Calculation of molecular motional correlation times from linewidths in nuclear magnetic resonance spectra of aggregated systems. Effect of particle size on spectra of phospholipid dispersions

Abstract

Equations are developed which relate NMR linewidths for aggregates of macromolecules to the molecular motional parameters τ c (correlation time) and A (“anisotropy factor”). The effects of molecular motion can be isolated from those of Brownian tumbling of the aggregates by measuring linewidths at two solution viscosities or aggregate sizes. Where molecular motion is anisotropic ( A > 0), e.g., in membranes, aggregated proteins, and lipid dispersions, NMR linewidths are not simply related to τ c Systems with the same τ c but different values of A may exhibit different linewidths, as for the lecithin 31P resonance above and below the chain melting transition. The reduced linewidths observed from sonicated, compared with unsonicated, lecithin dispersions can be explained solely by the increased particle tumbling rates. Any differences in molecular motion within the bilayers are only minor. All the linewidth behavior reported for lecithin dispersions, including variations with vesicle radius and the only slight dependence on solution viscosity, are predicted by the theory developed here. It is shown that changes in A are greater than changes in τ c both along lecithin chains, and for lecithin phosphate groups at the chain melting transition.