DEUTERIUM-ISOTOPE EFFECTS IN CONSTRAINED TRYPTOPHAN DERIVATIVES - IMPLICATIONS FOR TRYPTOPHAN PHOTOPHYSICS

The deuterium isotope effect on the fluorescence quantum yield and lifetime of the constrained tryptophan derivative 3-carboxy-1,2,3,4-tetrahydro-2-carboline, W(1), was determined as a function of pH and temperature. The isotope effect between pH 3.5 and 11 is attributed to a temperature-dependent quenching process common to all indoles. At room temperature the quantum yield ratio in D2O and H2O is 1.05 for W(1) zwitterion and 1.7 for W(1) anion. The temperature dependence of the fluorescence lifetime was determined for the zwitterion and the anion in H2O and D2O. The frequency factors A and activation energies E* in H2O are A = 6 x 10(16) s-1, E* = 12.6 kcal/mol for W(1) zwitterion and A = 5 x 10(16) s-1, E* = 11.8 kcal/mol for W(1) anion, compared to A = 8 X 10(16) s-1, E* = 13.1 kcal/mol for N-methylindole. The radiative rates, temperature-independent nonradiative rates, and activation energies E* of W(1) zwitterion, W(1) anion, and N-methylindole are insensitive to solvent isotope. The frequency factors A of these compounds are 2- to 3-fold larger in H2O than in D2O. The large deuterium isotope effect in W(1) anion at room temperature compared to W(1) zwitterion results from two factors: a smaller temperature-independent nonradiative rate and a larger isotopically sensitive temperature-dependent rate. Several mechanisms for the intrinsic deuterium isotope effect are discussed. Two mechanisms are consistent with available data for indoles: "invisible" or incomplete proton transfer from water to indole and formation of a water-indole charge-transfer exciplex.