Fluorescence and protein structure: X. Reappraisal of solvent and structural effects

Abstract

1. 1.|Quantum efficiencies of fiuorescence and emission maxima were measured in a number of solvents for tryptophanyl and tyrosyl peptides, as well as simpler derivatives of indole and phenol. Fluorescence was internally quenched by carbonyl groups but this occurred only when the molecule was solvated by a hydrogen-bonding solvent such as water or an alcohol. In non-polar organic solvents, quenching by the carbonyl group was lost and fluorescence rose to values of the parent indole and phenol compound. 2. 2.|Fluorescence quenching occurred with derivatives that contained unionized acid, amide, ester and peptide groups. These groups, when present on other solute molecules, did not quench; that is, external quenching of indole and phenol by amides, esters, etc., did not occur. An amino group alone had no effect on fluorescence, but in the free amino acids and peptides an α-amino group enhanced the quenching action of a neighboring carbonyl group. Fluorometric titrations showed that this enhancement of the carbonyl quenching was in the order α-NH 3 + > α-NH 2 > H. 3. 3.|The bearing that these observations may have on fluorescence of proteins was discussed. A preliminary classification scheme for exposed and buried tryptophanyl and tyrosyl residues was described and a range of values was selected for the fluorescence efficiency of each type of residue.