Characterization of the conical intersection of the visual pigment rhodopsin at the CASPT2//CASSCF/AMBER level of theory

Abstract

The branching plane associated with the conical intersection controlling the photochemical Z → E isomerization reaction of rhodopsin has been mapped using a CASPT2//CASSCF/AMBER quantum mechanics–molecular mechanics method. The nature of the derivative coupling and gradient difference vectors spanning the branching plane has been investigated, showing that the conical intersection is not only associated with the isomerization process but also to a charge transfer along the retinal backbone. Using a simple Landau–Zener model, the paper discusses the possible effects of the documented conical intersection topologies on the efficiency of the reactive process. It is argued that the peculiar shape of the conical intersection favours decay at structures that are geometrically displaced towards the photoproduct bathorhodopsin.

Acknowledgements

P. B. C. is grateful for a postdoctoral fellowship from the Spanish Ministerio de Educación y Ciencia. N.F is grateful for Marie Curie Fellowship HPMF-CT-2001-01769. Funds have been provided by the Università di Siena-Progetto di Ateneo (04_06 PRIN 04-06). The authors acknowledge helpful comments from Prof. L. Serrano-Andrés.

Notes

†Throughout the text we will continue using the labels h _IJ and g _IJ to indicate the branching plane vectors evaluated in the CI. However, the corresponding modes shall not be associated with the gradient difference and derivative coupling vectors at any point surrounding the tip of the conical intersection where these (unambiguously computed) vectors may feature a different direction. Indeed, the derivative coupling and the gradient difference vectors interchange twice when moving along a loop centred on the conical intersection point 12. In contrast, the branching plane is unambiguously defined 13.