Carboxylate Binding in Copper Histidine Complexes in Solution and in Zeolite Y: X- and W-band Pulsed EPR/ENDOR Combined with DFT Calculations

The complexes of copper with histidine exhibit a wide variety of coordination modes in aqueous solution. This stems from the three potential coordination sites of the histidine molecule and the existence of mono- and bis-complexes. The present work concentrates on the determination of the carboxylate binding mode, via the 13C hyperfine coupling of the carboxyl, in a number of copper complexes in frozen solutions. These are then used as references for the determination of the coordination mode of two zeolite encapsulated complexes. The 13C hyperfine coupling (sign and magnitude) was determined by a variety of advanced pulsed EPR and electron−nuclear double resonance (ENDOR) techniques carried out at conventional and high magnetic fields. These showed that while the carboxyl 13C isotropic hyperfine coupling of an equatorially coordinated carboxylate is negative with a magnitude of 3−4 MHz, that of a free carboxylate is small (∼1 MHz) and positive. To rationalize the experimentally determined ligand hyperfine couplings (1H and 13C) and further understand their dependence on the coordination mode and degree of protonation, density functional theory (DFT) calculations were carried out on a number of model complexes, representing the different Cu-histidine complexes studied experimentally. The exchange-correlation functional used for the calculation of the EPR parameters was B3LYP with triple-ζ plus polarization (TZP) quality basis sets. While the polarization agreement between the magnitudes of the calculated and experimental values varied among the various nuclei, sometimes exhibiting deviations of up to 40%, an excellent agreement was found for the sign prediction. This shows the unique advantage of combining high field ENDOR, by which the sign of the hyperfine can often be determined, with DFT predictions for structure determination.