PROPERTIES OF MIXED MONOLAYERS OF LECITHIN AND SPIN PROBE .1. STUDY OF THE INTERACTIONS AT THE AIR-WATER-INTERFACE

The perturbations induced in a lipid layer by the presence of a spin probe molecule (4-palmitamido-2,2,6,6-tetramethylpiperidine-l-oxyl, TAP) have been investigated in a model membrane. Mixed monolayers of probe and dipalmitoyl-dl-α-phosphatidylcholine (DPPC) were formed at the air-water interface and studied using classic surface techniques. In mixed films, surface pressure measurements showed that the spin probe induces a modification of the monolayer lateral compressibility giving rise to a more expanded structure. Two other methods, i.e., surface viscosity and enzymic hydrolysis, offered the possibility of detecting the propagation of the probe perturbation by observing modifications of the DPPC properties. The surface viscosity approach is based on a very specific property of the probe: The monolayer, even in a closepacked state, shows no detectable surface viscosity. In these conditions surface viscosity measurements carried out with mixed monolayers will depend only on the second component of the system, i.e., the lecithin. Any deviation from the predicted viscosity value indicates a change in the DPPC packing. Viscosity results indicate that the probe induces a more packed organization of the DPPC molecules. The changes in the enzymic hydrolysis kinetics of the mixed monolayers confirm these results. The phospholipases activity is strongly dependent on the lipid packing. Indeed, the probe molecule cannot be hydrolyzed and the inhibition observed must be interpreted in terms of a specific modification in the lipid organization induced by the probe. © 1979.