Hydrodynamic radii of protein-free and protein-containing reverse micelles as studied by fluorescence recovery after fringe photobleaching. Perturbations introduced by myelin basic protein uptake

Abstract

The determination of hydrodynamic radii of reverse micelles composed of sodium bis( 2-ethylhexyl) sulfosuccinate (AOT)/isooctane/water has been carried out using a nonperturbative technique: fluorescence recovery after fringe pattern photobleaching. The technique allows the precise measurement of hydrodynamic radii of protein-containing as well as protein-free micelles. This is achieved by the covalent labeling of a protein (myelin basic protein) and of a small dipeptide (Nacetylglycyl-L-lysine methyl ester) with a fluorescent dye (fluorescein isothiocyanate). The measurements were performed at low and high water content of the system as defined by the water-to-surfactant molar ratio wo = [H,O]/[AOT]. At wo = 5.6 and 50 mM AOT, the hydrodynamic radii were Rh = 29 8, and Rh = 44.5 8, for protein-free and protein-containing micelles, respectively. The radii do not seem measurably affected by the surfactant concentration or the amount of protein present. In contrast, at wo = 22.4, the effect of AOT concentration was prominent. Measurements carried out at various surfactant concentrations, extrapolated to [AOT] = 0, gave a single &, value of 51 8, for both protein-filled and protein-free micelles. All the results compared with previous measurements carried out by quasi-elastic light scattering were found to be in excellent agreement, provided the concentration effect of the surfactant was taken into account at high wo values. Moreover, under the latter experimental conditions, intermicellar interactions are significantly perturbed by the presence of the protein in the aqueous core of the micelles. The possible biological relevance of such attractive interactions involving myelin basic protein and the lipid monolayers of the myelin sheath is suggested.