N-14 ENDOR study of micellar solutions of sodium dodecylsulfate using small spin probes

Abstract

ENDOR (electron nuclear double resonance) spectroscopy has been applied to the study of the state of solubilization of small nitroxide spin probes in micelles produced in sodium dodecylsulfate solution. The intensity ratio of the pair of N-14 ENDOR lines observed by fixing the field on the low-field ESR (electron spin resonance) line was monitored as a measure of the motion of spin probes. Neutral spin probes, when solubilized in micelles, show a discontinuous change of ENDOR spectral parameters such as hyperfine splitting constant and peak intensity ratio. Above the critical micelle concentration, the spectrum features change only gradually as the number of solubilized probes is increased. Spin probes in micelles appear to experience environments that are less polar than those met in aqueous solutions. However, the solubilized state is stable and is insensitive to changes in the concentration of the amphiphile. An anionic spin probe, Fremy's salt, shows no indication of interaction with the micelles. The applicability of the use of ENDOR peak intensities for the study of spin probe motion is discussed.