DIFFUSION-CONTROLLED REACTIONS - TRANSIENT EFFECTS IN THE FLUORESCENCE QUENCHING OF INDOLE AND N-ACETYLTRYPTOPHANAMIDE IN WATER

Time-domain fluorescence quenching studies of indole and N-acetyltryptophanamide (NATA) in water were carried out at different temperatures using KI and acrylamide as quenchers. The fluorescence decay was nonexponential in all the cases as expected for a diffusion-controlled bimolecular reaction, and the data were analyzed according to different decay equations. The data for indole/KI were consistent with the Smoluchowski-Collins-Kimball (SCK) decay equation and the limiting values for the fundamental parameters - diffusion coefficient, D, reaction distance, R, and the absolute rate coefficient, ka - are reasonably close to the expected values. The SCK decay equation was tested by an "assumption-free" method and by using the "long-time" approximation. The results obtained by the two methods are consistent. Analysis of the quenched fluorescence data of indole/KI/H2O to fit the long-time SCK decay equation (mathematically similar to the Smoluchowski equation) gave reasonable values for D and the distance parameter, R′. It is generally observed that the values of D and R′ are correlated. The experimentally determined values for D and R′ for four fluorophore-quencher systems at different temperatures are reported in this paper and discussed. The results of D and R′ obtained at different temperatures are consistent with the identification that indole/KI system follows the SCK model and indole/acrylamide system is closer to the Smoluchowski model. The results of D and R′ for NATA/KI and NATA/acrylamide systems were not favorable for identifying these systems with either model. © 1990 American Chemical Society.