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A synthetic model system for the special pair in the photosynthetic reaction center of bacteria has been prepared from electrostatically paired porphyrin and phthalocyanine molecules. Use of rigorously dry propylene carbonate as the solvent allowed complex fonnation to occur between tetra-(N-methyl-4-pyridyl)porphyrin hexafluorophosphate (H2TMPyP(PF6)4) and tetraphenylphosphonium tetrasulfatophthalocyanine (Ph4P)4H2TSPc molecules. The stoichiometry of the complex was determined to be 1:1 by Job's method and the pair formation constant K1 was estimated to be 1x108.T he dependence of K1 on ionic strength was studied at various concentrations of tetra-n-butylammonium tetrafluoroborate BU4NBF4. With increasing ionic strength K1 decreases as expected according to the Debye-Huckellimiting law. Using an ionic strength of 2 mM (BU4NBF4) in PC, the extent of pairing was found to be almost 100%. Under these conditions tetrasulfatophthalocyanine and the electrostatically paired complex were oxidized at E1/2's of 0.453 and 0.350 V vs SCE (aq), respectively. From evaluation by the Nemst equation of the measured potential difference between paired and unpaired tetrasulfatophthalocyanine, the ratio of [C]K2 / [C+]K1 is estimated to be 55. K2 is the equilibrium constant for the formation of the singly-oxidized pair. The significance of the value of the ratio leads to an estimate of K2 = 6 x 109 at an ionic strength of 2.0 mM.



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