TD-DFT calculations of the potential energy curves for the trans–cis photo-isomerization of protonated Schiff base of retinal

One-dimensional potential energy curves for the isomerization of protonated Schiff base of retinal (PSBR) in bacteriorhodopsin (bR), i.e., isomerization from all-trans- to 13-cis-forms, have been calculated by means of time-dependent density functional theory (TD-DFT) calculations, in order to elucidate the mechanism of initial step in photo-absorption. The transition state of the isomerization in the first excited state is located at θ13–14 = 58°, where θ13–14 means twist angle around the C13C14 double bond of PSBR The potential barrier is formed by the avoided crossing between S1 (Bu-like) and S2 (Ag-like) states. The mechanism of the isomerization was discussed on the basis of theoretical results.