Carl F. Polnaszek

General method for multiparameter fitting of high-resolution EPR spectra using a simplex algorithm

A general method for fitting experimental electron paramagnetic resonance spectra to numerically simulated multiparameter model spectra has been developed. The goal of this work is to provide a quantitative and reliable method for evaluating spectral simulations and extracting the maximum information possible from experimental EPR spectra. The calculation involves a minimization of gx2, in which a downhill simplex algorithm is used as the search procedure for varying the parameters that determine the simulated model spectra. This method is applied to determine (1) magnetic tensors and linewidths from spectra of randomly oriented (powder) samples, (2) complex orientational distributions from spectra of oriented assemblies, and (3) exponential recovery times in time-domain EPR. The reliability of the calculation was verified by successful applications to simulated spectra for which the correct results were known, and by showing that the same results were obtained independently of initial assumptions or the convergence path followed, for both simulated and experimental spectra. Estimates of uncertainties in the fitted parameters were obtained by determining the standard deviations from multiple independent calculations with different initial values.

Nuclear magnetic relaxation dispersion measurement of water mobility at a silica surface

Abstract Nuclear magnetic relaxation rates are measured as a function of magnetic field strength corresponding to proton Larmor frequencies ranging from 0.01 to 42 MHz for silica gel samples with a nitroxide free radical covalently attached at the surface. The field dependence of the relaxation rate is interpreted using a translational model for the relaxation equation to yield a translational diffusion coefficient for the water, in the immediate vicinity of the radical attached to the surface, of 2.1 × 10 −6 cm 2 s −1 at 278 K for Si-4000 silica.

Simulation of the EPR spectra of the cholestane spin probe under conditions of slow axial rotation. Application to gel phase dipalmitoyl phosphatidyl choline

Abstract Simulations have been made of the EPR spectra of the cholestane spin probe undergoing anisotropic rotation about its long molecular axis in macroscopically unoriented samples such as multilayer dispersions of lipids. The simulations were performed using the stochastic Liouville method with the correlation time for rotation around the long axis, τ∥, as a variable, while the correlation time for rotation of the long axis, τ⊥, was maintained fixed at 3 × 10−7 sec. Empirical correlation time calibrations are introduced based on the outer splitting of the spectra, Amax (valid throughout the range τ∥ = 2 × 10−9to 7.5 × 10−8sec), and also on effective line height ratios of the motionally-averaged spectrum (valid for τ∥ 10−8 sec at 5°C to values corresponding to approximately 1–2 × 10−9 sec at temperature > 20°C, while still in the gel phase and then to values ≲ 10−10 sec above the main gel-fluid phase transition. In the latter case there is also appreciable motion of the long axis itself.

The magnetic field dependence of water proton T1 in aqueous solutions: implications for magnetic imaging contrast media

The magnetic field dependence of the water proton nuclear magnetic relaxation induced by a nitroxide radical both free and covalently bound to serum albumin has been measured from 0.23 mT to 0.7 T. The field dependence of the water proton 1/T1 shows only a minor dependence on the degree of radical immobilization, though the electron spin resonance (ESR) spectrum is very sensitive to the correlation time of the radical. The results and theory are described in more detail elsewhere (Journal of Chemical Physics, in press). The observed relaxation behavior arises because the long electron relaxation time of the nitroxide forces the correlation time for the magnetic interaction driving the water proton relaxation to become the correlation time for the relative translational motion of the water and the nitroxide. The relative insensitivity of the water proton relaxation to nitroxide immobilization should not change if the nitroxide is used as a contrast reagent for magnetic imaging in vivo.

EFFECT OF LINOLEIC ACID HYDROPEROXIDE ON HIGH AND LOW SPIN IRON IN MICROSOMAL CYTOCHROME P-450

Publisher Summary This chapter describes the effect of linoleic acid hydroperoxide (LAHP) on high- and low-spin iron in microsomal cytochrome P-450. LAHP, a product of microsomal lipid peroxidation, is known to decompose the heme moiety of cytochrome P-450 (P-450). Electron paramagnetic resonance (EPR) has been used extensively to study the high- and low-spin states of ferric P-450. In a study described in the chapter, EPR was used to determine if differences in the destruction of ferric P-450 by LAHP were related to the spin state of P-450. The results showed that the destruction of P-450 with increasing amounts of LAHP followed a concentration dependence. The exact results were not reproducible from animal to animal but were qualitatively similar. The chapter presents a typical set of results. The results indicate that the LAHP destruction of P-450 is not strongly dependent on spin state. This agrees with recent results obtained by optical methods at room temperature.