ELECTRON-SPIN ECHO MODULATION STUDIES OF DOXYLSTEARIC ACID SPIN PROBES IN FROZEN DIHEXADECYL PHOSPHATE AND DIOCTADECYLDIMETHYLAMMONIUM CHLORIDE VESICLES - INTERACTION OF THE SPIN PROBE WITH DEUTERATED WATER AND EFFECTS OF CHOLESTEROL ADDITION

Electron spin echo studies have been carried out for a series of x-doxylstearic acid (x = 5, 7, 10, 12, and 16) and 3-doxyl-5-alpha-cholestane spin probes in frozen deuterated aqueous solutions of anionic dihexadecyl phosphate (DHP) vesicles and cationic dioctadecyldimethylammonium chloride (DODAC) vesicles. Modulation effects due to interactions of the spin probe nitroxide group with D2O at the vesicle interface give distance-based information which can be used to draw conclusions about the ordering and chain packing of the vesicles. It is found that the normalized modulation depths are greater for DHP than DODAC vesicles, indicating that the DHP vesicle structure is less tightly packed with less chain bending for the doxyl moiety at x = 16. Upon addition of cholesterol to DHP vesicles, the degree of hydration of the spin probes near the interface is increased while the degree of hydration of the spin probes located deeper inside the bilayer is decreased. Upon addition of cholesterol to DODAC vesicles, the degree of hydration is decreased for all spin probes up to 50 mol % cholesterol concentration above which the vesicle begins to lose its structure and D2O can penetrate within the bilayer.