PARTIAL STACKING OF A WATER-SOLUBLE PORPHYRIN IN COMPLEX MONOLAYERS WITH INSOLUBLE LIPID

Mixed monolayers of a Prater-soluble tetracationic porphyrin (TMPyP) a nd an insoluble lipid matrix with anionic head groups (DMPA) Piers for med by the cospreading method. A ratio TMPyP:DMPA = 1:4 was selected b y investigation of pi-area isotherms and stability of the cospread mon olayer. Reflection spectroscopy has been used to infer the molecular o rganization of the porphyrin molecules in the complex monolayer at the air-water interface. A blue shift in the Soret band was observed with increasing surface pressure. The mixed monolayer of TMPyP:DMPA = 1:4 was transferred onto a glass substrate, and the molecular organization of the porphyrin was studied by transmission spectroscopy with plane polarized light (s and p) under various angles of incidence. The analy sis of the reflection spectra at the air-water interface and absorptio n spectrum of the mixed monolayer on glass leads to a model of partial stacking of the porphyrin molecules in the monolayer at the air-water interface when the surface pressure is increased. On the glass substr ate, the TMPyP molecules attached to DMPA are monomeric. Further, indi rect evidence far the partial stacking model in the case of the mixtur e TMPyP:DMPA = 1:4 was obtained from the investigation of the mixed mo nolayer TMPyP:DMPA:PME = 1:4:16 (PME, methyl palmitate at the air-wate r interface and transferred on glass. The presence of the neutral lipi d molecules facilitates dense packing of the insoluble monolayer matri x and the matching of surface charge densities of the matrix and the p orphyrin moiety. In this way, the intermolecular interactions between porphyrin molecules at the air-water interface under high surface pres sure are reduced and the stacking is avoided.