A Spirocyclohexyl Nitroxide Amino Acid Spin Label for Pulsed EPR Spectroscopy Distance Measurements

Site‐directed spin labeling and EPR spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide α‐amino acid and its N‐(9‐fluorenylmethoxycarbonyl) derivative, have been synthesized, which exhibit slow enough spin‐echo dephasing to permit accurate distance measurements by pulsed EPR spectroscopy at temperatures up to 125 K in 1:1 water/glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR spectroscopy to the study of structure and conformation of peptides and proteins.     

Single crystal EPR studies of Mn(II) doped into zinc ammonium phosphate hexahydrate (ZnNH4PO4·6H2O): A case of interstitial site for bio-mineral analogue

Single crystal EPR studies of Mn(II)-doped zinc ammonium phosphate hexahydrate (ZnNH₄PC₄·6H₂O) have been reinvestigated at room temperature. Single crystal rotations along the three orthogonal axes indicate that the spin Hamiltonian parameters for the interstitial site are:g _xx = 1.966,g _yy = 1.972,g _zz = 1.976;D _xx = -12.28 mT,D _yy = -2.09 mT andD _zz = 14.37 mT;A _xx = 9.06 mT,A _yy = 9.06 mT andA _zz = 11.09 mT;a = -0.11 mT. These parameters differ considerably from the previous report of Chand and Agarwal and indicate the orthorhombic nature of the paramagnetic impurity. The impurity is found to enter the lattice interstitially, in contrast to earlier prediction of substitutional position. The percentage covalency of the Mn-0 bond has been estimated.     

Calix[4]arene nitroxide tetraradical and octaradical

1,3-Alternate calix[4]arene with para-phenylene spacers connecting nitroxide monoradicals and high-spin (S = 1) diradicals provides tetraradical and octaradical scaffolds that possess conformations with slow electron spin relaxation rates (1/T(1)). Such scaffolds may facilitate tuning of relaxation rates that are more favorable for MRI or DNP applications.     

Unsymmetrical tetrasubstituted ureas from tertiary carbamoylimidazole: activation by AlMe3

An efficient and general method for the synthesis of unsymmetrical tetrasubstituted ureas from carbamoylimidazole is described. The conversion is achieved by the concurrent quarternization of the imidazole nitrogen and activation of amines with AlMe(3).     

Electron spin relaxation time of Ni(II) ion in hexapyrazole zinc(II) dinitrate at 300 K

Understanding the electron spin relaxation properties of paramagnetic species is a fundamental requirement to use them as a probe to measure distances between sites in biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Even though Ni(II) ion is an essential trace element for many species, relaxation properties are not well understood. Herein, the polycrystalline sample of Ni(II) ion magnetically diluted in Zn(Pyrazole)₆(NO₃)₂ (Ni/ZPN) has been studied in detail by EPR spectroscopy to explore the electron spin relaxation time. Progressive continuous-wave (CW) EPR power saturation study on Ni/ZPN at 300 K yielded 907 mW as the P_1/2 value. The cavity constant (K_Q) has been calculated using tempol in PVA-BA glass matrix and the product of electron spin-lattice relaxation time (T₁) and spin–spin relaxation time (T₂) for Ni/ZPN at 300 K has been reported for the first time.     

Identification of static JT in copper(II) doped hexaimidazole M(II) lattices: M=Co and Ni: an EPR study

Single crystal EPR studies on Cu(II) doped paramagnetic host lattices, hexaimidazole M(II) dichloride tetrahydrate (M=Co and Ni), isomorphous with M=Zn, have been carried out from room temperature to 77K to understand the nature of Jahn-Teller (JT) distortion in these paramagnetic host systems. The paramagnetic impurity, doped in the present two paramagnetic host lattices, shows anisotropic EPR spectra with superhyperfine from ligands, even at room temperature. An interesting observation noticed in the EPR spectra at room temperature is that there are more resonances corresponding to the second site in the paramagnetic hosts than in the diamagnetic host at 4.2K. This difference in behavior between the diamagnetic and paramagnetic host lattices indicates a change in the depth of the JT valleys. The spin Hamiltonian parameters are evaluated for Cu(II) ion in both the host lattices and the relaxation times have been calculated for the ion in cobalt host lattice only.     

Observation of three sites for vanadyl in a biomineral, zinc sodium phosphate hexahydrate: an EPR investigation

Single crystal EPR study has been carried at room temperature for VO(II) doped zinc sodium phosphate hexahydrate. Single crystal rotations in each of the three mutually orthogonal crystallographic planes namely bc, ac, and ab indicate three chemically inequivalent sites, with intensity ratios of 25:13:1. The spin Hamiltonian parameters obtained for the two intense sites are: Site I: g_xx=1.983, g_yy=1.985, g_zz=1.933; A_xx=7.39 mT, A_yy=7.15 mT, A_zz=19.03 mT; Site. II: g_xx=1.985, g_yy=1.985, g_zz=1.937; A_xx=7.36 mT, A_yy=7.25 mT, A_zz=18.67 mT. The two VO bond directions in the two sites are approximately at right angles to each other. The powder spectrum clearly indicates two chemically inequivalent sites, confirming the single crystal analysis. Admixture coefficients, Fermi contact, and dipolar interaction terms have also been evaluated.     

Single crystal EPR studies of paramagnetic ions doped zinc potassium phosphate hexahydrate; Part III: Mn(II)-a case of rhombic distortion

Single crystal electron paramagnetic resonance (EPR) studies of Mn(II) doped zinc potassium phosphate hexahydrate have been carried out at room temperature. Single crystal rotations along the three orthogonal axes indicate orthorhombic symmetry with spin-Hamiltonian parameters as: g(xx) = 1.9997; g(yy) = 1.9538; g(zz) = 1.9524, D(xx) = 15.49 mT; D(yy) = 0.22 mT; D(zz) = -15.71 mT, A(xx) = 11.70 mT; A(yy) = 10.53 mT; A(zz) = 10.42 mT and a = 0.8 x 10(-4) cm(-1). A large E term indicates considerable distortion from axial symmetry. The impurity is found to enter the lattice substitutionally. The distortion axis for the impurity has been identified along one of the Zn-O bond directions in the crystal.     

Single-crystal EPR investigation of Mn(II)-doped biomineral cobalt ammonium phosphate hexahydrate: a case of multiple substitutions

Single-crystal electron paramagnetic resonance (EPR) study of Mn(II)-doped cobalt ammonium phosphate hexahydrate has been carried out at room temperature. The impurity shows more than 30 line pattern EPR spectra along the three crystallographic axes, suggesting the existence of more than one type of impurity ion in the host lattice. The spin Hamiltonian parameters, estimated from the three mutually orthogonal crystal rotations, are: site 1: g _xx =1.989, g _yy =1.994, g _zz =1.999; A _xx =?8.97, A _yy =?9.52, A _zz =?9.71 mT; D _xx =?8.09 mT, D _yy =?6.05 mT, D _zz =14.14 mT; site 2: g _xx =1.988, g _yy =2.009, g _zz =2.019; A _xx =?9.11 mT, A _yy =?9.58 mT, A _zz =?9.93 mT; D _xx =?6.61 mT, D _yy =?6.11 mT, D _zz =12.72 mT. The angular variation studies further reveal that the Mn(II) impurities enter the lattice substitutionally. The other Mn(II) sites which are at interstitial locations are difficult to follow due to their low intensity. The variation of zero-field splitting parameter with temperature indicates no phase transition. The observation of well-resolved Mn(II) spectrum at room temperature has been interpreted in terms of ‘host spin-lattice relaxation narrowing’ mechanism.