Michael K. Bowman

Imaging radio frequency electron‐spin‐resonance spectrometer with high resolution and sensitivity for in vivo measurements

We report the development of a novel radio frequency electron‐spin‐resonance spectrometer designed to provide measurements with high molar sensitivity and resolution in vivo. Radio frequency (250 MHz) is chosen to obtain good penetration in animal tissue and large aqueous samples with modest sacrifice of sensitivity. The spectrometer has a lumped component resonator and operates in continuous‐wave mode. The spectrometer is capable of two‐dimensional imaging, and with a modest addition should be capable of three‐dimensional imaging. We demonstrate 3‐mm spatial resolution for DPPH samples. For 10‐ml samples of aqueous nitroxide, we demonstrate sensitivity (normalized to spectral width of 1 G) to 3×10−8‐M concentrations and spectral resolution of 0.1 G. Spectra from nitroxide spin label injected into a live mouse are shown.

Parameters of quadrupole coupling of 14N nuclei in chlorophyll a cations determined by the electron spin echo method

Modulation effects induced by interaction of an unpaired electron with 14N nuclei of pyrrole rings in the electron spin echo from chlorophyll a radical cations have been investigated. The modulation frequencies have been determined by Fourier analysis of the echo signal decay. The modulation frequencies approximate to the quadrupole resonance frequencies of 14N nuclei.

Selective isotopic labeling of a nitroxide spin label to enhance sensitivity for T2 oxymetry

Abstract The synthesis and application of a novel compound, a variant of a standard spin label which has been used for T2-based oxymetry, is described. The compound is 4-hydro-3carbamoyl-2,2,5,5-tetraperdeuteromethylpyrrolin-1-yloxy-d12 (mHCTPO). It was developed to optimize both the sensitivity of the T2 method to oxygen tension and the spectrometer signal for a given concentration of spin label. The compound is used with a very low frequency (250 MHz) electron paramagnetic resonance spectrometer for in vivo application and for imaging, but has application at X band. The compound provides a convenient spectral feature distinguishing broadening associated with self-interaction from that due to environmental oxygen.

Ratio analysis of electron spin echo modulation envelopes in disordered matrices and application to the structure of solvated electrons in 2-methyltetrahydrofuran glass

A new analysis for two pulse and three pulse electron spin echo modulation data is presented which is independent of the echo decay function. The experimental ratios of maxima to minima of the echo modulation curve are compared with simulated ones to obtain the number of interacting nuclei, their distances, and their isotropic coupling by a least squares fit. From the best simulation, the empirical echo decay function can be determined and the original experimental data can be simulated. This new method is applied to new three pulse and old two pulse [L. Kevan et al., J. Chem. Phys. 63, 409 (1975)] electron spin echo data on solvated electrons in 2‐methyltetrahydrofuran (MTHF) glass at 77 K. This analysis together with previous second moment data gives a revised model for electron solvation in this matrix based on a statistical orientation of the MTHF ring carbon planes toward the electron. In this statistical configuration model three equivalent MTHF molecules form the first solvation shell with their ri...

Time-resolved electron spin echo spectroscopy applied to the study of photosynthesis

A recent development in the research of photosynthesis has been the application of fast time-resolved electron paramagnetic resonance (EPR) [l-4]. The EPR method is particularly promising for systems which cannot readily be studied by optical techniques. The radicals which have been observed in photosynthetic systems by the magnetic resonance technique exhibit chemically-induced dynamic electron polarization (CIDEP) [5]. The mechanism which produces this spin polarization (a non-Boltzmann distribution of spin population) reflects the earlier fate of the radicals. The reported studies of photosynthetic systems by time resolved magnetic resonance have utilized what are now fairly standard EPR techniques [6]. They have been limited by lack of adequate time resolution, possible distortions or artifacts due to the use of magnetic field modulation [3,7], and the problem of positively identifying the observed radical species. We are applying time-resolved electron spin echo (ESE) spectroscopy, a pulsed form of EPR, to the study of photosynthetic systems. This new technique not only avoids many of the problems described above but also provides several new advantages. We report here on the first observation of spin-polarized signals in the blue-green alga Synechococcus Zividus using timeresolved ESE, and illustrate advantages of the technique.

