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 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 (aiso) could be fit to cosine-square dependences on dihedral angles, namely, for Cα-bound hydrogens, aiso = −1.09 + 8.21cos2θ MHz, and for amino hydrogens, aiso = −6.16 + 4.15cos2φ MHz. For the Cα 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 since 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. In order to provide a theoretical basis for the coupling variations, QM calculations performed at the Density Functional Theory (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.

Electron Paramagnetic Resonance Spectroscopic Study of Copper Hopping in Doped Bis(l-histidinato)cadmium Dihydrate

“…With this caveat, Table 2 shows good agreement between experimental fit and calculated hyperfine splittings, supporting the ligand assignments. Referring to Table 2 and Figure 1, the near copper histidine amide (N1) and imidazole (N2) nitrogen ligand a iso couplings of 29.8 MHz and 37.1 MHz, respectively, are similar to those previously reported by Electron Nuclear Double Resonance (ENDOR) studies for directly coordinated nitrogen in copper-doped amino acid crystal complexes (23.5 – 32.1 MHz) 15 . The more distant histidine amide (N1’) coupling, 20 MHz, is significantly lower than the coupling to N1, and is at the lowest end of this range.…”

“…The selection of N1' as the origin of this splitting over imidazole N2' was because its theoretical hyperfine components had a much better correspondence with the measured values. The resolved proton splitting was assigned to the C β carbon-bound H1, as its relatively large a iso of 10.1 MHz can be predicted using the results from a previous survey of ENDOR measured couplings in similar systems 15 . Using the Cu-N1-C β -H1 dihedral angle (θ≈175°) with an empirical cosine-square formula found by Colaneri et al 15 gave an a iso of 7.1 MHz, which is close to but somewhat lower than 10.1 MHz.…”

“…The resolved proton splitting was assigned to the C β carbon-bound H1, as its relatively large a iso of 10.1 MHz can be predicted using the results from a previous survey of ENDOR measured couplings in similar systems 15 . Using the Cu-N1-C β -H1 dihedral angle (θ≈175°) with an empirical cosine-square formula found by Colaneri et al 15 gave an a iso of 7.1 MHz, which is close to but somewhat lower than 10.1 MHz. However, the DFT calculated value a iso = 9.8 MHz confirms this assignment.…”

Detection of Copper(II) Ions Using Glycine on Hydrazine-Adsorbed Gold Nanoparticles via Raman Spectroscopy

“…The GLY-Cu 2+ complex based on reference [19]. The increase of [Cu 2+ ] results in more positive redox properties of the GLY-Cu 2+ complex and stronger CN intensities.…”

“…GLY was predicted to bind the Cu 2+ ion as a dimeric form [19]. Using an infrared spectroscopy tool, GLY was reported to dissociate to produce the cyanide group under alkaline pH conditions, when a positive voltage was applied to a gold electrode surface [20].…”