Wojciech Froncisz

Broadening by strains of lines in the g‐parallel region of Cu2+ EPR spectra

Linewidths of the mI=−3/2 and −1/2 hyperfine lines in the g‐parallel region of several square‐planar copper complexes with oxygen ligands in frozen solutions have been measured at 2.6, 3.8, and 8.9 GHz. A theory has been developed in terms of distributions of molecular bonding parameters that satisfactorily accounts for the observed dependence of linewidths on mI and the microwave frequency. It has been found that distributions of the bonding parameters are strongly correlated, indicating that copper complexes when subjected to strains during freezing distort in a well‐defined, predictable manner. It is found that the mI=−3/2 line generally is narrowest near 6 GHz and the mI=−1/2 line near 2 GHz. By proper selection of the microwave frequency, improved spectral resolution can be obtained. Complexes with nitrogen ligands also have been examined. Proper selection of microwave frequency often permits observation of superhyperfine structure, and even when it cannot be seen, analysis of the linewidth variation...

Continuous and stopped flow EPR spectrometer based on a loop gap resonator

A continuous and stopped flow EPR spectrometer based on a new loop gap resonator operating at X band is described. The important features of the instrument are: (1) very small amounts of material are consumed in both stopped and continuous flow modes of operation, (2) dead times on the order of 4 ms for stopped flow and 1 ms for continuous flow are realized with a very narrow age distribution in the sample compartment, and (3) the dead times and, hence, sample age are highly reproducible and independent of viscosity due to the use of a positive displacement syringe ram. The performance of the instrument is evaluated using the decay of ascorbate radical generated by Ce(iv) oxidation and reduction of nitroxides by ascorbic acid.

Q‐band loop‐gap resonator

The upper frequency for loop‐gap resonators intended for use in electron‐spin‐resonance spectroscopy has been extended to Q band (35 GHz). A practical structure is described containing sample support, frequency tuning, and variable coupling. A typical sample volume is 39 nL. High‐energy densities (15 GW−1/2) were achieved. As found previously at X band, Q‐band loop‐gap resonators permit observation of the dispersion with minimal demodulation of phase noise originating in the klystron. Theoretical calculations of the resonant frequency, Q, and the filling factor are found to be in good agreement with experiment.

The loop-gap resonator. II. Controlled return flux three-loop, two-gap microwave resonators for ENDOR and ESR spectroscopy

Abstract In a previous paper ( I, W. Froncisz and J. S. Hyde, J. Magn. Reson. 47 , 515 (1982) ), one-loop, multigap microwave resonators were described with emphasis on applications to ESR spectroscopy. In the present paper, n -loop, m -gap resonators are introduced, where there are n parallel loops with a shared gap connecting adjacent loops, there being ( n − 1) = m gaps in all, and an analysis of three-loop, two-gap resonators is presented. Calculations of resonant frequencies are compared with experiment. The structure exhibits two resonant modes, a fundamental mode where magnetic flux is excluded from the central loop, and a higher frequency mode in which flux links all three loops. This latter mode is used for ESR and ENDOR spectroscopy. The sample fills the central loop with a filling factor of about 0.4, and one side loop is used for inductive coupling. A 25% tuning range can be achieved by introduction of a metallic conductor into the intense magnetic field of the other side loop. Lines of magnetic flux are well controlled in the three-loop, two-gap resonator, permitting juxtaposition of radiofrequency coils suitable for ENDOR without interference between the coils and the microwave fields.

Inductive (flux linkage) coupling to local coils in magnetic resonance imaging and spectroscopy

Abstract An engineering analysis of inductive (flux linkage) coupling to local coils used for NMR imaging and spectroscopy is presented. It is emphasized that the concept of “resonance” must be carefully defined because of complexity introduced into the equivalent circuit by the coupling loop inductance and the mutual inductance between the coupling and receiving loops. The condition of match to the transmission line is unique and coincides with the maximum in receiver voltage. The effect of matching on frequency detuning is analyzed, and it is shown that the coupling loop should be as small as possible and the coupling coefficient as large as possible. A convenient variable inductive coupler is described in which a variable capacitor is displaced on odd number of quarter wavelengths from the coupling loop. The variable coupler has been implemented using a voltage controlled tuning capacitor (varactor).

Saturation recovery EPR and ELDOR at W-band for spin labels.

