The coordination of sterically hindered nitrogenous bases to octaethylporphinatocobalt(II), (Co(OEP)), has been studied by epr techniques. It is shown that, despite reports in the literature to the contrary, 2,4,6-trimethylpyridine (s-collidine) and 2,6-dimethylpyridine (2,6-lutidine) do not coordinate to cobalt porphyrins. On the basis of progressive purification of the s-collidine it is shown that previous reports of its coordination result from the presence of impurities in the material. We have also found that 2-methylpyridine (2-picoline) at best coordinates weakly. This work, along with the results of independent structural studies, shows that the trigger mechanism for the allosteric linkage of hemoglobin cannot apply unmodified to cobalt-substituted hemoglobin.
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