Aspects of Structure and Bonding in Copper - Amino Acid Complexes Revealed by Single-Crystal EPR/ENDOR Spectroscopy and Density Functional Calculations

Authors

Michael J. Colaneri, State University of New York at Old Westbury
Jacqueline Vitali, Cleveland State UniversityFollow
Jack Peisach, Albert Einstein College of Medicine

Document Type

Article

Publication Date

5-1-2009

Publication Title

J. Phys. Chem. A

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 Cu²⁺ doped into α-glycine, triglycine sulfate, α-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_α-bound hydrogens, a_iso = −1.09 + 8.21 cos² θ MHz, and for amino hydrogens, a_iso = −6.16 + 4.15 cos² φ MHz. For the C_α 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 Cu²⁺-doped α-glycine because the initial study determined the ¹⁴N coupling parameters without holding its nuclear quadrupole tensor traceless. This new treatment of the data was needed to correctly align the ¹⁴N 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.

Repository Citation

Colaneri, Michael J.; Vitali, Jacqueline; and Peisach, Jack, "Aspects of Structure and Bonding in Copper - Amino Acid Complexes Revealed by Single-Crystal EPR/ENDOR Spectroscopy and Density Functional Calculations" (2009). Physics Faculty Publications. 149.
https://engagedscholarship.csuohio.edu/sciphysics_facpub/149

Original Citation

J. Phys. Chem. A, 2009, 113 (19), pp 5700–5709

DOI

DOI: 10.1021/jp811249s

Volume

113

Issue

19