Photostationary state compositions of retinal and related compounds included in beta-lactoglobulin. Effects of protein host on isomer distribution of polyene substrates.

The UV-visible absorption spectra and photostationary state compositions of retinal (or the related C-18 ketone, 3-dehydroretinal and the C-22 aldehyde) imbedded in the binding cavity of beta-lactoglobulin (BLG) are consistent with the view that the carbonyl group of these polyenes are hydrogen-bonded with the protein host, most likely with the lone protonated lysine residue in the binding pocket. Patterns of variation in photochemical behavior of the imbedded chromophore versus that in solution are discussed in terms of possible specific protein-substrate interactions. The results are also compared with that of the methyl ether of retinol where similar hydrogen bonding is not possible.