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The XAS model of dissolved Cu(II) and its significance to biological electron transfer

Patrick Frank1,2,5, Maurizio Benfatto3, Britt Hedman2 and Keith O Hodgson1,4

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The standard model for dissolved Cu(II) portrays the complex ion as an axially elongated, equatorially planar octahedron. Using EXAFS and MXAN analyses of copper K-edge XAS spectra, new structural models for dissolved [Cu(aq)]2+ and [Cu(amm)]2+ have been determined. These structures uniformly depart from the octahedral model in favour of an axially elongated square pyramidal core. MXAN results also indicate that the equatorial ligands need not be coplanar with copper. Further structural elements include a -z axially localized scatterer at ~3 Å. Even more distant scatterers imply second shell solvent organization, which can vary with the medium. Preliminary results from new extended, k = 18 Å-1, higher resolution copper K-edge XAS data sets are reported. The low symmetry of dissolved Cu(II) ion contradicts the central thesis of the rack-induced bonding hypothesis of copper electron transfer proteins. The asymmetry of biological copper is not a frozen vibronic excited state enforced by a rigid protein scaffold, but is entirely in harmony with the structural ground state of the dissolved aqueous Cu(II) complex ion.


PACS

87.15.M- Spectra of biomolecules

87.15.B- Structure of biomolecules

87.15.A- Theory, modeling, and computer simulation

82.39.Jn Charge (electron, proton) transfer in biological systems

87.14.E- Proteins

87.15.N- Properties of solutions of macromolecules

Subjects

Biological physics

Chemical physics and physical chemistry

Dates

Issue 1 (2009)



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