Effect of Urea on the Organization and Dynamics of Triton X-100 Micelles:  A Fluorescence Approach

Chaotropes such as urea perturb the organization of molecular assemblies formed by the hydrophobic effect. We have monitored the change in the organization and dynamics of Triton X-100 micelles induced by urea utilizing wavelength-selective fluorescence and related approaches using the environment-sensitive fluorescent membrane probes NBD-PE ( N -(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dipalmitoyl- sn -glycero-3-phosphoethanolamine) and DPH (1,6-diphenyl-1,3,5-hexatriene). NBD-PE is a phospholipid in which the headgroup is covalently attached with the fluorescent NBD group. On the other hand, DPH is an extensively used fluorescent membrane probe which partitions into deeper hydrocarbon region of membranes and micelles. Our results utilizing several sensitive fluorescence parameters (such as emission maximum, anisotropy, lifetime, and quenching) show increased polarity around the NBD group of micelle-bound NBD-PE in the presence of varying concentrations of urea. This could possibly be due to increased water penetration in Triton X-100 micelles induced by urea. Data from experiments using the deeper hydrocarbon region probe DPH support this observation. Interestingly, the extent of red edge excitation shift (REES) was invariant in the presence of varying concentrations of urea. Fluorescence quenching measurements of the micelle-bound NBD-PE using the aqueous quencher Co 2+ indicate increased accessibility of the NBD group at higher urea concentrations. These results could be due to an increase in solvation of the polar headgroups by urea−water mixture than water alone, along with an increase in water penetration.