Complement-induced decrease in membrane mobility: introducing a more sensitive index of spin-label motion

Abstract

Spin-labeling was used to investigate complement-induced changes in lipid organization of antibody-sensitized sheep erythrocyte membranes. The spectrum of methyl 5-doxylstearate incorporated into the lipid component of sheep erythrocyte membranes is typical of a membrane bilayer. The membranes from complement-lysed erythrocytes have a small, but statistically significant, reduction in fluidity when compared to membranes from osmotically-lysed erythrocytes, as indicated by a small increase in T.dblvert.''. In theory, measurements of the widths of the outer hyperfine extrema should be more sensitive to motion than the separation of the outer hyperfine extrema (2T.dblvert.''). The half-width at half-height of the outer hyperfine extrema apparently show a several-fold greater percentage change than T.dblvert.''. The sign and magnitude of these changes are in general agreement with previous predictions. Our results imply that motional corrections to the S formalism of Hubbell, Gaffney and McConnell are necessary because spin-label motion appears to be explicitly represented in this type of ESR spectra.