AFFINITY-CONTROLLED TRIPLET ENERGY TRANSFER FROM p-BENZOYLBENZYL-BOVINE SERUM ALBUMIN TO LOW MOLECULAR WEIGHT QUENCHERS

Abstract— Methods for the controlled synthesis of modified protein photosensitizers which maximize affinity-controlled energy transfer are discussed. Modified proteins containing covalently linked benzophenone groups were prepared by the reaction of bovine serum albumin with p -benzoylbenzyl bromide under conditions limiting the number and locations of the introduced benzophenone chromophores. The mechanism of energy transfer responsible for quenching of triplet photochemical reactions of the modified proteins was explored using water soluble quenchers. In addition, methods were employed to determine the magnitude of the contribution of the affinity-controlled mechanism for energy transfer in these systems. The utility of sodium- cis -8-methylene-4,9-decadienoate as a triplet energy transfer indicator for macromolecular systems was demonstrated using the modified proteins as sensitizers. The trienoic acid was prepared starting with the known 4-methylene-5-hexenal by the Wittig reaction with 3-ethoxycarbonylpropylidene triphenylphosphorane followed by saponification. Triplet sensitized irradiation of this trienoic acid using p -benzoylbenzyltriethylammonium chloride as sensitizer led to production of endo- and exo-1-vinyl-5-(3-carboxyethyl)bicyclo2.1.1]hexane along with the trans acid. Characterization of the bicyclohexane products was made on the basis of spectroscopic evidence. Results demonstrate that quenching of the intramolecular photoreactions of the modified proteins by trienoic acid must be a result of triplet energy transfer, since irradiation of these modified proteins in the presence of the trienoic acid salt led to the characteristic triplet photoproduct mixture.