Aspects of Structure and Bonding in Copper-Amino Acid Complexes Revealed by Single-Crystal EPR/ENDOR Spectroscopy and Density Functional Calculations
Aspects of Structure and Bonding in Copper-Amino Acid Complexes Revealed by Single-Crystal EPR/ENDOR Spectroscopy and Density Functional Calculations
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DOI：
10.1021/jp811249s
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发表时间：
2009-05-14
影响因子：
2.9
通讯作者：
Peisach, Jack
中科院分区：
文献类型：
--
作者：
Colaneri, Michael J.;Vitali, Jacqueline;Peisach, Jack
This work deduces from a series of well-defined copper-doped amino acid crystals, relationships between structural features of the copper complexes, and ligand-bound proton hyperfine parameters. These were established by combining results from electron paramagnetic resonance (EPR)/electron-nuclear double resonance (ENDOR) studies, crystallography, and were further assessed by quantum mechanical (QM) calculations. A detailed evaluation of previous studies on Cu2+ doped into alpha-glycine, triglycine sulfate, alpha-glycylglycine, and L-alanine crystals reveal correlations between geometric features of the copper sites and proton hyperfine couplings from amino-bound and carbon-bound hydrogens. Experimental variations in proton isotropic hyperfine coupling values (a(iso)) could be fit to cosine-square dependences on dihedral angles, namely, for C-alpha-bound hydrogens, a(iso) = - 1.09 + 8.21 cos(2) theta MHz, and for amino hydrogens, a(iso) = -6.16 + 4.15 Cos(2) phi MHz. For the C, hydrogens, this dependency suggests a hyperconjugative-like mechanism for transfer of spin density into the hydrogen Is orbital. In the course of this work, it was also necessary to reanalyze the ENDOR measurements from Cu2+-doped alpha-glycine because the initial study determined the N-14 coupling parameters without holding its nuclear quadrupole tensor traceless. This new treatment of the data was needed to correctly align the N-14 hyperfine tensor principal directions in the molecular complex. To provide a theoretical basis for the coupling variations, QM calculations performed at the DFT level were used to compute the proton hyperfine tensors in the four crystal complexes as well as in a geometry-optimized Cu2+(glycine)(2) model. These theoretical calculations confirmed systematic changes in couplings with dihedral angles but Greatly overestimated the experimental geometric sensitivity to the amino hydrogen isotropic coupling.
