Photophysics of indole, tryptophan and N-acetyl-L-tryptophanamide (NATA): heavy atom effect

Previously reported flash photolysis studies showed that the triplet state lifetime of aqueous indoles is μs long (12.5 μs for tryptophan [10]), while other recently reported phosphorescence lifetimes of aqueous indoles, determined from photon counting phosphorescence techniques, vary from μs (approximately 40 μs [11]) to ms (5 ms for indole [12]). This study was motivated to explain the discrepancy regarding the intrinsic triplet state lifetime of aqueous indole and its derivatives: tryptophan and N-acetyl-L-tryptophanamide (NATA). For this purpose, a new methodology based on both fluorescence and phosphorescence decay kinetics incorporating the heavy atom effect have been applied in order to determine some quantitative parameters of the photophysics of indole and its derivatives. Additionally, we have also determined the triplet state lifetimes of the studied indoles using flash photolysis in which contributions from both a first order component and a second order component (from triplet-triplet annihilation) have been taken into account in the triplet state depopulation. The measured phosphorescence lifetime of the indoles examined measures between the values reported by Fischer and Strambini and is consistent with the triplet state lifetime determined from flash photolysis. We hope that the results obtained in this paper would be helpful for deriving structural and dynamical information from phosphorescence data of tryptophan residues in proteins.