Aggregate formation in mixed monolayers at the air–water interface of metal-complex tetracationic water-soluble porphyrins attached to a phospholipid matrix

The conformational behaviour of mixtures of the metallated Ni-TMPyP/DMPA and Co-TMPyP/DMPA, with molar ratio 1∶4, has been investigated at the air–water interface. In order to characterize the interactions of the metallated TMPyP derivatives with DMPA, different surface methods have been used. Thus, the surface pressure–surface area, π–A, and surface potential–surface area, ΔV–A, isotherms were measured and valuable information about the presence of the metallated porphyrin molecules underneath the lipid matrix of DMPA, as well as the stability and the hysteresis phenomena of these monolayers was obtained. To reveal the molecular organization of the metallated porphyrins in the mixed monolayers, the reflection spectra of the mixed monolayers at the air–water interface were measured. Additional information on the organization of Ni-TMPyP and Co-TMPyP was obtained by using Brewster angle microscopy (BAM). An analysis of the molecular organization of Ni-TMPyP and Co-TMPyP in a mixed monolayer containing DMPA as lipid anchor, by using the oscillator strength and in accord with the extended dipole approximation, leads us to propose different organization models of these metal-complex porphyrins at the air–water interface depending upon the coordination number of the metal ion.