Structural order of lipids and proteins in membranes: evaluation of fluorescence anisotropy data

The limiting long-time value of fluorescence anisotropy in membranes is correlated with the orientational order parameter, which characterizes the structural anisotropy of membranes. Existing experimental results for diphenylhexatriene in lipid bilayers are evaluated for the order parameter of lipid order. Steady-state measurements of fluorescence anisotropy can provide the order parameter in good approximation. Proteins in a fluid lipid phase increase the lipid order parameter so determined. Upon comparison with the order parameter from deuterium magnetic resonance, it is concluded that proteins increase the order of the surrounding lipids in off-normal directions. Order parameters of protein order obtained from the limiting value of protein fluorescence anisotropy are discussed with respect to the influence of lipid order on protein order.