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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.1-3 In 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.5-7 The second aspect is concerned with the extrinsic Cotton-effects induced when optically inactive substances are dissymmetrically attached to optically active macromolecules.8' 22 In this work, we have investigated the extrinsic circular dichroism (CD) exhibited by the prosthetic group in the visible region and the near ultraviolet, as well as the intrinsic CD of the opsin moiety in the range of 200-240 my (nanometers, nm). We have recorded both these effects for several visual pigments from different species, before and after bleaching them by exposure to white light. A preliminary report9 described a CD associated with the main absorption band in the visible part of the spectrum and its disappearance upon bleaching. This result was confirmed by Williams10 and by Kito and Takezakill who measured rotatory dispersion. The occurrence of rotatory dispersion in the short-wave ultraviolet, and its reduction after bleaching, has also been demonstrated,3' 11 whereas a change in ultraviolet absorption12 probably represents a related phenomenon. Materials and Methods.-Extracts were made of retinas of several species by the usual procedures,13 by extraction of alum-hardened retinas, or of rod outer segments, with 2 per cent digitonin solution; some extracts were also made employing cetyltrimethyl ammonium bromide or the nonionic detergent Triton X-100. The obtained pigments were characterized by the usual spectrophotometric techniques, and the optical densities at the wavelength of maximum absorption for the a-band were determined from the spectral absorbances before and after bleaching in the presence of hydroxylamine.13 Circular dichroism; The work described in this paper was done with the prototype of the Jasco recording dichrograph, modified so as to eliminate baseline drift. The

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