Potential Energy Curves for the Isomerization of Protonated Schiff Base of Retinal on the Triplet State Surface

Abstract

Potential energy curves (PECs) for the trans–cis isomerization of protonated Schiff base of retinal (PSBR) have been calculated by means of the time-dependent density functional theory (TD-DFT) method at the B3LYP/3-21G* level. The isomerization from all trans to 13-cis forms of PSBR in bacteriorhodopsin (bR) in the ground state (S 0) and low-lying triplet states (T 1–T 10) were considered in the present calculation. The excitation energies from S 0 to T 1 in all trans and 13-cis forms were calculated to be 1.16 and 1.17 eV, respectively. The shape of PEC for the ground state (S 0) showed that the activation barrier is located in where θ13–14 is twist angle around the double bond of PSBR. The shape of PEC for the triplet state (T1) was very similar to that of S 0. In the first excited triplet state, PEC (T2 state), two potential barriers are located at and 112°, while the energy minimum is located at The mechanism of the photo-isomerization on the low-lying triplet states is discussed on the basis of theoretical results.