Fluorescent probes in model membranes. II. Monolayer studies of 2,2'-(vinylenedi-p-phenylene)bisbenzoxazole, d-3-aminodesoxyequilenin and N-octadecylnaphthyl-2-amino-6-sulfonic acid with host-lipid tetradecanoic acid.
Film studies at the air-water interface have been carried out for pure films of 2,2'-(vinylenedi-p-phenylene)bisbenzoxazole (VPBO), d-3-aminodesoxyequilenin (EQ) and N-octadecylnapthyl-2-amino-6-sulfonic acid (ONS), and for mixed films with tetradecanoic acid for the first two fluorescent probes. Pure film isotherms indicate highly rigid non-monomolecular films for both VPBO and EQ, revealing the presence of strong intermolecular forces. In mixed films with tetradecanoic acid VPBO rapidly segregates with resultant film loss over a wide concentration range. EQ, however, can be stabilized by the host-lipid at low concentrations. This, coupled with an ability to only slightly affect the host-lipid liquid-condensed/liquid-expanded phase change, suggests that EQ can be regarded as "non-perturbing" and should be retained in condensed lipid phases. ONS, because of its unusual polar headgroup, resembled hexadecanoic acid more than octadecanoic acid. While difficulties in spreading ONS precluded the study of mixed films, the indications are that it would be a satisfactory expanded lipid state probe if mixing can be brought about.