CO bond angle changes in the photolysis of carboxymyoglobin.

The Fe-C-O bond angle in carboxymyoglobin is bent (127+-4C) having a structure identical, within the error, to the "pocket" porphyrin model compound FePocPiv(1-MeIm)(CO) (Collman, et al., 1983a). On photolysis this angle decreases by 5-10C. In addition, correlation is observed between the increase in the length of the Fe-C bond and the decrease of the Fe-C-O angle. These results suggest that the rate limiting step in recombination is the thermal motion of CO in the pocket to achieve an appropriate bonding angle with respect to the iron.