Aggregation of Fe(III)TPPS(4) on Biological Structures Is pH-Dependent, Suggesting Oxo-Bridging in the Aggregates.

Interaction of the Fe(III) derivative of tetra(4-sulfonatophenyl)porphyrin (TPPS(4)), and diamagnetic ZnTPPS(4) and metal-free TPPS(4), with the simplest models for membranes and protein reaction centers, aqueous (AM) and reversed (RM) ionic micelles, was studied by high-resolution (1)H NMR and proton magnetic relaxation measurements. AM were much more sensitive than RM to the bulky porphyrins, seemingly, because of the more restricted motion of surfactant chains in AM. TPPS(4) and its derivatives were incorporated into the AM of cationic cetyltrimethylammonium chloride (CTAC) or zwitterionic lysophosphatidylcholine (LPC) near the terminal part of their hydrocarbon chains, as evidenced by a strong upfield shift of the corresponding peaks. At a FeTPPS(4)/CTAC molar ratio greater than 0.05 and a pH > 4, FeTPPS(4) partly formed nonparamagnetic aggregates, which dissociated into monomers at pH < 4. In CTAC RM, FeTPPS(4) 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 ZnTPPS(4) and TPPS(4). It is supposed that charged porphyrins may interact with cellular membranes. Characteristic pH dependence of the FeTPPS(4) aggregation in micelles suggests that aggregated units are bound through a &mgr;-oxo-bridge. Similar mechanisms may be operative in other systems, such as porphyrin-protein.