine, density functional theory, transition state, perturbation theory, solvent effect, hydroxyl, hydroxy radical