9.10 MNDO-PSDCI and the Analysis of the Photophysical Properties of Visual Chromophores and Retinal Proteins

Vision in vertebrates is a complex and involved process. Small organic molecules, known as chromophores, become electronically excited upon the absorption of a photon of light. In the excited state exists a barrier-less potential energy surface for some torsional movement about a double bond. The conformational change in the chromophore moiety induces a corresponding conformational change in the covalently bound polypeptide, thereby triggering a messenger cascade within the photoreceptor cell that results in the transmission of signals to the appropriate neurons. These signals are transmitted along the optic nerve to the visual centers of the brain, which allows the brain to process the visual input and perceive a visual image. The study of chromophores and photoreceptors in vitro is dominantly focused on theoretical computational calculations and spectroscopy. The focus of this chapter is to elucidate these methods, with emphasis on their application to the study of biological systems.