Recombination of carbon monoxide and microperoxidase

The studies of Hasinoff [53] on the recombination rate of C irbon monoxide and the heme units after photodissociation of carboxy ferrous microperioxidase come close to satisfying the requirements for observing the effects of anisotropic reactivity and rotational diffusion on the rate of a translational diffusion-limited reaction. In Chap. 2, Sect. 5.6, the details of this study were briefly mentioned. Hasinoff found that the rate of recombination was substantially diffusion-limited in all three aqueous solvents used at 260 K, but at higher temperatures, the rate of reaction of the encounter pair, feact was a significant factor in determining the overall rate of recombination (see Fig. 9). The observed rate coefficient of recombination, feobs was separated into the rate coefficient of diffusive formation of encounter pairs, and the rate coefficient of reaction of encounter pairs, feact with the Collins and Kimball expression, eqn. (26) 

Hasinoff noted that the rate coefficient of formation of encounters pairs, feo, was smaller than predicted from the Smoluchowski—Stokes—Einstein rate coefficient [eqn. (29)]. In aqueous glycerol, this reduction was by 

14 times, in aqueous polyethylene glycol by 0.30 times, and in aqueous ethylene glycol by 0.11 times. Hasinoff compared these reductions in rate of diffusive rate of formation of encounter pairs with three theories of anisotropic reactivity due to Weller [262], Schmitz and Schurr [257] and 

Hill [263]. All of these could reasonably explain the measured reduction of the encounter pair formation rate coefficient. Being based on a more rigorous foundation, the Schmitz and Schurr model is to be preferred. From this analysis, Hasinoff [53] suggests that the heme unit in microperoxidase has a reactive site which subtends a semi-angle of 31° at the heme centre. 

It would be interesting to study the effect of changing the size of peptides associated with the heme unit, but only in the plane of the heme unit. The rotational relaxation time would be markedly affected but not the anisotropic reactivity of the heme unit. 

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