Intramolecular electron transfer azurin systems

Azurin as a Model System for Intramolecular Electron Transfer... 

Selectively modified and characterized ruthenated proteins have been prepared for cytochrome c (141-144), cytochrome c-551 (145), azurin (146, 147), plastocyanin (147), HiPlP (/4S), and myoglobin (77, 149-154). Intramolecular electron transfer rates, k, measured in these systems are listed in Table 111 along... 

Although electron transfer rates within myoglobin appear to follow an exponential dependence on distance, the derived rate expression is not directly transferable to other electron transfer proteins. A particularly striking comparison is between the c cytochromes and the copper proteins plastocyanin and azurin. Intramolecular electron transfer rates are at least 10-100 times slower in the copper proteins compared to the c cytochromes, even though the distances and driving forces for the reactions are comparable. The origin of this behavior is unclear, but it does suggest caution in the quantitative transfer of rate expressions between different systems. 

Direct evidence for long range electron-transfer in biological systems was first observed by Gray et al.50,51) and Isied et al.481 using [Ru(NH3)5]3+ substituted metallo protein. Histidine-83 of blue copper (azurin) was labeled with Ru(III)(NH3)5 50). Flash photolysis reduction of the His-83 bound Ru(III) followed by electron-transfer from the Ru(II) to Cu2+ was observed with a rate constant of 1.9 s 1. The result shows that intramolecular long distance (approx. 1 nm) electron-transfer from the Ru(II) to the Cu2 + of the azurin takes place rapidly. 

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