The Triplet-State Lifetime of Indole in Aqueous and Viscous Environments: Significance to the Interpretation of Room Temperature Phosphorescence in Proteins

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Abstract
The interpretation of room temperature phosphorescence studies of proteins requires an understanding of the mechanisms governing the tryptophan triplet-state lifetimes of residues fully exposed to solvent and those deeply buried in the hydrophobic core of proteins. Since solvents exposed tryptophans are expected to behave similarly to indole free in solution, it is important to have an accurate measure of the triplet state lifetime of indole in aqueous solution. Using photon counting techniques and low optical fluence (J/cm2), we observed the triplet-state lifetime of aqueous, deoxygenated indole and several indole derivatives to be approximately 40 μs, closely matching the previous reports by Bent and Hayon based on flash photolysis (12 μs; Bent, D. V.; Hayon, E. J. Am. Chem. Soc.1975, 97, 2612−2619) but much shorter than the 1.2 ms lifetime observed more recently (Strambini, G. B.; Gonnelli, M. J. Am. Chem. Soc.1995, 117, 7646−7651). However, we have now been able to reproduce the long lifetime reported by the latter workers for aqueous indole solutions and show that it likely arises from geminate recombination of the indole radical cation and solvated electron, a conclusion based on studies of the indole radical cation in water (Bent and Hayon, 1975). The evidence for this comes from a fast rise in the phosphorescence emission and measurements of a corresponding enhanced quantum yield in unbuffered solutions. This species can be readily quenched, and the corresponding fast rise disappears, leaving a monoexponential 40 μs decay, which we argue is the true indole triplet lifetime. The work is put in the context of room temperature phosphorescence studies of proteins.