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

Ultrafast fluorescence dynamics of Tryptophan-Tryptophan (Trp-Trp/Trp₂) 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 Trp₂, N-tert-butyl carbonyl oxygen-N′-aldehyde group-l-tryptophan-l-tryptophan (NBTrp₂), l-tryptophan-l-tryptophan methyl ester (Trp₂Me), and N-acetyl-l-tryptophan-l-tryptophan methyl ester (NATrp₂Me) 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 Trp₂ and its derivatives also exhibit a new decay with a lifetime of ∼100 ps, while single-Trp fluorescence decay in dipeptides/proteins shows 20-30 ps. The intramolecular interaction lifetime constants of Trp₂, NBTrp₂, Trp₂Me and NATrp₂Me 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.