A spin-label study of the viscosity profile of sarcoplasmic reticular vesicles.

Sarcoplasmic reticular vesicles were prepared from both lobster and rabbit muscle. A variety of water-soluble, lipid-soluble, and alkylating spin labels were used to treat the sarcoplasmic reticular vesicles. All spin label analyses were carried out with and without NiCl₂. Nickel is used to remove spin label signal originating from outside of, on the surface of, or localized in the outermost part of the outer bilayer half of the sarcoplasmic reticular membrane.We conclude that the hydrocarbon portion of sarcoplasmic reticular vesicles has symmetry in regard to the physical properties that limit spin label motion; however, we find that the membrane interface limits spin label motion more on the inner surface than the outer surface and that the trapped aqueous volume which is sampled by water soluble spin labels inside the vesicle enclosure limits spin label motion much more than the average aqueous medium outside.