Frog photoreceptor membranes contain 54,000 g of protein

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1. Retinol and stilbene are both isomerized when they are illuminated anaerobically in the presence of flavins. 2. Triplet quenchers (e.g. oxygen, potassium iodide and paramagnetic ions) inhibit the reaction more efficiently than they quench flavin fluorescence. At 770C in a diethyl ether-isopentane-ethanol (5:5:2) glass retinol quenches flavin phosphorescence, but not its fluorescence. 3. For the stilbene reaction cis/trans photostationary-state mixtures are obtained with different flavins and these are linearly related to the phosphorescence transition energies of the flavins used. 4. The reaction involves the triplet state of flavin and a scheme for the reaction is suggested. 5. The dependence of the rate of reaction on substrate concentration is explicable in terms of this scheme. 6. The photobleaching of rhodopsin sensitized by flavin is also demonstrated.Flavins and carotenoids are found together in many photoreceptors

The angles between the two principal long wavelength absorption oscillators of FMN and some other flavins have been measured by two methods. A novel technique involving dichroic bleaching gave a value (approx. 35") consistent with that obtained from the polarisation of fluorescence extrapolated to infinite viscosity.Flavins trapped in polyacrylamide films display E-type delayed fluorescence and phosphorescence depending on the temperature. The variations in lifetimes and intensities of these two delayed emissions with temperature have been studied. The Arrhenius activation energies have been analysed in terms of a scheme that gives information about the interrelationships between different electronic states of flavins.Studies on the mechanism of flavoprotehs and on the photochemical and photobiological properties of flavins depend on extensive use of spectral data, and on an understanding of the properties of the excited states of these molecules. The spectral properties of flavin radical intermediates have been widely investigated [i, 21 but no comprehensive study of flavin spectra is available. Hemmerich and his collaborators have collected a large amount of spectral information on the effects of structural changes in the isoalloxazine ring [3,4] ; these results and other data have been summarised in a review [ 5 ] . Qualitative data on the effects of solvents on the absorption spectrum of 3-methyl lumiflavin [6] The other derivatives were gifts from the following: 3-methyl lumiflavin, 2-and 4-morpholino lumiflavins, isoriboflavin and isoperacetylriboflavin, Professor P. Hemmerich ; 9-(N,N-dimethylaminoethyl)-isoalloxazine and 6-chloro,7-methoxy,9-(ethyl-2'Npyrrolidin0)-isoalloxazine, Professor T. WagnerJauregg ; 6-chloro,9-(N,N-dimethylaminopropyl)-isoalloxazine and B-rnethyl,9- (N,N-diethylaminoethyl)

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