A Spin Label Study of the Influence of Cholesterol on Phospholipid Multibilayer Structures

Abstract

Cholesterol increases the degree of order indicated by the spin-labelled derivative of cholestane (3-spiro-(2′-(N-oxyl-4′,4′-dimethyloxazoIidine))-cholestane) in a hydrated multibilayer structure of egg lecithin. Estimates of the angle between the long axis of the label and the lamellar plane were obtained using a simple model for the effect of this angle on the angular dependence of the hyperfine splittings of the spin label. The deviation from perpendicularity was ~28 + 5° in the absence of cholesterol. This deviation decreased to a limiting value of ~10 ± 3° at 25 mole% cholesterol. The limiting deviation was smaller when dipalmitoyl lecithin was substituted for egg lecithin, indicating the importance of the nature of the acyl residues in this effect.The rotational frequency of the spin label about its long axis depended upon whether or not the multibilayer structure had been in contact with an aqueous phase. Before wetting, this frequency was « 75 × 106 s−1; after wetting it was » 75 × 106 s−1.