Distance-dependent fluorescence quenching of tryptophan by acrylamide.

We used GHz frequency-domain fluorometry to investigate the time-dependent intensity decays of N-acetyl-L-trytophanamide (NATA) when collisionally quenched by acrylamide in propylene glycol at 20 degrees C. The intensity decays of NATA became increasingly heterogeneous in the presence of acrylamide. The NATA intensity decays were not consistent with the Collins-Kimball radiation boundary condition (RBC) model for quenching. The steady-state Stern-Volmer plots show significant upward curvature. At low temperature in vitrified propylene glycol (-60%), where translational diffusion cannot occur during the lifetime of the excited state, quenching of NATA by acrylamide was observed. The Smoluchowski and RBC quenching models do not predict any quenching in the absence of translational diffusion. Hence, these frequency-domain and steady-state data indicate a through-space quenching interaction between NATA and acrylamide. The rate for quenching of NATA by acrylamide appears to depend exponentially on the fluorophore-quencher separation distance. Comparison of the time-resolved and steady-state data provides a sensitive method to determine the distance dependence of the fluorophore-quencher interaction. The distance-dependent rate of quenching also explains the upward curvature of the Stern-Volmer plot, which is often observed for quenching by acrylamide. These results suggest that the distance-dependent quenching rates need to be considered in the interpretation of quenching data of proteins by acrylamide.