Interaction of Cetyltrimethylammonium Bromide and Its Gemini Homologue Bis(cetyldimethylammonium)butane Dibromide with Xanthine Oxidase

The interaction of xanthine oxidase (XO), a key enzyme in purine metabolism, with cetyltrimethylammonium bromide (CTAB) and bis(cetyldimethylammonium)butane dibromide (C16C4C16Br2) has been studied using tensiometry, spectrofluorometry, spectrophotometry, and circular dichroism at pH 7.4 and 25 degrees C. The tensiometric profiles of CTAB and C16C4C16Br2 in the presence of XO exhibit a single break at a lower surfactant concentration termed as C-1 compared to their CMC in the buffered solution and show the existence of interaction between the surfactants and the enzyme. The results of the multitechnique approach showed that, although both CTAB as well as C16C4C16Br2 interact with the XO, C16C4C16Br2 interacts more strongly than its conventional single chain counterpart. Fluorescence and absorption measurements revealed that, compared to CTAB, C16C4C16Br2 is more effective in unfolding the enzyme. Change in XO activity by the surfactants was in concurrence with the structural alterations monitored by circular dichroism and showed structural stabilization of XO at higher surfactant concentrations, consistent with the aggregation results. This stabilization has been explained in light of strong tendency of C16C4C16Br2 for micellar growth and membrane/water stabilization of proteins by membrane-like fragments provided by higher concentrations of C16C4C16Br2. The results are related to the stronger electrostatic and hydrophobic forces in C16C4C16Br2, owing to the presence of two charged headgroups and two hydrophobic tails.