Excited State Property of Hardly Photodissociable Heme-CO Adduct Studied by Time-Dependent Density Functional Theory

摘要

While most of CO-bound hemes are easily photodissociated with a quantum yield of nearly unity,we occasionally encounter a CO-heme which appears hardly photodissociable under the ordinary measurement conditions of resonance Raman spectra using CW laser excitation and a spinning cell.This study aims to understand such hemes theoretically,that is,the excited-state properties of the five-coordinate heme-CO adduct (5cH) as well as the 6c heme-CO adduct (6cH) with a weak axial ligand.Using a hybrid density functional theory,we scrutinized the properties of the ground and excited spin states of the computational models of a 5cH and a water-ligated 6cH (6cH-H_2O) and compared these properties with those of a photodissociable imidazole-ligated 6cH (6cH-Im).Jahn-Teller softening for the Fe-C-O bending potential in the a_1-e excited state was suggested.The excited-state properties of 6cH-Im and 5cH were further studied with time-dependent DFT theory.The reaction products of 6cH-Im and 5cH were assumed to be quintet and triplet states,respectively.According to the time-dependent DFT calculations,the Q excited state of 6cH-Im,which is initially a pure pi-pi state,crosses the Fe-CO dissociative state (2A') without large elongation of the Fe-CO bond.In contrast,the Q state of the 5cH does not cross the Fe-CO dissociative state but results in the formation of the excited spin state with a bent Fe-C-O.Consequently,photoisomerization from linear to bent Fe-C-O in the 5cH is a likely mechanism for apparent nonphotodissociation.