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Aspects of structure and bonding in copper-amino acid complexes revealed by single-crystal EPR/ENDOR spectroscopy and density functional calculations.

ABSTRACT: 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 Cu(2+) 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(alpha) hydrogens, this dependency suggests a hyperconjugative-like mechanism for transfer of spin density into the hydrogen 1s orbital. In the course of this work, it was also necessary to reanalyze the ENDOR measurements from Cu(2+)-doped alpha-glycine because the initial study determined the (14)N coupling parameters without holding its nuclear quadrupole tensor traceless. This new treatment of the data was needed to correctly align the (14)N 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 Cu(2+)(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.

SUBMITTER: Colaneri MJ

PROVIDER: S-EPMC2896622 | biostudies-literature | 2009 May

REPOSITORIES: biostudies-literature

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Aspects of structure and bonding in copper-amino acid complexes revealed by single-crystal EPR/ENDOR spectroscopy and density functional calculations.

Colaneri Michael J MJ Vitali Jacqueline J Peisach Jack J

The journal of physical chemistry. A 20090501 19

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 Cu(2+) doped into alpha-glycine, trigl ...[more]

PMID: 19378965

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Project description:The chemical interplay of nitrogen oxides (NO's) with hemoglobin (Hb) has attracted considerable recent attention because of its potential significance in the mechanism of NO-related vasoactivity regulated by Hb. An important theme of this interplay-redox coupling in adducts of heme iron and NO's-has sparked renewed interest in fundamental studies of FeNO(x) coordination complexes. In this Article, we report combined UV-vis and comprehensive electron paramagnetic resonance (EPR) spectroscopic studies that address intriguing questions raised in recent studies of the structure and affinity of the nitrite ligand in complexes with Fe(III) in methemoglobin (metHb). EPR spectra of metHb/NO(2)(-) are found to exhibit a characteristic doubling in their sharper spectral features. Comparative EPR measurements at X- and S-band frequencies, and in D(2)O versus H(2)O, argue against the assignment of this splitting as hyperfine structure. Correlated changes in the EPR spectra with pH enable complete assignment of the spectrum as deriving from the overlap of two low-spin species with g values of 3.018, 2.122, 1.45 and 2.870, 2.304, 1.45 (values for samples at 20 K and pH 7.4 in phosphate-buffered saline). These g values are typical of g values found for other heme proteins with N-coordinated ligands in the binding pocket and are thus suggestive of N-nitro versus O-nitrito coordination. The positions and shapes of the spectral lines vary only slightly with temperature until motional averaging ensues at approximately 150 K. The pattern of motional averaging in the variable-temperature EPR spectra and EPR studies of Fe(III)NO(2)(-)/Fe(II)NO hybrids suggest that one of two species is present in both of the alpha and beta subunits, while the other is exclusive to the beta subunit. Our results also reconfirm that the affinity of nitrite for metHb is of millimolar magnitude, thereby making a direct role for nitrite in physiological hypoxic vasodilation difficult to justify.

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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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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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