Fluorescence and the structure of proteins. VII. Hydrogen-bonded phenols as fluorescence sinks.

Abstract

Abstract The low fluorescence of tyrosyl residues in some proteins suggests that sinks for loss of energy must exist, and it has been proposed that these sinks are tyrosyl residues hydrogen-bonded to carbonyl groups. To test this proposal, compounds such as HOC 6 H 4 CH 2 CH 2 C 6 H 4 OCH 3 were prepared and tested for fluorescence and for the degree of fluorescence lost upon hydrogen-bonding the phenolic group to amides. Because the formation of a stable bond between phenols and amides can occur only in non-polar solvents, the following distinction has been made. In non-polar solvents, a hydrogen-bonded phenolic group can serve as an efficient energy sink for loss of excitation energy from a nearby phenolic ring but not from an indole ring. In aqueous solution, a phenolic group can serve as an energy sink during the period of its collision with a carboxylate ion. The latter sink is quite inefficient unless the carboxylate ion concentration is high.