The thermodynamic constants of oxygen binding to cobalt "picket fence" porphyrin complexes, mejo-tetra(a,a,a,ao-pivalamidophenyl)porphyrinatocobalt(II)-1 -methylimidazole and 1,2-dimethylimidazole, are reported. In contrast to previously studied cobalt porphyrins, these complexes bind oxygen with the same affinity as cobalt substituted myoglobin and hemoglobin. Solvation effects are discussed as the source of this difference. The use of sterically hindered axial bases as models of T state hemoglobin is discussed.

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02 binding to a series of ferrous and cobaltous "picket fence" porphyrins-is reported. N-Methylimidazole and covalently attached imidazoles give 02 binding to ferrous porphyrins with AH' =--16.2 kcal/mol (-67.7 kJ/mol) and ASO = -40 eu (standard state, 1 atmosphere OaJ. Similar studies with cobaltous porphyrins yield AH' = -12.8 kcal/mol (-53.5 kJ/mol) and AS' = -39 eu. These values match well those of myoglobin and isolated subunits of hemoglobin and their cobalt reconstituted analogues. 1,2-Dimethylimidazole has been successfully used to mimic the presumed restraint of T state hemoglobin. In direct analogy to the decreased cooperativity shown by cobalt-substitute hemoglobin, model cobalt porphyrins show a smaller decrease in 02 affinity than the analogous iron porphyrins when the axial base is hindered. Thermodynamic data are presented. The molecular mechanism of cooperativity in hemoglobin is discussed. The mechanism of cooperativity in hemoglobin (Hb) is of continuing interest (1-3). Model porphyrins capable of reversible oxygen binding have contributed to a fuller understanding of myoglobin (Mb) and other monomeric hemoproteins (4-6). Very little work, however, has appeared on models for the low affinity, T state, of Hb. The effect of steric restraint built into the porphyrin (7,8) or the axial base (9-13) has not been fully explored. We wish to report a full study of 02 binding to both iron and cobalt "picket fence" porphyrins with hindered and unhindered imidazoles. In addition, we make here a preliminary report on the synthesis and characterization of "picket fence" porphyrins with covalently attached axial bases. MATERIALS AND METHODSmeso-Tetra(a,a,a,a-o-pivalamidophenyl)porphyrin (H2T-PivPP) was prepared as described (4). Cobalt was inserted by use of anhydrous CoC12 in a solution of tetrahydrofuran with a trace of 2,6-lutidine at 500 under N2. Further purification consisted of chromatographic separation on Woelm neutral alumina. Details are presented elsewhere (14). meso-were prepared by the reaction of 4-(N-imidazolyl)butyl chloride and 5-(N-imidazolyl)valeryl chloride, respectively, with meso-tri(aaao-pivalamidophenyl)-(3-o-aminophenylporphyrin. Extreme care must be taken to prevent exposure of the porphyrins to 02 and light due to singlet molecular oxygen production catalyzed by the metal-free porphyrin (15) and trapping by the attached imidazole (16). Chemical structures of these porphyrins are shown in Fig. 1. The meso-tri(a,a,a-o-pivalamidophenyl)-fl-o-aminophenylporphyrin was prepared by the reaction of the readily available meso-tetra(aa,a,a-o-aminophenyl)-porphyrin (4) with limited amounts of pivaloyl chloride [(CHF)3COC1] and isolated by column chromatography. Iron was easily inserted under inert atmosphere by use of anhydrous FeBr2 in 1:1 benzene/tetrahydrofuran with a trace of 2,6-lutidine, followed by chromatographic filtration through a short plug of alumina. All intermediates and porphyrins described above have been well characterized by elemental analysis, UV/visible, nuclear ...

The carbon monoxide affinities of iron complexes of meso-tetra (aa,a,a-o-pivalamidophenyl)porphyrin (the "picket fence" porphyrin) and of a "picket fence" porphyrin derivative with an appended axial base have been measured in solution and compared with the CO affinities of various hemoproteins. The model complexes bind CO with much greater affinity than normal hemoproteins; the role of the steric bulk of distal residues in lowering the CO affinities of the hemoproteins is discussed. The significance of this lowered CO affinity is described with regard to endogenous CO. A discussion of mutant hemoglobins lacking distal residues that sterically inhibit the binding of CO is presented. The use of pressure units versus concentration units in equilibrium expressions is analyzed.The oxygen affinities of myoglobin (Mb) and both the lowaffinity ("T") and high-affinity ("R") states of hemoglobin (Hb) have been reproduced by model iron(II) porphyrin complexes (1, 2). Simple iron(II) porphyrins irreversibly oxidize when exposed to oxygen at room temperature in solution (3-6); this led to the development of "picket fence" (7) and "tailed picket fence" (unpublished results) porphyrins whose steric bulk inhibits the oxidation process. The equilibria of interest in the study of such systems are: FePB +2 02 FePB(02), where P represents the porphyrinato ligand and B represents an axial base. For sterically hindered bases, such as 1,2-dimethylimidazole (Me2Im), K2 is much less than K1 (8-10) and direct solution measurements of K02 are possible. For unhindered bases, such as 1-methylimidazole (N-MeIm), however, K2 is greater than K1 (10), precluding direct measurement of Ko2 by using a simple porphyrin and external axial base. A system enforcing five-coordination about iron (i.e., effectively reducing K2 to zero) has been developed to permit such measurements (unpublished results).Carbon monoxide is a biological ligand for Hb and Mb (11-13), and thus the binding of CO to model systems is also of interest. We wish to report a study of the binding of CO to iron(II) "picket fence" porphyrins with hindered and unhindered imidazoles as axial bases. Although these "picket fence" porphyrins displayed the same 02 affinities as Hb and Mb (1), their CO affinities are significantly greater than those of the hemoproteins. We suggest that steric interactions with CO in the hemoproteins serve to lower the CO affinities, a claim supported both by conclusions reached from the examinationThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 6035 of mutant hemoglobins lacking such interactions and by the results from the "picket fence" systems reported here.MATERIALS AND METHODS Meso-tetra(a,a,a,a-o-pivalamidophenyl)porphyrinato iron(II) (FeTPivPP) and meso-tri(a,a,a-o-pivalamidophenyl)- 13-o-5-(1-imidazolyl)valeramidophenylporphyrinato iron(II)[FePiv3(5CImP)Por], Fig. 1, were prepare...

The synthesis of "picket fence" porphyrin derivatives bearing covalently attached imidazole and pyridine nuclei is described.The oxygen and carbon monoxide affinities of the iron(II) complexes of these porphyrins are reported. Electronic effects due to replacement of imidazole by pyridine have been isolated from steric constraints. These electronic effects, deriving from a decreased basicity of the axial base, give rise to 40-fold and 13-fold reduction, respectively, in oxygen and carbon monoxide affinities upon replacing imidazole by pyridine. Kinetic measurements of oxygen and carbon monoxide association rates suggest that these changes are manifested primarily in ligand dissociation rates.