Long Range Electron Transfer In Metalloproteins:

Electrontransfer reactions are of major importance for both photosynthesis and respiration. These processes occur along electrontransport chains located on cellular organelles. In many ways, biological electrontransfer reactions are unique. They not only span relatively large molecular distances, but also occur at a rapid rate. In recent years, researchers have been trying to elucidate the precise nature of these "long range electron transfer" reactions. Some of these investigations have sought to define the parameters affecting electrontransfer rates. Among others, the more important influences may include protein structure, donoracceptor distance, reaction driving force, and reorganization energy. These factors' relevance may have evolved over time for the purpose of providing electrontransfer reactions with control, efficiency, and specificity.

Longrange electron transport (LRET) is a central and essential component of biological energy conversion systems. The process occurs in both photosynthetic and mitochondrial electrontransport chains. For the most part, these chains consist of electrontransport complexes bound to biological membranes. These complexes may be composed of flavins, ironsulfur clusters, hemebound iron, and manganese and copper ions. Often, they will span the membranes; their electrontransport being coupled to concomitant movement of protons. The resultin

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eptor distance on LRET reaction rates. Timeaveraged Co(diAMsar)heme c edgetoedge distances were determined to range from approximately 14 A to 20 A. Assuming a simple exponential distance dependence for the electron transfers, this range should have given a 200fold range of rates. Moreover, the calculated timeaveraged dominant pathways for electron transfer in each derivative predicted a 2000fold range of rates. Regardless though, rate constants for the intramolecular electron transfer (ket) from Co(II)(diAMsar) to the Fe(III) of heme c in each derivative ranged from 1.0 to 3.2 s1 (at 25 degrees Celsius and pH 7.0) (1:9909). The data gathered by Conrad and his colleague thus indicates that the rate of LRET reactions is "nearly independent" of the Co(diAMsar) attachment site (1:9915). Other important influences on LRET reaction rates include the driving force of the reaction (i.e., the free energy change) and the reorganization energy (i.e., lambda). In fact, these two factors can be related according to the Marcus relation as follows: G = (G' + lambda)2/4(lambda), where G is the activation free energy and G' is its reaction free energy. Farver et al. (1992) specifically investigated the effects of driving force
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