Applications of Electron Spin Echo Modulation

Abstract

Electron spin echoes are a consequence of particular resonance microwave pulse sequences in pulsed electron spin resonance (ESR) experiments. The pulse sequences reorient the electron magnetic dipoles such that they dephase and then rephase to reform a macroscopic magnetic moment which is detected as a burst of microwave energy called an echo. Spin echoes were first discovered for nuclear spins in early nuclear magnetic resonance experiments by Hahn while a graduate student at the University of Illinois in 1950 (1). Nuclear spin echoes were initially demonstrated for protons in various liquids such as glycerin and water. Interestingly, this original paper on nuclear spin echoes also reports observations of modulation of the spin echo response reflecting nuclear spin -nuclear spin coupling. The nuclear modulation effect was first demonstrated for protons in liquid ethanol corresponding to interactions between different groups of nonequivalent protons which have different molecular environments in the molecule.The first electron spin echoes were observed by Blume at Columbia University in 1958 for solvated electrons in sodium-ammonia solutions (2). With electron spin echoes the analogous modulation effect occurs between interactions of the electron spins with surrounding nuclear spins in different local magnetic environments. Hence, this is related to an electron-nuclear dipolar hyperfine interaction which is averaged to zero by rapid molecular tumbling in liquids. Consequently, in the original electron spin echo experiments in liquid ammonia no nuclear modulation effects were seen. The first observation of nuclear modulation effects in electron spin echo signals was reported in 1961 by Mims, Nassau and McGee at Bell Laboratories (3a) who observed the electron spin echo of cerium(III) doped into calcium tungstate modulated by the 183 W nuclei in the solid single crystal and by Cowan and Kaplan at Lockheed Research Laboratories who observed 14 N modulation of cerium(III) in lanthanum magnesium nitrate (3b). Subsequently, Mims and coworkers described electron spin echo modulation in detail and developed it as a new and useful scientific technique.The basic theory of the nuclear modulation effect on electron spin echo intensities was first given by Rowan, Hahn and Mims (4) in 1965 who described how to calculate the expected nuclear modulation pattern from a given geometrical model. Their description referred to single crystal experiments for a particular 641 Foundations of Modern EPR Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 10/04/15. For personal use only.

Multifrequency Pulsed Electron Paramagnetic Resonance Study of the S2 State of the Photosystem II Manganese Cluster

“…The two-pulse frequency-domain spectra in Figure c clearly show the bound histidine ∼5 MHz peak (labeled *), a smaller positive feature at ∼2 MHz (labeled #), as well as a negative feature at ∼7 MHz. This negative feature may be a combination peak because two-pulse ESEEM spectra contain modulation frequencies that are combinations of the sums and differences of fundamental frequencies . The dominant peak at 15 MHz, present in Figure c, is due to weakly coupled protons.…”

ESR, ENDOR, and ESEEM Spectroscopy Study on the Local Structure and Motion of Reactants: Highly Selective Tunneling Radical Abstraction in a Neopentane−Ethane Mixture

“…The ESE signals were measured by using π/2−π two-pulse sequences. − The periodic modulation on the envelope signals, ESEEM, arises from magnetic interactions between the unpaired electron of the neopentyl or ethyl radicals and the deuterons of perdeuterated matrix neopentane molecules. Large magnetic interactions such as hyperfine couplings of α- or β-protons in ethyl or neopentyl radicals are not detectable in ESEEM spectra. , Simulations of the ESEEM spectra were done by the second-order perturbation theory described elsewhere. − The parameters for the simulation are summarized in Table . The periodic modulation on the envelope signals can be expressed as a function of the distances between the unpaired electron and surrounding matrix deuterons and the number of matrix deuterons.…”

“…The electron spin−echo (ESE) signals of neo-C 5 D 12 (98%)−C 2 H 6 (2%) mixture X-ray-irradiated at 4 or 77 K were measured with a X-band JEOL JES-PX1050 pulse-ESR spectrometer in Advanced Science Research Center of Japan Atomic Energy Research Institute at the temperature range 4−50 K. The ESE signals were observed by the π/2−π two-pulse sequences. − …”

“…The former information is very important for the intramolecular structure of the radical, and the latter one, the so-called matrix ENDOR, will determine the distances between the radical and matrix protons . ESEEM spectroscopy also detects magnetic dipole−dipole interactions between the radical and matrix protons or deuterons. − Since the ESEEM signals include both information about distances between the radical and matrix protons or deuterons and numbers of the matrix protons or deuterons, the number of matrix protons or deuterons from the radical at the same distance can be strictly determined by combining the results of matrix ENDOR. The purpose of this study is to elucidate the highly selective tunneling reaction in the solid neopentane−ethane mixtures in relation to the local structures around reactants and their local motion.…”

Influence of the 33 kDa Manganese-Stabilizing Protein on the Structure and Substrate Accessibility of the Oxygen-Evolving Complex of Photosystem II

“…ESEEM is a pulsed EPR technique that measures relatively weak magnetic coupling between an electron spin, here the one associated with the S 2 -state multiline signal of the Mn cluster, and nuclear spins of nearby ligand atoms out to a distance of ca. 5 Å. Three-pulse ESEEM utilizes a pulse sequence that leads to formation of a “stimulated” spin echo; unlike its two-pulse counterpart, this technique offers the advantage to suppress the modulation effect of an unwanted class of nuclei ( , ). We also performed a refined EXAFS analysis, using an improved technology compared to that of Cole et al (), to detect possible changes of the Mn cluster structure in the absence of the 33 kDa protein.…”