The phosphorescence lifetime (τ) of tryptophan (Trp) residues in proteins in aqueous solutions at ambient temperature can vary several orders of magnitude depending on the flexibility of the local structure and the rate of intramolecular quenching reactions. For a more quantitative interpretation of τ in terms of the local protein structure, knowledge of all potential quenching moieties in proteins and of their reaction rates is required. The quenching effectiveness of each amino acid (X) side chain and of the peptide backbone was investigated by monitoring their intramolecular quenching rate (kobs) in tripeptides of the form acetyl-Trp-Gly-X-CONH2 (WGX), where Trp is joined to X by a flexible Gly link. The results indicate that among the various groups present in proteins only the side chains of Cys, His, Tyr and Phe are able to quench Trp phosphorescence at a detectable rate (kobs > 40 s−1), with the quenching effectiveness for rotationally unrestricted side chains ranking in the order Cys ≫ His > Tyr ≫ Phe ∼ His. For the aromatic side chains the corresponding contact rate at 20°C is estimated to be between 3–4 × 109 s−1 for Cys (as determined by Lapidus et al.), 0.8–8 × 106 s−1 for His , 0.37–3.7 × 106 s−1 for Tyr and 0.2–2 × 105 s−1 for Phe and His. In the cases of His and Tyr, kobs drops sharply with increasing pH, with midpoint transitions about 1 pH unit above the pKa, indicating that quenching is almost exclusive to the protonated form. From the temperature dependence of the rate, obtained in 50/50 propylene glycol/water between −20°C and 20°C, the reaction is characterized by activation energies of about 5 kcal·M−1 for His and Tyr and 8 kcal·M−1 for Phe. An analysis of the groups in contact with Trp residues in proteins that exhibit long phosphorescence lifetimes at ambient temperature leads to the conclusion that the contact rate of the peptide group and of the remaining side chains is lower than 0.1 s−1, showing that these moieties are practically inert with respect to the triplet-state lifetime. It shows further that the immobilization of the aromatic side chains within the globular fold cuts their quenching effectiveness drastically to contact rates < 2 s−1, a phenomenon attributed to the low probability of forming a stacked exciplex with the indole ring. All evidence suggests that, except in the case of nearby Cys or Trp residues, whose interaction with the triplet state reaches beyond van der Waals contact, the emission of buried Trp residues is unlikely to be quenched by surrounding protein groups.
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Vol. 81 • No. 3