CIRCULAR DICHROISM OF VISUAL PIGMENTS IN THE VISIBLE AND ULTRAVIOLET SPECTRAL REGIONS

Abstract

Current ideas concerning the chemical structure of the visual pigments involve the suggestions that the prosthetic group, whether retinal, or retinal2, is attached primarily to the opsin as a Schiff base, and that this prosthetic group is held rigidly in some specific conformational relationship to the protein.1-3In this model, the shape of the prosthetic group, thought to be that of the 11-cis isomer, is considered to be critical in holding the protein and retinal in the functionally correct relationship.The action of light is held to consist primarily of an isomerization of the retinal from the 11-cis to the all-trans configuration; this loosens the specific attachment of the prosthetic group to the protein, following which the latter undergoes a change in structure.3 24 We have investigated, among possible experimental approaches, whether these specific conformations might be detected in terms of optically active absorption bands.Such an examination involves two different aspects.In polypeptides and proteins, the occurrence of helical structures has been successfully correlated with intrinsic Cotton-effects measured in terms of optical rotatory dispersion4 5 or circular dichroism.5-7The second aspect is concerned with the extrinsic Cotton-effects induced when optically inactive substances are dissymmetrically attached to optically active macromolecules.8' 22In this work, we have investigated the extrinsic circular dichroism (CD) exhibited by the prosthetic group in the visible region and the near ultraviolet, as