The solubilization site of 5,10,15,20-tetrakis-(2,6-dichlorophenyl)-porphyrin-Mn(III) in DPPC vesicles: A spectrophotometric and tensiometric study

The solubilization site of 5,10,15,20-tetrakis-(2,6-dichlorophenyl)-porphyrin-Mn(III) (MnTDPPC) in l-α-dipalmitoylphosphatidyl choline (DPPC) vesicles was determined in a systematic UV–vis spectrophotometric study. Measurements of the MnTDPPC absorbance have been conducted at constant temperatures (in the interval 30 °C ≤ T ≤ 48 °C) and varying concentrations of phospholipid. In the entire temperature range, the absorption due to MnTDCCP was found to increase with increasing DPPC concentration, until it reaches a plateau value. The data have been analyzed using a pseudo-phase model yielding the binding constant of MnTDCPP to vesicular aggregates. No significant temperature dependence of the partition coefficient of MnTDCPP between the aqueous phase and the lipophilic domain of DPPC vesicles could be detected in the studied temperature interval (which also includes the main transition temperature of DPPC double layers, Tm = 41.3 °C), unambiguously indicating that MnTDCPP is confined in the palisade layer of DPPC bilayers. The nature of the interactions between the different reaction components have been investigated by tensiometric measurements, which provide further support for hydrophobic interaction between MnTDCPP and the DPPC vesicles. This solubilization locus makes this porphyrin – which is used as an important compound in biomimetic models for the cytochrome P450 model systems – accessible for electron donation by suitable, water soluble electron donors.