Spin restricted self consistent field-Xα-scattered wave (SCF-Xα-SW) calculations are presented for a number of approximations to the blue copper active site in plastocyanin. The results of these calculations indicate that the bonding at the site is quite covalent and that substantial electron delocalization occurs. A comparison of Xα calculations for free imidazole and the blue copper site indicates that the splitting of the imidazole π1 and π2 and n and π2 levels is expected to significantly increase upon complexation. Similarly, the 2e and 3a1 valence levels of the methylthiolate ligand split into three levels upon complexation to copper. The energy separation between these levels is calculated to decrease relative to the splitting between the 2e and 3a1 levels in free methylthiolate. All three valence levels have significant electron delocalization onto the copper, and the bonding is best described as involving one π- and two σ-type interactions. The 4a1 (sulfur pz) orbital of the axial thioether ligand at the blue copper site is computed to undergo a small but significant bonding interaction with the dzz orbital on the copper. On the basis of comparison to D4h CuCl42- and CuCl64- model complexes, this interaction is found to effect the energy of the dz2 → dx2-y2 ligand field transition. Formalisms are developed to calculate g and A values from the output of the Xα calculation. The g values provide an experimental calibration for the amount of delocalization present in the ground state wave function. These calculations indicate that the unpaired electron in the blue copper site spends about 40% of the time in a dx2-y2 orbital on the copper and about 36% of the time on the p π orbitral of the thiolate sulfur. A rhombic splitting of 0.017 in the g values is calculated, the magnitude of which is confirmed by experimental observation of this splitting in spinach plastocyanin with Q-band EPR. The rhombic character of the EPR appears to relate to electron delocalization of the ground state wave function over the p π orbitals of the sulfur. Hyperfine values are calculated for the blue site and for D4h CuCl42-. The difference between the values for the two complexes is found to relate in large measure to the increased delocalization in the blue site relative to CuCl42-. pz mixing at the blue site is not found and is shown to be insufficient as an explanation for the reduced Az in the blue site relative to D4h CuCl42-. These studies provide a reasonable, experimentally calibrated, approximation of the orientation and delocalization of the ground state wave function involved in electron transfer.
ASJC Scopus subject areas
- Colloid and Surface Chemistry