Multipurpose High Frequency Electron Spin Resonance Spectrometer for Condensed Matter Research

We describe a quasi-optical multifrequency ESR spectrometer operating in the 75-225 GHz range and optimized at 210 GHz for general use in condensed matter physics, chemistry and biology. The quasi-optical bridge detects the change of mm wave polarization at the ESR. A controllable reference arm maintains a mm wave bias at the detector. The attained sensitivity of 2x10^10 spin/G/(Hz)1/2, measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz and 300 K, is comparable to commercial high sensitive X band spectrometers. The spectrometer has a Fabry-Perot resonator based probe head to measure aqueous solutions, and a probe head to measure magnetic field angular dependence of single crystals. The spectrometer is robust and easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.Comment: submitted to Journal of Magnetic Resonanc

Structural investigation of a high-affinity Mn-II binding site in the hammerhead ribozyme by EPR spectroscopy and DFT calculations. Effects of neomycin B on metal-ion binding

Electron paramagnetic resonance spectroscopy and density functional theory methods were used to study the structure of a single, high-affinity Mn-II binding site in the hammerhead ribozyme. This binding site exhibits a dissociation constant K-d of 4.4 muM in buffer solutions containing 1 M NaCl, as shown by titrations monitored by continuous wave (cw) EPR. a combination of electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) experiments revealed that the &lt;LF&gt;paramagnetic manganese(II) ion in this binding site is coupled to a single nitrogen atom with a quadrupole coupling constant kappa of 0.7 MHz, an asymmetry parameter eta of 0.4, and an isotropic hyperfine coupling constant of A(iso)(N-14) = 2.3 MHz. All three EPR parameters are sensitive to the arrangement of MNII ligand sphere and can therefore be used to determine the structure of the binding site. A possible location of this binding site may be at the G10.1, A9 site found to be occupied by Mn-II in crystals (MacKay et al., Nature 1994, 372, 68 and Scottet al., Science 1996, 274, 2065). To determine whether the structure of the binding site is the same in frozen solution, we performed DFT calculations for the EPR parameters, based on the structure of the Mn-II site in the crystal. Computations with the BHPW91 density function in combination with a 9s7p4d basis set for the manganese (ii) center and the Igio-II basis set for all other atoms yielded values of kappa((1)4N) = +0.80 MHz, eta - 0.324, and A(iso) (N-14) = +2.7 MHz, in excellent agreement with the experimentally obtained EPR parameters, which suggests that the binding site found in the crystal and in frozen solution are the same. In addition, we demonstrated by EPR that Mn-II is released from this site upon binding of the aminoglycoside antibiotic neomycin B (K-d = 1.2 muM) to the hammerhead ribozyme. Neomycin B has previouisly been shown to inhibit the catalytic activity of this ribozyme.</p