Electron spin resonance studies of phosphatidylcholine interacted with cholesterol and with a hopanoid in liposomal membrane.

The effects of bacteriohopane-32-ol (Monol) on liposomal membrane composed of dipalmitoylphosphatidylcholine (DPPC) or egg yolk phosphatidylcholine (egg PC) were compared with those of cholesterol (Chol) in the change of fluidity using a spin label. The fluidity change close to the polar head groups caused by temperature increase in the DPPC membrane containing Monol was different from that of Chol. Chol had a condensing effect on DPPC membrane, whereas Monol had only a slight effect except when used at 20 mol%. Near the hydrophobic end, Chol incorporation into DPPC led to fluidization below transition temperature (Tm) and condensation above Tm. Monol incorporation into DPPC had only a fluidizing effect below Tm. On the other hand, in egg PC membrane Chol had the condensing effect at any temperature, whereas Monol had only slight effect. These results suggest that Monol may have a role in supporting constant membrane fluidity under drastic conditions.