Retinal Isomerization Almost Perfectly Efficient

Polli et al. excited retinal in rhodopsin and then followed the molecule as it returned to its electronic ground state.  By monitoring stimulated emission and absorption of light from the molecule, they mapped out the energy gap between the ground and excited electronic states as a function of time after excitation.  Their data revealed an initial decrease and a subsequent increase of the energy gap, consistent with passage through a conical intersection (an intersection of a 3N-dimensional 'landscape' that plots the total energy of a collection of N atoms as a function of the atomic positions during transitions from excited states to ground states of a molecule).   The authors also simulated the excited-state dynamics of retinal in rhodopsin, which agreed with the measured data and allow an inference of the time-evolution of the retinal geometry after excitation.  In this molecular 'movie' of the first step in vision, retinal in a crowded protein environment reaches its conical intersection seam within 75 femtoseconds (this is astonishingly short, essentially the same time as that predicted by theoretical simulations of retinal in the gas phase).  This indicates that the binding pocket for retinal in rhodopsin must be ideally organized to both promote and accommodate the observed conformational change, and indicates which of the geometries along the seam of the conical intersections is responsible for the ultrafast de-excitation in rhodopsin.  The conical intersection topography is strongly 'peaked'--spectral signatures of part of the molecular wavepacket remaining on the excited state are largely absent from the experimental data--showing how the passage of retinal through the conical intersection is nearly perfectly efficient.

References:
Conical intersection dynamics of the primary photoisomerization event in vision
Dario Polli, Piero Altoè, Oliver Weingart, Katelyn M. Spillane, Cristian Manzoni,
Daniele Brida, Gaia Tomasello, Giorgio Orlandi, Philipp Kukura, Richard A. Mathies, Marco Garavelli, Giulio Cerullo. Nature 467, 440–443 (2010) 10.1038/nature09346

Physical chemistry: Seaming is believing
Todd J. Martinez. Nature 467:7314, 412 (2010) doi:10.1038/467412a