title: Solid-state (67)zn NMR spectroscopy in bioinorganic chemistry. Spectra of four- and six-coordinate zinc pyrazolylborate complexes obtained by management of proton relaxation rates with a paramagnetic dopant.
abstract: Solid-state (67)Zn NMR spectra of model compounds for metalloproteins, such as [H(2)B(3,5-Me(2)pz)(2)](2)Zn (pz denotes pyrazolyl ring), have been obtained using low temperatures (10 K) to enhance the Boltzmann factor in combination with cross polarization (CP) from (1)H to (67)Zn. Attempts to observe spectra of other model compounds, such as [H(2)B(pz)(2)](2)Zn, were hindered by long relaxation times of the protons. To decrease the proton relaxation times, the high-spin six-coordinate complex [HB(3,4,5-Me(3)pz)(3)](2)Fe has been investigated as a dopant. NMR and EPR measurements have shown that this Fe(II) dopant effectively reduces the (1)H spin lattice relaxation time, T(1), of the zinc samples in the temperature range 5-10 K with minimal perturbations of the (1)H spin lattice relaxation time in the rotating frame, T(1)(rho). Using this methodology, we have determined the (67)Zn NMR parameters of four- and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful models for systems of biological importance. The (67)Zn NMR parameters are contrasted to the corresponding changes in the (113)Cd NMR parameters for the analogous compounds. Further, these investigations have demonstrated that a temperature-dependent phase transition occurs in the neighborhood of 185 K for [HB(3,5-Me(2)pz)(3)](2)Zn; 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 (18.8 T) solid-state (67)Zn 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, for example, (25)Mg, (43)Ca, and (67)Zn.
authors: Macromolecular Structure & Dynamics Directorate, WR Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
authors: Andrew S
authors: AS
authors: Lipton
authors: Terri A
authors: TA
authors: Wright
authors: Michael K
authors: MK
authors: Bowman
authors: Daniel L
authors: DL
authors: Reger
authors: Paul D
authors: PD
authors: Ellis
mesh_terms: D001881:Borates / Q000737:chemistry*; D002104:Cadmium / Q000737:chemistry; D003080:Cold Temperature; D004578:Electron Spin Resonance Spectroscopy; D007554:Isotopes; D008667:Metalloproteins / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D018716:Molecular Mimicry; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D009942:Organometallic Compounds / Q000737:chemistry*; D011720:Pyrazoles / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*; D015033:Zinc Isotopes
chemical_list: D001881:Borates; D007554:Isotopes; D008667:Metalloproteins; D009942:Organometallic Compounds; D011720:Pyrazoles; D015033:Zinc Isotopes; D002104:Cadmium; D015032:Zinc