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The nature of the aqueous solution axial ligands of the water-soluble iron(ll1) porphyrinates [Fe(TMPyP)15+ and [Fe(TPPS)I3has been examined as a function of pH by 'H N M R spectroscopy. The N M R shifts provide indirect evidence for the existence of [FC(TPPS)(H,O)~]~. [Fe(TPPS)(H20)(OH)le, and [(Fe(TPPS)),0l8-in the [Fe(TPPS)]* system and [Fe(TMPyP)(H20)2]5+, [ FC(TMP~P)(H,O)(OH)]~', [(Fe(TMPyP))20]8+, and [Fe(TMPyP)(0H),l3+ in the [Fe(TMPyP)I5+ system. Estimates of the dibtribution of the species from the NMR spectra allows approximation of the various equilibrium constants in the [Fe(TPPS)]'systcm. The pH-dependent equilibria for [Fe(TPPS)])-and [Fe(TMPyP)15+ correspond qualitatively with previously published rcsults. Attempts to prepare crystalline species at various pH's were successful only for the high pH form of [Fe(TMPyP)15+. A single-crystal structure determination shows that this species has the idealized composition [(Fe(TMPyP))20](C10,)8.4H20. The five-coordinate iron atom has an axial Fe-0 bond distance of 1.750 (2) A, while the average Fe-N, bond distance is 2.080 (8) A; the iron(ll1) atom is 0.47 A out of the porphyrin plane. Crystal data: a = 32.06 (2) A, 6 = 18.492 (9) A, c = 17.17 (2) A, / 3 = 108.6 (I)", monoclinic, space group C2/c, V = 9650 (2) AS, Z = 4, Fe2Cls03,N,6CssH80, 5188 observed data, R,(F,) = 0.125. R2(F,,) = 0.140, all observations at 1 18 K. Water molecules and hydroxide ions are potentially important ligands for iron in hemoproteins, and protein crystal structures have shown such coordinated ligands in the ligand binding pockets of a number of heme proteins.'-5 It would thus seem desirable to define thc aqueous interaction of the iron porphyrinates over a significant pH range. The aqueous solution chemistry of nat-( I ) Cytochrome P-450: Poulos, T. L.; Finzel, 8. C.; Howard, A. J. Biochemisrry 1986. 25, 5314. (2) Cytochrome c peroxidase: Finzel, B. C.; Poulos, T. L.; Kraut, J. J. Biol. Chem. 1984. 259, 13027. Poulos, T. L.; Freer, S. T.; Alden, R. A.; Edwards, S. L.; Skogland, U.; Takio, K.; Eriksson, B.; Xuong, N.; Yonetani, T.; Kraut, J.