Solvent Reorientational Dynamics and Fluorescence Quenching of Tryptophan

Abstract

We have studied the temperature dependence of the relative quantum yield of fluorescence for D-tryptophan in water, polar and nonpolar glasses in the expectation that such experiments will lead to an understanding of the nature of nonradiative processes in solutions. At any temperature

$$In({B_0}/B - 1) = In(A{\tau_0}) - E/(R\;T) $$

((1))

, where ${B_0}\;,\;{\tau_0}$ are the low temperature limits of quantum yield, B, and lifetime, τ, and E, A are the activation energy and frequency factor of thermally activated quenching. In Figure 1 are shown results of the left side of equation (1) against 1/T. When $In({B_0}/B - 1)$ was plotted against 1/T for tryptophan with 95%-ethanol, propanol, glycerine and ethylene glycol as solvents, it was seen that results are within the experimental errors equal and viscosity independent. For this hydrophilic solvents which had possibly a little impurity of water.E=1.10 kcal/mole at 7<230 K and 3.31 kcal/mole at T>230 K. The activation energy is 4.5 kcal/mole for high temperature region (T>230 K) with hydrophobic isobutanol as solvent and agree approximately with that for hydrophilic solvents at low temperatures for 10³/ T between 4.8 and 5.6. The reverse picture takes place for nonpolar 2-methylbutane but activation energies are similar to those for hydrophilic solvents: E=3 kcal/mole at T<147 K and E=1.03 kcal/mole at T>147 K.