A reference arm W-band (94 GHz) microwave bridge with two sample-irradiation arms for saturation recovery (SR) EPR and ELDOR experiments is described. Frequencies in each arm are derived from 2 GHz synthesizers that have a common time-base and are translated to 94 GHz in steps of 33 and 59 GHz. Intended applications are to nitroxide radical spin labels and spin probes in the liquid phase. An enabling technology is the use of a W-band loop-gap resonator (LGR) [J.W. Sidabras, R.R. Mett, W. Froncisz, T.G. Camenisch, J.R. Anderson, J.S. Hyde, Multipurpose EPR loop-gap resonator and cylindrical TE(011) cavity for aqueous samples at 94 GHz, Rev. Sci. Instrum. 78 (2007) 034701]. The high efficiency parameter (8.2 GW(-1/2) with sample) permits the saturating pump pulse level to be just 5 mW or less. Applications of SR EPR and ELDOR to the hydrophilic spin labels 3-carbamoyl-2,2,5,5-tetra-methyl-3-pyrroline-1-yloxyl (CTPO) and 2,2,6,6,-tetramethyl-4-piperidone-1-oxyl (TEMPONE) are described in detail. In the SR ELDOR experiment, nitrogen nuclear relaxation as well as Heisenberg exchange transfer saturation from pumped to observed hyperfine transitions. SR ELDOR was found to be an essential method for measurements of saturation transfer rates for small molecules such as TEMPONE. Free induction decay (FID) signals for small nitroxides at W-band are also reported. Results are compared with multifrequency measurements of T(1e) previously reported for these molecules in the range of 2-35 GHz [J.S. Hyde, J.-J. Yin, W.K. Subczynski, T.G. Camenisch, J.J. Ratke, W. Froncisz, Spin label EPR T(1) values using saturation recovery from 2 to 35 GHz. J. Phys. Chem. B 108 (2004) 9524-9529]. The values of T(1e) decrease at 94 GHz relative to values at 35 GHz.

Direct Evidence of Nitrogen Coupling in the Copper(II) Complex of Bovine Serum Albumin by S-Band Electron Spin Resonance Technique

ESR spectra of the tight binding Cu(II) complex of bovine serum albumin (BSA) has been studied using S-band. At physiological pH, only one form of copper binding to BSA was detected from the ESR spectra. From previous X-band ESR spectra, nitrogen superhyperfine splittings were observable in the g perpendicular region; however, the resolution of the g parallel region was not sufficient to confirm the exact donor atoms of the complex. Using low-frequency ESR (2-4 GHz) at 77 K, we have resolved the nitrogen superhyperfine structure in the g parallel region. A computer simulation method has been developed for distinguishing between three and four nitrogen donor atoms. The Hyde-Froncisz theory of g and A strain broadening has been modified to use a field-swept calculation for the line shape. The observed intensity pattern and the computer simulation of such spectra positively confirm the structure of Cu(II) ion coordinated to four in-plane nitrogen atoms in frozen aqueous solutions of Cu(II)-BSA complexes at physiological pH. This is the first time that this binding site has been confirmed on the protein instead of a protein fragment or model compound. This work is another example of the usefulness of the S-band ESR technique for characterizing the metal-protein interactions when random variation in g factors cause line broadening in conventional X-band ESR spectra.

Dispersion electron spin resonance with the loop‐gap resonator

The loop‐gap resonator, a novel microwave lumped‐circuit structure, when used as a sample resonator in electron‐spin‐resonance spectroscopy, permits the direct detection of dispersion signals with greatly decreased demodulation of the FM noise that originates in the microwave oscillator. The improvement arises from two factors: 65 times higher energy density for a given input power and 12 times lower resonator Q compared with a typical cavity resonator. The signal‐to‐noise ratio for the dispersion signal of DPPH is predicted to be improved by a factor of 12×65=780; experimentally a factor of 700 was realized.

Multipurpose loop-gap resonator

Abstract A three-loop-two-gap resonator is described that has properties that are similar to the widely used X-band rectangular TE102 multipurpose cavity. Both structures accommodate the same cavity accessories interchangeably: variable temperature Dewar insert, liquid nitrogen Dewar, electrochemical cell, tissue cell, and flat cell. However, optimum performance with the loop-gap resonator (LGR) is achieved with 1 mm flat cells rather than 0.4 mm cells used in the cavity. The lower Q of the LGR decreases the demodulation of phase noise when tuned to the dispersion. The geometry is flexible and it is suggested that it can serve as a convenient general purpose resonator for ESR spectroscopy.

ELECTRON PARAMAGNETIC RESONANCE DETECTION BY TIME-LOCKED SUBSAMPLING

A detection method for electron paramagnetic resonance spectroscopy is described that permits simultaneous acquisition of multiple in- and out-of-phase harmonics of the response to magnetic-field modulation for both dispersion and absorption: (i) conversion of the microwave carrier to an intermediate frequency (IF) carrier; (ii) subsampling of the IF carrier by an analog-to-digital converter four times in K IF cycles where K is an odd integer; (iii) dividing the digital words into two streams, odd indexes in one and even in the other, followed by sign inversion of every other word in each stream; and (iv) feeding the two streams to a computer for the digital equivalent of phase-sensitive detection (PSD). The system is broadbanded, in the frequency domain, with narrow banding for improved signal-to-noise ratio occurring only at the PSD step. All gains and phases are internally consistent. The method is demonstrated for a nitroxide spin label. A fundamental improvement is achieved by collecting more informa...