Theoretical study of two photochemical pathways of l-tyrosine isomerization

The pathways of L-tyrosine isomerization have been investigated by using density functional theory (DFT) and time-dependent DFT (TD-DFT) at 6-311++G(d,p) level in order to understand experimental observations of isomerization exposed to UV lights. The calculated results indicate that the l-tyrosine isomerization may proceed via two independent photochemical pathways, symmetrical and unsymmetrical. Because of the high energy barrier of the isomerization in S0 state, the l-tyrosine must be excited to S1 state by UV lights to start the reaction. For symmetrical pathway, the first transition state of l-tyrosine isomerization, formed by the proton migration to the carboxyl oxygen, transitions from S1 to T1 and continues the isomerization reaction until form the d-tyrosine. Meanwhile, the d-tyrosine transitions from T1 to S0 state accompanying the release of phosphorescence. There exists two correlative enol intermediates in symmetrical pathway. For the two proton transfer steps, the calculated energy barriers are 31.38 and 11.3 kcal/mol in T1 states, respectively. The unsymmetrical pathway occurs via one-shift proton transfer in S1 state. Subsequently, the transition of intermediate from S1 to T1 takes place for the nearly energy profiles, and it follows the remainder steps as the symmetrical pathway to complete the isomerization reaction. The unsymmetrical pathway goes through different intermediates and transition states from the symmetrical pathway.