Theoretical Study On The Electronic Structure Of L-Tyrosine-Pillared Layered Double Hydroxides

L-tyrosine plays an important role in the physiological functions of lives. The study of its racemization is benefit for the search of the life origin and the storage of amino acid,and also used to estimate the age of fossils.This work explores the isomerization pathway of the tyrosine molecule and the influence of MgAl-LDHs layer on the reaction by using density functional theory.1.The mechanisms of L-tyrosine racemization in the ground and lowest triplet excited states have been investigated by density functional theory (DFT).The geometries for reactant,product,intermediate and transition states have been completely optimized at B3LYP/6-311++G(d,p)level of theory.All transition states have been verified by vibrational analysis and the internal reaction coordinate calculations.The results show that every reaction process follows three-step mechanism with enol form as intermediate species.The energy barriers of L-tyr→IM1 in the ground state are 291.58 and 201.88 kJ·mol^-1separately,and the energy barriers in the first triplet excited states are 63.45 and 129.76 kJ·mol^-1,respectively.Because of the small energy barrier in T₁ state and the substantial energy barrier in S₀ state,there is enough energy to overcome this barrier and thus it is possible that racemization occurs in the lowest triplet excited state.The potential energy surfaces of the equilibrium and transition states in the system were calculated by TDDFT at B3LYP/6-311++G(d,p)level.The initial excitation usually provides molecules with enough energy before reaction occurring,which makes the molecules into a higher energy level,and the energy released from the vibration makes it easier for molecules to overcome the energy barrier,so the photochemical reaction occurs consequently.The calculated results have shown that the electrons of L-tyr molecule could be excited into the S₁ state under illuminating of the ultraviolet light with the wavelength less than 261.53 nm. Since the potential energy surfaces of the S₁ state and the T₁ state are close and the life of the triplet state is relatively long,the system could relax the energy from the S₁ state to the T₁ state through the intersystem crossing.2.The geometries of tyrosine ion absorbed onto MgAl-LDHs layer are optimized by density functional theory at the B3LYP/6-31G(d,p)level.The frontier molecular orbitals,vibrational frequencies and supermolecular interactions between the host slabs and the guest anions of LDHs are also calculated.The calculated interatomic distances from the interlayer anion to the slab denote that there exist hydrogen bond interactions between the hosts and guests.The charge analysis for the natural bond orbitals(NBO)shows that the electrostatic interaction is an important component of supermolecular interactions between the slab and the anion.According to the molecular orbital calculations,it can be seen that the electrons transfer from the highest occupied molecular orbital(HOMO)of the anion to the lowest unoccupied molecular orbital(LUMO)of slab in the adsorption process.The combination energies of the anions's being assembled to tyrosine-LDH are -1135.12(L-tyrosine)and -1137.15(D-tyrosine)kJ·mol^-1,respectively.Because the proton of chirality carbon atoms is hard to transfer due to the effect of super-molecule assemble,the tyrosine ion in the hydrotalcite tends to keep the chirality character.