FLUORESCENCE LIFETIME MEASUREMENTS ON SOME TRYPTOPHAN-CONTAINING DIKETOPIPERAZINES

Fluorescence decay measurements were made on six tryptophan-containing diketopiperazines (cyclo(-L-Trp-L-His-), cyclo(-L-Trp-D-His-), cyclo( -L-Trp-L-Asp-), cyclo(-L-Trp-D-Asp-), cyclo(-L-Trp-L-Glu-) and cyclo(- L-Trp-D-Glu-); Trp, tryptophan; His, histidine; Asp, aspartic acid; Gl u, glutamic acid). For each compound measurements were made at two pH values so as to determine the extent of the change in lifetime over th e range of titration of the titratable group. The peptides were dissol ved in either H2O or 98% D2O. The decay curves were fitted quite satis factorily by two exponentials. The lifetime of the slow component decr eased with pH from about 4 ns to about 0.9 ns depending on the peptide and pH. The lifetime of the fast component ranged from 0.2 ns to 0.5 ns depending on the peptide and pH. The lifetime of the slow component of the compound containing the L enantiomer of a given dipeptide was always longer than the lifetime of the slow component of the compound containing the D enantiomer. It was concluded that the fluorescence li fetimes correlate with the protonation of nearby titratable sites, by virtue of either their quenching capability or charge, and possibly wi th the distance from these sites.