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

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 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 cos ² θ MHz, and for amino hydrogens, a _iso = −6.16 + 4.15 cos ² 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 Cu ²⁺ -doped α-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.