PNNL

Abstract

Solid state 67ZNn NMR spectra of model compounds for metalloproteins, such as [H2B(3, 5-Me2pz)2]2Zn (pz denotes pyrazolyl ring), have been obtained using low temperatures (10 K) to enhance the Boltzmann factor in combination with cross polarization (CP) from 1H to 67Zn. Attempts to observe spectra of other model compounds, such as [H2B(pz)2]2 Zn, were hindered by long relaxation times, of the protons. In order to decrease the proton relaxation times, the high spin six coordinate complex [HB(3, 4, 5-Me3pz)3]2Fe has been investigated as a dopant. NMR and EPR measurements have shown that this Fe(II) dopant effectively reduces the 1H spin lattice relaxation time, T1, of the zinc samples in the temperature range 5 to 10 K with minimal perturbations of the 1H spin lattice relaxation time in the rotating frame, T1p. Using this methodology, we have determined the 67Zn NMR parameters of 4-and 6-coordinate zinc (II) poly (pyrazolyl)borate complexes that are useful models for systems of biological importance. These investigations have demonstrated that a temperature dependent phase transition occurs in the neighborhood of 185 K for [HB(3, 5,-Me2pz)3]2Zn. Further, the other poly (pyrazolyl)borate complexes we investigated did not show this temperature dependent behavior. This conclusion is confirmed by a combination of room temperature high field (18.8T) solid state 67Zn NMR spectroscopy and low temperature X-ray methods. The utilization of paramagnetic dopants should enable low temperature cross polarization experiments to be performed on a wide variety of nuclides that are important in bioinorganic chemistry, e.g., 25Mg. 43Ca, and 67Zn.