Some peculiarities of the electronic transfer in redox enzymes

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Abstract
The new properties of clusters—the polynuclear Fe, Cu, Mo—containing metal‐protein complexes have been discussed. The properties arose as a result of strong electron interaction and of the multiorbitals system existence in which the orbitals energetically fall close together. The clusters are characterized by (i) a high electronic capacity, (ii) an ability to multielectronic transfer without essential rearrangement of nuclei configuration, (iii) a high degree of donor‐acceptor energetical levels fitting at tunnel transfer, (iv) high possibilities for avoiding of reaction pathways being quantum mechanically forbidded, and (v) an ability to provide smooth reaction energetic relief in coordinated sphere. The analysis of data on spin exchange between paramagnetic centers (binuclear transition‐metal complexes, nitroxile biradicals, triplet exited chromophores) showed that in the range of spinexchanged constants Kex = 1014−1 sec −1 of the distances between the centers r = 3−17 Å the approximate relation Kex =1017 exp (−2.3r)sec−1 takes place. This relation may be considered as a criterion of nonspecific electron density transfer through nonconducting medium. The quantitation of exchange triplet‐triplet energy migration permits one to estimate the degree of quantum‐mechanical electron density penetration through biological matrix. By means of measurement of spin‐lattice relaxation rate for oxidized primary donor in bacterial photosynthetic system—bacteriochlorophyll cation (BChl+ )—it is shown that the distance between BChl+ and primary acceptor (complex FeQ) is about 34 Å. The proposed two‐step photoelectron transfer model explains the effective charge separation by relatively slow tunnel recombination of the charges BChl+ FeQ−. As spin and Mössbauer labeling experiments showed the conformation mobility of surrounding protein and membrane matrix with frequency more than 107 sec−1 is required for photoelectron output from primary photosynthetic cell in chromotophores and reaction center to secondary acceptor.