Delocalization plastocyanin

Figure 8. Proposed electron transfer pathway in blue copper proteins. The plastocyanin wave function contours have been superimposed on the blue copper (type 1) site in ascorbate oxidase (40). The contour shows the substantial electron delocalization onto the cysteine Spir orbital that activates electron transfer to the trinuclear copper cluster at 12.5 A from the blue copper site. This low-energy, intense Cys Sp - Cu charge-transfer transition provides an effective hole superexchange mechanism for rapid long-range electron transfer between these sites (2, 3, 28).

From the assignment of the hyperhne shifted signals of Cu(II) plastocyanin, azurin, and stellacyanin (Bertini et al., 1999, 2000), information on the electron delocalization onto the metal ligands was gained hy calculating the contact and pseudocontact contrihutions to the hyperhne shifts. In any case, since the magnetic anisotropy of the Cu(ll) ion is low, the observed shifts can be approximated to the contact contribution, which can be used as an initial criterion to compare the electron spin density on the different nuclei and in the various proteins. 

As the axial ligand is weakly bound in BCP (Randall et al., 2000), the spin density delocalized on it is small. Indeed, in azurin the resonances of the axial methionine protons do not experience a significant hyperfine shift contribution. Electron delocalization onto a Hy of the axial Met has been detected in plastocyanin (signal F in Fig. 3B), suggesting some covalency for the Cu-S(Met) bond. The absence of spin density on the axial Gin ligand in stellacyanin has been attributed to the fact that the y-CH2 Gin geminal couple is four bonds away from the metal ion, whereas the equivalent protons in a bound Met residue (such as in plastocyanin) are only three bonds away (Bertini et al., 2000). 

Electron Delocalization in Cu-Proteins. Fernandez and Vila have prepared a review. Hansen and Led have used proton Ri s to map electron spin delocalization in a type I Cu protein (oxidized plastocyanin from Anabaena variabilis). Their method relies on the dependence of Ri on the spatial distribution of the unpaired electron spin. 

---