X-ray absorption spectroscopic studies of the blue copper site: Metal and ligand K-edge studies to probe the origin of the EPR hyperfine splitting in plastocyanin

Journal Article · Wed Jan 27 04:00:00 EST 1993 · Journal of the American Chemical Society; (United States)

DOI:https://doi.org/10.1021/ja00055a057· OSTI ID:6528181

Shadle, S E; Penner-Hahn, J E; Schugar, H J; Hedman, B; Hodgson, K O; Solomon, E I ^[1]

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1. Stanford Univ., CA (United States)

X-ray absorption spectra for the oxidized blue copper protein plastocyanin and several Cu(II) model complexes have been measured at both the Cu K-edge and the ligand K-edges (Cl and S) in order to elucidate the source of the small parallel hyperfine splitting in the EPR spectra of blue copper centers. Assignment and analysis of a feature in the Cu K-edge X-ray absorption spectrum at [approximately]8,987 eV as the Cu 1s [yields] 4p + ligand-to-metal charge-transfer shakedown transition has allowed for quantitation of 4p mixing into the ground-state wave function as reflected in the 1s [yields]3d (+4p) intensity at [approximately]8,979 eV. The results show that distorted tetrahedral (D[sub 2d])CuCl[sub 4][sup 2[minus]] is characterized by <4% Cu 4p[sub z] mixing, while plastocyanin has only Cu 4p[sub xy] mixing. Thus, the small parallel hyperfine splitting in the EPR spectra of D[sub 2d]CuCl[sub 4][sup 2[minus]] and of oxidized plastocyanin cannot be explained by 12% 4p[sub z] mixing into the 3d[sub x[sup 2]-y[sup 2]] orbital as had been previously postulated. Data collected at the Cl K-edge for CuCl[sub 4][sup 2[minus]] show that the intensity of the ligand pre-edge feature at [approximately]2,820 eV reflects the degree of covalency between the metal half-occupied orbital and the ligands. The data show that D[sub 2d]CuCl[sub 4][sup 2[minus]] is not unusually covalent. The source of the small parallel splitting in the EPR of D[sub 2d]CuCl[sub 4][sup 2[minus]] is discussed. Experiments at the S K-edge ([approximately]2,470 eV) show that plastocyanin is characterized by a highly covalent Cu-S(cysteine) bond relative to the cupric-thiolate model complex [Cu(tet b)(o-SC[sub 6]H[sub 4]CO[sub 2])][center dot]H[sub 2]O. The XAS results demonstrate that the small parallel hyperfine splitting in the EPR spectra of blue copper sites reflects the high degree of covalency of the copper-thiolate bond. 34 refs., 12 figs., 3 tabs.

• OSTI ID: 6528181
• Journal Information: Journal of the American Chemical Society; (United States), Journal Name: Journal of the American Chemical Society; (United States) Vol. 115:2; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400101 -- Activation
Nuclear Reaction
Radiometric &amp; Radiochemical Procedures
550200* -- Biochemistry
59 BASIC BIOLOGICAL SCIENCES
ABSORPTION SPECTROSCOPY
AMINO ACIDS
BREMSSTRAHLUNG
CARBOXYLIC ACIDS
COMPLEXES
COPPER COMPLEXES
CYSTEINE
ELECTROMAGNETIC RADIATION
ELECTRON SPIN RESONANCE
FUNCTIONS
MAGNETIC RESONANCE
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC SULFUR COMPOUNDS
PROTEINS
RADIATIONS
RESONANCE
SELF-CONSISTENT FIELD
SPECTROSCOPY
SYNCHROTRON RADIATION
THIOLS
TRANSITION ELEMENT COMPLEXES
WAVE FUNCTIONS
X-RAY SPECTROSCOPY