Oxygen Binding to Cobalt Porphyrins

The thermodynamic constants of oxygen binding to cobalt “picket fence” porphyrin complexes, meso-tetra(a,o,a,oo-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. In studies of myoglobin (Mb) and hemoglobin (Hb), the replacement of the neutral iron porphyrin prosthetic group with different metalloporphyrins has proved to be a useful technique.2 Artificial hemoglobins containing ~ i n c , ~ . ~ mangan e ~ e , ~ * ~ o p p e r , ~ and nickel9 have been reconstituted, and their properties compared with those of the native iron proteins. These artificial systems, however, are incapable of reversible oxygenation. In contrast, cobalt substituted hemoglobin and myoglobin (CoHb and CoMb)l0 are functional,I1-l3 although their oxygen affinities are 10-100 times less than those of native H b and Mb. CoHb exhibits cooperativity in oxygen binding, though to a lesser degree than Hb. The extent of this cooperativity is conveniently expressed as AG;, , the free energy difference between the intrinsic binding of the first and the fourth 0 2 to Hb.13b,c For CoHb, AG:l is roughly one-third that of H b under comparable condition^.'^^ Because of the different stereochemical and electronic factors involved in binding oxygen to a cobalt porphyrin, the observation of cooperativity in CoHb has been variously used either to q u e ~ t i o n ~ 9 ~ ~ J ~ or s ~ p p o r t ’ ~ ~ , ’ ~ the elegant proposal of Perutz concerning the molecular mechanism of cooperativity in natural Hb.16 At the heart of this proposal is the assumption, based on earlier ideas of Hoard” and Williams,18 that the high-spin iron in the unligated, low 0 2 affinity form of H b (T state) lies out of the porphyrin plane, and that on binding 0 2 , the iron becomes low spin and moves into the plane. The resulting motion of the proximal imidazole (0.6 A) then causes conformational changes in the protein which produce a higher 0 2 affinity quaternary form of the protein (R state). In the deoxy form of coboglobin, cobalt is low spin rather than high spin and the best estimates from simple cobalt model systems indicate that the proximal imidazole in CoHb moves -0.4 A upon oxygenation’4,’9,20 as opposed to the 0.6 8, for Hb. The resulting motion of the proximal histidine upon binding 0 2 will therefore only be two-thirds as great. This seems to be qualitatively consistent with the lowered AGil of CoHb. This same reasoning argues that a real tension of the Co0002-7863/78/1500-2761$01.00/0 “is bond may exist in the deoxy (T) form of CoHb. The best current explanation of cooperativity is a “restraint” theory:’5a by holding the proximal imidazole in place, the T form of the native protein restrains the five-coordinate, deoxy metalloporphyrin from becoming six coordinate, without necessarily inducing any strain in the deoxy form of native H b itself. In deoxy CoHb, however, the Co atom is already closer to the mean porphyrin plane than is Fe and is likely to have created a real strain in the C O N H ~ ~ bond, which would be demonstrated as a lengthening of this bond as in structures of CoTPP( 1 ,2-diMeIm)l0 compared with CoTPP(N-MeIn an effort to further clarify the nature of 0 2 binding to CoHb, and, by implication, to native Hb, many workers in the last several years have investigated the reaction of simple cobalt(I1) porphyrins with oxygen.2,11,21-29 This work has been frustrated in most cases to date by the surprisingly low affinity of such simple cobalt(I1) porphyrins for oxygen in the absence of the lobin protein environment. For example, whereas CoHb toluene has PPI2 (25 “C) of 15 500 Torr.30 In a previous paper,28 the synthesis of the cobalt derivative of the “picket fence” porphyrin, CoTpivPP (1 in Figure 1) was reported, along with its N-methylimidazole (N-MeIm) adduct, 2. It was pointed out that the five-coordinate cobalt porphyrin 2 has a fairly high 0 2 affinity at room temperature in solution, but no thermodynamic data were obtained. We have since found that the method previously used for introduction of cobalt into the “picket fence” prophyrin resulted in rotation of the pivalamidophenyl groups (“pickets”) to give a statistical mixture of atropisomers, and have therefore developed a milder technique that does not cause this rotational isomerization. In this paper, we report the improved synthesis, along with the thermodynamic constants for oxygen binding to CoTpivPP(N-MeIm), 2, in the solid state and in solution. In the solid state and in toluene, this simple porphyrin is found to bind oxygen as well as CoMb. In addition, a decrease in oxygen affinity has been observed when N-MeIm is replaced by a 1m). l0~9 has Pl f2 8 (22 “C) of 50 Torr, CoT-p-OCH3PP(N-MeIm)Io in