Tracking the Excited State Time Evolution of the Visual Pigment with Multiconfigurational Quantum Chemistry

The primary event that initiates vision is the photoinduced isomerization of retinal in the visual pigment Rhodopsin. Here we use a scaled quantum-mechanics/molecular-mechanics potential that reproduces the isomerization path determined with multiconfigurational perturbation theory to follow the excited state evolution of bovine Rhodopsin. The analysis of a 140 femtosecond trajectory provides an unprecedented description of the electronic and geometrical changes that prepare the system for decay to the ground state. The data uncover a complex change of the retinal backbone that, at ca. 60 femtosecond delay, is converted into a space saving “asynchronous bicycle-pedal or crankshaft” motion leading to a conical intersection on a 110 femtosecond time scale. It is shown that the twisted structure achieved at decay features a momentum that provides a natural route towards the photorhodopsin structure recently resolved using femtosecond-stimulated Raman spectroscopy. The asynchronous bicycle-pedal motion is then quenched during the photorhodopsin to bathorhodopsin relaxation yielding a single isomerized double bond. While promptly excited during the initial excited state relaxation, hydrogen-out-of-plane wagging does not seem to play a critical role in the decay