Effect of N-Alkyl Trimethylammonium Bromide (CnTAB) Cationic Surfactant Hydrophobicity on Cellulase Intrinsic Fluorescence

Abstract

Aims: To determine the effect of the hydrophobic forces involved in the interaction of cellulase (endo-β-1,4-glucanase from Aspergillus niger), and cationic surfactant n-alkyl trimethyl ammonium bromides (CnTAB), with varying chain lengths (n = 10, 12, 14, 16, 18), steady-state fluorescence spectroscopy using tryptophan as a probe was employed. Study Design: Using intrinsic tryptophan emission, a Stern-Volmer analysis of steady-state fluorescence spectroscopy data was used. Place and Duration of Study: Department of Chemistry, The Cooper Union between January 2009 and April 2010. Methodology: Intrinsic cellulose fluorescence was quenched by the addition of cationic surfactants. This quenching, which might occur by dynamic or static collisional mechanisms, was measured by monitoring the decrease in fluorescence intensity as a function of the surfactant concentration. A Stern-Volmer plot was then generated to obtain values for KSV, the Stern-Volmer constant for the interaction. Results: No appreciable quenching occurred for the lower chain CnTABs (C10TAB and C12TAB). A linear form of the Stern-Volmer equation modeled the behavior of cellulase quenching by C14TAB, C16TAB, C18TAB, and the KSV values were determined to be 5.9 x 10-5 μM-1 (59 M-1), 1.1 x 10-4 μM-1 (110 M-1), and 1.4 x 10-4 μM-1 (140 M-1), respectively, with R2 values greater than 0.90 but less than 0.95. The data for the experiments involving C12TAB and C14TAB disagreed significantly with earlier published work in that they differed significantly in the extent of quenching and emission wavelength shift observed from cellulase fluorescence upon addition of the surfactants. Conclusion: None of the results obtained in this experiment were able to confirm the biphasic behavior of endoglucanase. The KSV values obtained from the results are sufficient to claim that higher chain surfactants are more effective quenchers of cellulase fluorescence and consequently, that the hydrophobic forces play a great role in cellulase-CnTAB interactions. A modified form of the equation (having a second order with respect to the concentration of the quencher) distinguishing and modeling both dynamic and static quenching can possibly be applied in the future studies to improve the accuracy of the Stern-Volmer model. Although this would enable to distinguish and perhaps more accurately model the two types of quenching exhibited by the same fluorophore, it would require additional experiments such as measuring fluorescence lifetimes (τ) and/or testing for the effect of temperature.