Hydration of tryptophan probed by triplet lifetime isotope effect

Water plays an essential role in the structures, dynamics, and functions of proteins. The experimental determination of the presence of water in proteins remains a challenge. Tryptophan and its derivatives are well-studied molecules whose photophysical properties are sensitive to the local environment. In particular, the lifetime of the triplet state is impacted by the presence of nearby water via coupling to high-frequency vibrational modes of solvent. The ratio of the indole triplet lifetimes in D2O and H2O (τD/τH), called the triplet lifetime isotope effect (3LIE), is 1.6. The feasibility of using measurements of 3LIE to assess hydration of tryptophan residues in proteins was explored, with focus on a membrane-associated model compound, tryptophan octyl-ester; soluble proteins, human serum albumin and ribonuclease T1; the membrane peptide melittin; a leucine-rich synthetic membrane peptide we call leucimer; and the membrane protein outer membrane protein A. The results indicate that while there is variation in the triplet lifetimes depending on the local environment, the value of 3LIE reflects the absence (3LIE = 1.0) or presence (3LIE > 1.0) of nearby water. Molecular dynamics simulations support this interpretation. The mechanism and number of water O-H bonds that couple to the triplet state were explored. These findings suggest that measurements of 3LIE can be applied to directly monitor changes in the hydration of proteins.