title: Lipid structural order parameters (reciprocal of fluidity) in biomembranes derived from steady-state fluorescence polarization measurements.
abstract: This paper presents an interpretation of fluorescence polarization measurements in lipid membranes which are labelled with the apolar probe 1,6-diphenyl-1,3,5-hexatriene. The steady-state fluorescence anisotropy, rs, is resolved into a fast decaying or kinetic component, rf, and an infinitely slow decaying or static component, r infinity. The latter contribution, which predominates in biological membranes, is exclusively determined by the degree of molecular packing (order) in the apolar regions of the membrane; r infinity is proportional to the square of the lipid order parameter. An empirical relation between rs and r infinity is presented, which is in agreement with a prediction based on a theory of rotational dynamics in liquid crystals. This relation enabled us to estimate a lipid structural order parameter directly from simple steady-state fluorescence polarization measurements in a variety of isolated biological membranes. It is shown that major factors determining the order parameter in biomembranes are the temperature, the cholesterol and sphingomyelin content and (in a few systems) the membrane intrinsic proteins.
mesh_terms: D000818:Animals; D002417:Cattle; D002462:Cell Membrane / Q000648:ultrastructure*; D002784:Cholesterol; D005260:Female; D005454:Fluorescence Polarization / Q000379:methods; D006801:Humans; D008051:Lipid Bilayers; D008055:Lipids / Q000032:analysis; D008433:Mathematics; D008560:Membrane Fluidity; D008563:Membrane Lipids / Q000032:analysis*; D051379:Mice; D008856:Microscopy, Fluorescence / Q000379:methods; D008895:Milk, Human; D011247:Pregnancy; D051381:Rats; D013109:Sphingomyelins
chemical_list: D008051:Lipid Bilayers; D008055:Lipids; D008563:Membrane Lipids; D013109:Sphingomyelins; D002784:Cholesterol