Covalent dimers, electronic coupling

Figure 6. Electron-transfer pathway in the azurin dimer mutant (43). The path connects Sy of Cys42 with Nj of His46, which is one of the copper ligands, and consists of 17 covalent bonds resulting in a very effective electronic coupling of the two redox centers. Calculations were based on the Beratan and Onuchic model (6, 7). Coordinates were taken from the PDB, code IJVO.

In covalent dimers, the bridging element between the two chromophores may restrict the respective orientations of the two chromophores, and thus, influences the degree of their electronic coupling. In the absence of abridge, the degree of electronic coupling between two porphyrins linked directly at the meso position can still be controlled by the peripheral substitution. Indeed, the peripheral substitution can affect the nature of HOMO frontier orbital. 

Of course, the influence of the bridge on the degree of electronic coupling appears to be an excellent way to promote electron transfer from a donor to an acceptor. Fukuzumi, Kadish, and Crossley have reported the use of covalently linked chromophores in which electronic coupling is enhanced by a bridge that allows a coplanar orientation of the donor and the acceptor. In addition to the choice of a biphenyl type link, in which each phenyl is fused with the tetrapyrrolic rings, the subtlety of the hybrid dimer 23 depicted in Figure 13.24 is the use of a nonneutral dyad. 

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