Phosphorescence lifetime of tryptophan in proteins

This investigation enquires into the factors that are responsible for the wide range of room-temperature Trp phosphorescence lifetimes (tau) in proteins. By exploiting the enhanced sensitivity and time resolution of phosphorescence measurements, experiments were conducted to evaluate the triplet quenching potential of each amino acid side chain. From the magnitude of the Stern-Volmer rate constant it is concluded that, among the amino acids, quenching reactions at 20 degrees C are quite effective with His, Tyr, Trp, cysteine, and cystine, with rate enhancements of 20 and 50 times when the side chains of Tyr and His, respectively, are in the ionized form. The distance dependence of the quenching interaction, estimated from the quenching of internal Trp residues in proteins separated from the amino acid in solution by a protein spacer of various thickness, emphasizes the very short-range nature of the process. The importance of these side chains, and to some extent that of the peptide linkage, as intrinsic quenchers of Trp phosphorescence in proteins was also confirmed with short synthetic peptides prepared appositely with only one type of these residues. Finally, very short (microseconds) phosphorescence lifetimes of Trp residues in proteins were shown to be invariably associated with the presence of Tyr or Cys in the immediate neighborhood of the chromophore. From a survey of the amino acids that are nearest neighbors to Trp in proteins and the corresponding value of tau it was established that, in the absence of His, Tyr, Trp, and Cys, tau is > or = 1 ms and appears to reflect mainly the local fluidity of the protein structure. Otherwise, tau can be much shorter, and for bulky His, Tyr, and Trp side chains it seems to depend dramatically on the mutual chromophore-quencher orientation. In these cases the triplet decay kinetics is shown to be a complex function of temperature, pH, and flexibility of the protein site.