ORGANIZATION OF A WATER-SOLUBLE PORPHYRIN IN MIXED MONOLAYERS WITH PHOSPHOLIPIDS STUDIED BY BREWSTER-ANGLE MICROSCOPY

The organization of a cationic water-soluble porphyrin, TMPyP, in a co mplex monolayer containing an anionic phospholipid, DMPA, at the air-w ater interface has been directly inferred by Brewster angle microscopy , BAM. In a previous work (Martin, M. T.; Prieto, I.; Camacho,L.; Mobi us, D. Langmuir 1996, 12, 6554) the organization of this mixed monolay er TMPyP/DMPA, in the ratio 1:4, was explained on the basis of a model where the TMPyP molecules are in a monomer-dimer equilibrium dependin g on the surface pressure, that is, on the surface density of the porp hyrin, by analysis of pi-A isotherms and reflection spectra results. T he morphology of the mixed monolayer was recorded at several surface p ressures. The images are different from those obtained for the lipid m atrix. At 5 mN/m a homogeneous and continuous film was observed. Howev er, for surface pressures > 8 mN/m the coexistence of two phases is re corded, and small domains with higher brightness than that of the surr ounding area are formed. The density of the bright domains increased u ntil covering nearly the whole surface at 35 mN/m. The BAM results hav e been related with those obtained by reflection spectroscopy at the a ir-water interface. The appearance of the bright domains has been attr ibuted to the formation of the dimer phase II where the DNPA molecules are dense-packed with an areaA(DMPA,II) = 0.40 nm(2), and the homogen eous area has been attributed to phase I with porphyrin monomers. In p hase I, the DMPA molecules are in a liquid-expanded state with the are a A(DMPA,I) = 0.84 nm(2).