Q-Cycle Bypass Reactions at the Qo Site of the Cytochrome bc1 (and Related) Complexes

Publisher Summary This chapter presents an overview of Q-cycle bypass reactions at the Q 0 site of the cytochrome bc 1 and related complexes. The availability of high-resolution crystal structures, a large array of mutant strains in a number of species, and new spectroscopic approaches should greatly facilitate elucidation of the mechanisms by which the various bypass reactions are prevented. To aid this approach, this chapter describes the application and limitations of assays for detection of the various bypass reactions. A method is described for probing conformational changes in the ISP head domain, which are thought to be critical for preventing some of the bypass reactions. The chapter describes the Q-cycle and its bypass reactions. The chapter also elaborates about estimating the concentrations of cyt bc 1 and b6f complexes. The chapter presents the estimates of cyt bc 1 and b6f concentrations and measurements of the bifurcated oxidation of QH2 and probes the involvement of ISP domain movements in restricting bypass reactions. A simple method for estimating changes in the orientation and ordering of membrane-bound anisotropic EPR signals is also presented in the chapter.

Singlet and triplet absorption spectra of carotenoids bound in the reaction centers of Rhodopseudomonas sphaeroides R26

The singlet and triplet state absorption spectra are reported for two carotenoids, methoxyneurosporene and spheroidene, incorporated into the reaction center protein of the photosynthetic bacterial carotenoidless mutant Rhodopseudomonas sphaeroides R26. The spectra for the two different carotenoid molecules are identical suggesting a strong interaction between the protein and the different chromophores. Combined effects of electrochromic band shifts and carotenoid structural changes are invoked to account for the spectral observations.

An electron spin echo envelope modulation study of lithium nuclear hyperfine and quadrupole coupling in the A(Ti–Li) center of α‐quartz

Computerized microwave pulse generation and detection with subsequent Fourier transformation of electron spin echo envelope modulation spectra has been used to measure the small 6,7Li nuclear superhyperfine and quadrupole splittings of a lithium‐compensated titanium(3+) center in single crystal α‐quartz. Comparison of the 7Li hyperfine matrices A determined from standard cw‐EPR and from ESEEM show excellent agreement. The precision is better by an order of magnitude for the latter, with uncertainties on the order of one part in a thousand. Use of a three‐dimensional display technique yielded the relative sign of matrices A and Q for 7Li.

Electron spin echo studies of the internal motion of radicals in crystals: Phase memory vs correlation time

An electron spin echo (ESE) study of the internal motion of the CH2 protons in irradiated zinc acetate dihydrate crystals shows that quantitative measurements of the motional correlation time can be obtained quite directly from pulsed measurements. In the slow motional limit, the motional correlation time is equal to the phase memory time determined by ESE. In the fast motional limit, the motional correlation time is proportional to the no motion spectral second moment divided by the ESE phase memory time. ESE offers a convenient method of studying motion, electron transfer, conductivity, etc. in a variety of systems too complicated for study by ordinary EPR. New systems for study by ESE include biological samples, organic polymers, liquid solutions of radicals with unresolved hyperfine, etc. When motion modulates large anisotropic hyperfine couplings, ESE measurements of the phase memory time are sensitive to modulation of pseudosecular hyperfine interactions.

Temperature dependence of a process competing with S/sub 2/--S/sub 1/ internal conversion in indole and phenol in aqueous solutions

The drop in phi/sub f/ with decreasing excitation wavelength, previously attributed to ionization from the S/sub 2/ state, increases substantially with increasing temperature for both indole and phenol in aqueous solution. Addition of 2-propanol to the solvent reduces the drop in phi/sub f/ but not its activation energy which on the other hand depends significantly on the solute. phi/sub eaq/-measured upon excitation of the S/sub 2/ state of indole in H/sub 2/O increases from 0.09 at 25/sup 0/C to 0.17 at 65/sup 0/C. The results are interpreted in terms of an intermediate charge transfer to solvent state (CTTS) populated from S/sub 2/. In the 2-propanol-water solvent the formation of this CTTS state appears to be governed primarily by interaction of the excited solute with water molecules. (auth)