LIGHT-INDUCED FREE RADICALS OF RETINAL RETINOL AND RHODOPSIN

Light-induced electron paramagnetic resonance signals that disappeared upon melting were elicited from trapped paramagnetic photoproducts of all- trans-, 9-cis-, and 13-m-retinal, all-trans retinol, and rhodopsin. Solutions of these substances in organic solvents or 2% aqueous digitonin were irradiated by visible light at —196°. At about 1 mM concentration of the three retinals and from rhodopsin, electron paramagnetic resonance singlet spectra were obtained at g ≃ 2.004, with line widths of about 23 gauss and with some suggestion of further unresolved spectral detail. In general the electron paramagnetic resonance signal was greater with increasing illumination and with decreasing polarity of the solvent. At higher concentrations an electron paramagnetic resonance photosignal also was obtained from retinol, evincing an over-all, roughly symmetrical three-line spectrum plus additional hyperfine structure, with a total width of about 210 gauss but unaccompanied by a half-field signal. Room temperature fluorescence was observed from retinol but not from the aldehydes. The data are taken to indicate strong localization of excitation energy and of unpaired electron distribution in retinol, with relative delocalization in the retinaldehydes. Retinal and rhodopsin free radicals are viewed as probable products of the light-induced triplet states previously noted by others, and they are discussed as possible participants in the first (isomerizing) step in the photochemical conversion of rhodopsin. Photochemical creation of radical ions also is proposed as the initial step in the charge separation that appears to be associated with at least some components of the early receptor potential of the electroretinogram (recorded from illuminated retinae in vivo). © 1968, American Chemical Society. All rights reserved.