Reaction of carbon monoxide with microperoxidase

11—30% of the estimated theoretical rate coefficient based upon the Smoluchowski—Stokes—Einstein eqn. (29). This reduction may well be due to the highly anisotropic reactivity of the heme unit in the peroxidase (see Chap. 5 for a discussion of anisotropic reactivity and rotational diffusion, and Chap. 5 Sect. 4.4 for comments on this system). 

In the preceding sections of this chapter, a theory of diffusion-limited chemical reactions has been described for two cases, (a) where the reaction of the encounter pair is much faster than its formation and (b) where these rates are of comparable magnitude. Some experimental evidence for both these cases has been described. During this discussion, a number of other difficulties in the interpretation of diffusion-limited reactions were indicated. This section details the complications and when they may be expected. The following chapters serve to amplify these comments. Chapter 8 provides a resume and conclusion as well as recommendations for future areas of both experimental and theoretical study. 

Logan [54] has discussed the applicability of the simple partially reflecting boundary condition (5) as used by Collins and Kirhball [4] and Noyes [5]. He suggested that, in the limit of a very fast activation reaction Noyes was implying in his analysis that the rate of reaction 

This discussion highlights the difficulty of deciding at what separation A and B form an encounter pair and then whether this reacts or separates. Noyes [5] and Wilemski and Fixman [51] have taken the encounter distance to be that separation which, if reduced slightly, will lead to reaction. Where these authors disagree is that Noyes [5] only allows reaction to occur in a very narrow range of separation distances about Jt (which is the usual assumption) and Wilemski and Fixman [51] assume that any separation distance less than the encounter distance, i , can lead to reaction between A and B and that A and B can diffuse through each other till their centres of mass coincide (Chap. 9, Sect. 4). Neither assumption is good, but the differences in predicted rate coefficients are so small that an experimental test of these theories could not be definitive. 

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The remainder of this section considers several experimental studies of reactions to which the Smoluchowski theory of diffusion-controlled chemical reaction rates may be applied. These are fluorescence quenching of aromatic molecules by the heavy atom effect or electron transfer, reactions of the solvated electron with oxidants (where no longe-range transfer is implicated), the recombination of photolytically generated radicals and the reaction of carbon monoxide with microperoxidase. 

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