Solvent effects on the fluorescence quenching of tryptophan by amides via electron transfer. Experimental and computational studies.

Hybrid quantum mechanical/molecular mechanics (QM-MM) calculations [Callis and Liu, J. Phys. Chem. B 2004, 108, 4248-4259] make a strong case that the large variation in tryptophan (Trp) fluorescence yields in proteins is explained by ring-to-backbone amide electron transfer, as predicted decades ago. Quenching occurs in systems when the charge transfer (CT) state is brought below the fluorescing state (1L(a)) as a result of strong local electric fields. To further test this hypothesis, we have measured the fluorescence quantum yield in solvents of different polarity for the following systems: N-acetyl-L-tryptophanamide (NATA), an analogue for Trp in a protein; N-acetyl-L-tryptophan ethyl ester (NATE), wherein the Trp amide is replaced by an ester group, lowering the CT state energy; and 3-methylindole (3MI), a control wherein this quenching mechanism cannot take place. Experimental yields in water are 0.31, 0.13, and 0.057 for 3MI, NATA, and NATE, respectively, whereas, in the nonpolar aprotic solvent dioxane, all three have quantum yields near 0.35, indicating the absence of electron transfer. In alkyl alcohols the quantum yield for NATA and NATE is between that found for water and that found for dioxane, and it is surprisingly independent of chain length (varying from methanol to decanol), revealing that microscopic H-bonding, and not the bulk dielectric constant, dictates the electron transfer rate. QM-MM calculations indicate that, when averaged over the six rotamers, the greatly increased quenching found in water relative to dioxane can be attributed mainly to the larger fluctuations of the energy gap in water. These experiments and calculations are in complete accord with quenching by a solvent stabilized charge transfer from ring to amide state in proteins.