Crystal Field Applications to Biochemical and Geochemical Systems: Electronic Structures of Blue Copper Proteins and Iron Group Olivines

Author: Hare, Jeffrey Wayne

Year: 1976

Degree: Dissertation (Ph.D.)

Advisor: Unknown, Unknown

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/7f37-6553

Abstract

Part I, Chapter I

A helical secondary structure has been found to be associated with the metal site in bean plastocyanin. It is likely that such structure is also present in other blue copper proteins. This short section of helix is distorted by metal (copper or cobalt) incor poration, which is probably the result of coordination to an amide nitrogen or oxygen of the peptide backbone. The perturbed helical structure at the site remains conformationally rigid upon reduction of holoplasto-cyanin to the copper(I) derivative. Finally, a combination of known physical data leads to a picture of the metal site where copper(II) is bound by histidine-37, an amide nitrogen or oxygen or one or more of the following residues which make up the helix, and by cysteine-84 and histidine-87 which provide the final coordinating positions.

Part I, Chapter II

New low energy electronic transitions at ~10000 and 5000 cm-1 have been identified as ligand field bands of the blue site of the single copper proteins stellacyanin, plastocyanin and azurin from low temperature absorption as well as CD and MCD studies in the near-infrared. The well known visible series of absorption bands are assigned as charge transfer transitions of π and σ character from sulfur to copper and of π character from deprotonated amide nitrogen to copper, based on comparison of visible absorption and CD intensities. Ligand field calculations have shown that the observed d-d excitation energies are consistent with a tetrahedral model distorted approximately 4 to 8° towards a square plane. A model of the blue site, which incorporates recent physical evidence for specific ligands, is presented. The reduction potentials have been shown to be influenced, but not dominated, by ligand field effects.

Part I I

The absorption bands of iron, cobalt and nickel olivines have been separated with respect to metal site on the basis of previous assignments, the absorption spectra of synthetic and natural triphylites, and the polarized spectra of magnesium-nickel and magnesium-cobalt olivines. An effective C2v site symmetry has been shown to adequately describe the band splittings, but it fails to completely describe the observed polarization properties particularly for Co(II). Ligand field calculations have resulted in values of B, Dq and the low symmetry radial parameters for both the M1 and M2 sites. Calculation of LFSE's and the ligand field contribution to the distribution coefficients of the cation exchange reactions from these parameters and comparison to KD's calculated from x-ray diffraction data has led to the conclusion that ligand field stabilization is the major driving force for cation ordering in olivine.

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