[61] Protein-Chromophore Interactions as Spectroscopic and Photochemical Determinants

This chapter describes various protein–chromophore interactions that serve as spectroscopic and photochemical determinants. Studies on pigment spectra have demonstrated that appropriately positioned charged or polar amino acids are responsible for producing the colors of visual pigments. Electrostatic interactions also appear to play an important role in the light transduction mechanism. A related problem, not considered in this work, concerns the nature of the photoisomerization in the excited state of the chromophore. The quantum yield for this process is 0.67 in the pigments, an order of magnitude greater than the solution value. It is not unlikely that the same charges responsible for wavelength regulation and energy storage play a catalytic role in the photochemistry. Electrostatic interactions are believed to play a central role in many enzyme mechanisms) and it is to be expected that their relative contribution will be even greater in the low dielectric medium provided by the interior of membranes. Thus, the emphasis on these factors in membrane proteins such as rhodopsin and bacteriorhodopsin is not unwarranted.