Wavepacket splitting and two-pathway deactivation in the photoexcited visual pigment isorhodopsin

Isorhodopsin is the visual pigment analogue of rhodopsin. It shares the same opsin environment but it embeds 9-cis retinal instead of 11-cis. Its photoisomerization is three times slower and less effective. The mechanistic rationale behind this observation is revealed by combining high-level quantum-mechanical/molecular-mechanical simulations with ultrafast optical spectroscopy with sub-20 fs time resolution and spectral coverage extended to the near-infrared. Whereas in rhodopsin the photoexcited wavepacket has ballistic motion through a single conical intersection seam region between the ground and excited states, in isorhodopsin it branches into two competitive deactivation pathways involving distinct conical intersection funnels. One is rapidly accessed but unreactive. The other is slower, as it features extended steric interactions with the environment, but it is productive as it follows forward bicycle pedal motion. Follow the right path! In the visual pigment analogue isorhodopsin, the photo-excited wavepacket branches in two competitive deactivation pathways involving two distinct conical intersections. This scenario is deduced from high-level QM/MM simulations of the photoinduced dynamics and is responsible both for the recorded non-exponential two-component decay and for the reduced isomerization quantum yield with respect to rhodopsin. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.