The Peptide Bond Quenches Indole Fluorescence

Abstract

The effect of the peptide bond on protein fluorescence is an important unresolved question in tryptophan photophysics. Definitive evidence for the peptide group as a weak quencher of indole fluorescence was obtained from solute quenching studies with a series of model compounds. Two amides are required for detectable quenching of 3-methylindole fluorescence and the quenching rate depends on the distance between amides. The bimolecular rate constants kq of malonamide, N-acetylasparagine, N-acetylglycinamide, and N-acetylglutamine are 33 × 107, 8.8 × 107, 6.6 × 107, and 2.2 × 107 M-1 s-1, respectively. Transient absorption and temperature dependence of the fluorescence lifetime measured in the absence and presence of quencher gave strong circumstantial evidence for electron transfer as the quenching mechanism. Triplet yields were measured for five indole derivatives using transient absorption. Intersystem crossing rates were calculated from triplet yield and fluorescence lifetime data. The intersystem cross...