A Model for the Explanation of the Thermally Induced Increase of the Overall Fluorescence in Tryptophan-X Peptides

In the range of temperature 10–35°C, Trp-X dipeptides show an unusual increase of fluorescence intensity in solution at pH 7. This effect has been recently studied by means of steady-state fluorescence. Although a model involving the deprotonation at the ground state of the zwitterion was proposed, the activation energy for that process could not rule out the involvement of excited state. In order to understand the mechanism of the thermal-induced increase of fluorescence, we present here time-resolved fluorescence experiments on Trp-X and X-Trp dipeptides at different pH and excitation wavelength. The fluorescence lifetimes (τi) decrease in accord to thermal quenching, with activation energies (Ei) ranging from 4.0 to 6.4 kcal/mol. Under those circumstances where the anomaly was detected the preexponential factors of the longer-lived component increased as well as their fractional fluorescence. This component can be assigned to the anion species. Because of its larger (three- to fourfold) fluorescence quantum yield, compared to that of the corresponding zwitterion, the large increase of the concentration of the anion leads to an increase of the overall emission despite the thermal quenching. Also the decay-associated spectra well account for the red shift of the emission fluorescence spectrum, which accompanies the anomaly. Our model well fits the experimental data using a simple equation which combines Vanʹt Hoff and Arrhenius equations; it also explains the presence of the anomalous thermal quenching exclusively in Trp-X dipeptides excited above 290 nm and at pH around neutrality.