Fluorescence kinetics of Trp-Trp dipeptide and its derivatives in water via ultrafast fluorescence spectroscopy

Ultrafast fluorescence dynamics of Tryptophan-Tryptophan (Trp-Trp/Trp(2)) dipeptide and its derivatives in water have been investigated using a picosecond resolved time correlated single photon counting (TCSPC) apparatus together with a femtosecond resolved upconversion spectrophotofluorometer. The fluorescence decay profiles at multiple wavelengths were fitted by a global analysis technique. Nanosecond fluorescence kinetics of Trp(2), N-tert-butyl carbonyl oxygen-N'-aldehyde group-L-tryptophan-L-tryptophan (NBTrp(2)), L-tryptophan-L-tryptophan methyl ester (Trp(2)Me), and N-acetyl-L-tryptophan-L-tryptophan methyl ester (NATrp(2)Me) exhibit multi-exponential decays with the average lifetimes of 1.99, 3.04, 0.72 and 1.22 ns, respectively. Due to the intramolecular interaction between two Trp residues, the water relaxation lifetime was observed around 4 ps, and it is noticed that Trp(2) and its derivatives also exhibit a new decay with a lifetime of similar to 100 ps, while single-Trp fluorescence decay in dipeptides/proteins shows 20-30 ps. The intramolecular interaction lifetime constants of Trp(2), NBTrp(2), Trp(2)Me and NATrp(2)Me were then calculated to be 3.64, 0.93, 11.52 and 2.40 ns, respectively. Candidate mechanisms (including heterogeneity, solvent relaxation, quasi static self-quenching or ET/PT quenching) have been discussed. (C) 2015 Published by Elsevier B.V.