Ve. Yushmanov, Aggregation of Fe(III)TPPS4 on biological structures is pH-dependent, suggesting oxo-bridging in the aggregates, INORG CHEM, 38(8), 1999, pp. 1713-1718
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.