Aggregation of Fe(III)TPPS4 on biological structures is pH-dependent, suggesting oxo-bridging in the aggregates

Interaction of the Fe(III) derivative of tetra(4-sulfonatophenyl)porphyrin (TPPS4), and diamagnetic ZnTPPS4 and metal-free TPPS4, with the simplest mo dels for membranes and protein reaction centers, aqueous (AM) and reversed (RM) ionic micelles, was studied by high-resolution H-1 NMR and proton magn etic relaxation measurements. AM were much more sensitive than RM to the bu lky porphyrins, seemingly, because of the more restricted motion of surfact ant chains in AM. TPPS4 and its derivatives were incorporated into the AM o f cationic cetyltrimethylammonium chloride (CTAC) or zwitterionic lysophosp hatidylcholine (LPC) near the terminal part of their hydrocarbon chains, as evidenced by a strong upfield shift of the corresponding peaks. At a FeTPP S4/CTAC molar ratio greater than 0.05 and a pH > 4, FeTPPS4 partly formed n onparamagnetic aggregates, which dissociated into monomers at pH < 4. In CT AC RM, FeTPPS4 was predominantly aggregated, the transition to the monomer form occurring upon acidification of the water RM interior to about pH -1. No similar pH dependencies were found for ZnTPPS4 and TPPS4. It is supposed that charged porphyrins may interact with cellular membranes. Characterist ic pH dependence of the FeTPPS4 aggregation in micelles suggests that aggre gated units are bound through a mu-oxo-bridge. Similar mechanisms may be op erative in other systems, such as porphyrin-protein.