Electron distribution copper centers

C (66). If electron transfer from type 1 to type 3 copper couples the two halves of the enzyme cycle, as proposed for laccase, then this intramolecular redox reaction must be extremely rapid to account for the effects of trace dioxygen on the reduction of the type 1 copper. Consequently, despite the fact that an ambiguous assignment of a type 1 to type 3 transfer is not possible in this example, facile intramolecular electron transfer processes probably ensure a rapid distribution of electrons among the type 1 and type 3 copper centers, at least in some of the enzyme molecules. The equilibrium distribution, and quite conceivably the relative rates of approach to this state, should be influenced by the oxidation-reduction potentials, which, as described earlier in this chapter (Figure 5), favor electron occupancy of the type 3 copper pairs at 10.0°C. 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

The overall picture describing the electron delocalization in Cua resembles that found in Type I sites most of the delocalized unpaired spin density is found on the Cys ligands. The electron spin density on each P-CH2 Cys proton is about half of that observed on the equivalent protons in blue copper proteins (Bertini etal., 1996, 1999). The unpaired electron is distributed over the two copper ions and the two Cys ligands. The observed values for the hyperfine shifts are consistent with the fact that the hyperfine couplings found in the P-CH2 Cys protons in Type I sites are twice as large as those observed in Cua centers. However, as already discussed, the Cu(H)-Cys covalency in Type I sites can be severely altered by the strength of the Cu(H)-axial ligand interaction. This... 

The surface matrix in Fig. 3.1c shows the alternative sign of charge distribution both within the domains and in the domain boundaries. Therefore, the surface is fully covered with dipoles in a way that stabilizes not only the domains but also the domain boundaries. Hence, the surface undergoes the tensile stress due to electrostatic attraction among the charged bodies. The atomic valences of the copper atoms within the domain (0 Cu" or Cu" ) differ from that at the domain boundary (Cu ). The surface reaction in the -derived phase takes place without involvement of the second atomic layer at the precursor 0 stage. There are no atoms missing in the short-ordered c(2 x 2)-20 surface but only electron repopulation and polarization. VLEED optimization revealed an off-centered 0 pyramid with a position of the adsorbate (ZX) 0.40 A. DOj 0.18 A is about 5 % of the fourfold hollow dimension) with respect to the fourfold hollow [1]. 

AOx, CP, and tree and fungal laccases (Figure 8.5) [69]. Comparisons among the ground-state distribution of the Cu—S(cys) stretching vibration in the T1 copper site in each of these proteins show that they differ in strength. The Cu—S(cys) bond is one of the three essential coordination complexes of copper that defines the T1 site in MCOs and other copper proteins and is the major factor in electron transfer from the substrate to the trinuclear center [40,85]. Hence, RR spectroscopy can provide valuable information on how subatomic structural variations at this coordination can drive the functional properties of electron transfer at the T1 site. 

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