Hyperfine interactions electron spin echo envelope modulation

A large variety of hyperfine spectroscopy methods exist that allow the detection of hyperfine and nuclear quadrupole interactions electron spin-echo envelope modulation (ESEEM), ENDOR, and ELDOR-detected NMR (electron-electron doubleresonance detected nuclear magnetic resonance) [13]. Although there are cases in which ESEEM and ENDOR perform equally well, ESEEM-like methods tend to be... 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

Electron nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) are two of a variety of pulsed EPR techniques that are used to study paramagnetic metal centers in metalloenzymes. The techniques are discussed in Chapter 4 of reference la and will not be discussed in any detail here. The techniques can define electron-nuclear hyperfine interactions too small to be resolved within the natural width of the EPR line. For instance, as a paramagnetic transition metal center in a metalloprotein interacts with magnetic nuclei such as H, H, P, or these... 

Previous lower-frequency electron spin echo envelope modulation (ESEEM) studies showed a histidine nitrogen interaction with the Mn cluster in the S2 state, but the amplitude and resolution of the spectra were relatively poor at these low frequencies. With the intermediate frequency instruments we are much closer to the exact cancellation limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. We will report the results on 14N and 15N labeled PSII at these two frequencies, along with simulations constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral determination of the histidine ligation to the cluster in the S2 state. 

Wamcke, K., Babcock, G. T., and McCracken, J., 1994, Structure of the YD tyrosine radical in photosystem II as revealed by H electron spin echo envelope modulation (ESEEM) spectroscopic analysis of hydrogen hyperfine interactions. J. Am. Chem. Soc. 116 7332n 7340. 

DPPH = 2,2-diphenyl-1-picrylhydrazyl ENDOR= electron-nuclear double resonance EPR = electron paramagnetic resonance ESE = electron spin echoes ESEEM = electron spin echo envelope modulation EFT = fast fourier transformations FWHM = fidl width at half maximum HYSCORE = hyperfine sublevel correlation nqi = nuclear quadrupole interaction TauD = taurme/aKG dioxygenase TWTA = traveling wave tube amphfier ZFS = zero field sphtting. 

The following sections provide a more detailed description of the hyperfine interaction as measured by ENDOR spectroscopy, a description of ENDOR instrumentation, and the types of ENDOR experiments that can be performed. Finally, examples of the application of ENDOR spectroscopy to a variety of biomolecules are described. In this brief review many statements are made without reference for details the reader is referred to the variety of more extensive works for the theory of EPR and hyperfine interactions and reviews of applications of continuous wave (cw) and pulsed ENDOR and ESEEM (electron spin echo envelope modulation) techniques. ... 

Electron spin echo spectroscopy (ESE) monitors the spontaneous generation of microwave energy as a function of the timing of a specific excitation scheme, i.e. two or more short resonant microwave pulses. This is illustrated in Fig. 7. In a typical two-pulse excitation, the initial n/2 pulse places the spin system in a coherent state. Subsequently, the spin packets, each characterized by their own Larmor precession frequency m, start to dephase. A second rx-pulse at time r effectively reverses the time evolution of the spin packet magnetizations, i.e. the spin packets start to rephase, and an emission of microwave energy (the primary echo) occurs at time 2r. The echo ampHtude, as a fvmction of r, constitutes the ESE spectrum and relaxation processes lead to an irreversible loss of phase correlation. The characteristic time for the ampHtude decay is called the phase memory time T. This decay is often accompanied by a modulation of the echo amplitude, which is due to weak electron-nuclear hyperfine interactions. The analysis of the modulation frequencies and ampHtudes forms the basis of the electron spin echo envelope modulation spectroscopy (ESEEM). 

Hyperfine splitting due to interaction with ligand nuclei with 7 > 0 reflects the extent of spin delocalization onto neighboring atoms and can be used to characterize the types and numbers of such nuclei. In cases where these couplings are too small to be resolved in the EPR spectra, electron nuclear double resonance (ENDOR) or electron spin echo envelope modulation (ESEEM) can be used to measure the couplings as discussed in Chapter 2.3. Modern calculational tools are approaching the capabilities required to calculate g and A values from electronic wave functions. However, much of the spectroscopy that has been performed to date has used empirical correlations to interpret g and A values. 

Not all the information can be obtained by the basic CW experiment that is considered by many chemists as all there is to EPR. Elucidating geometric structure or small spin densities requires the separation of small hyperfine couplings or dipole-dipole couplings between electron spins from larger interactions. This can be achieved by double resonance experiments, such as electron nuclear double resonance (ENDOR) [8,9] and electron electron double resonance (ELDOR) spectroscopy and further pulse-EPR techniques [10] such as electron spin echo envelope modulation (ESEEM). Pulse-EPR techniques may also provide more information on dynamic processes than simple CW experiments and may access longer time scales. 

ENDOR techniques work rather poorly if the hyperfine interaction and the nuclear Zeeman interaction are of the same order of magnitude. In this situation, electron and nuclear spin states are mixed and formally forbidden transitions, in which both the electron and nuclear spin flip, become partially allowed. Oscillations with the frequency of nuclear transitions then show up in simple electron spin echo experiments. Although such electron spin echo envelope modulation (ESEEM) experiments are not strictly double-resonance techniques, they are treated in this chapter (Section 5) because of their close relation and complementarity to ENDOR. The ESEEM experiments allow for extensive manipulations of the nuclear spins and thus for a more detailed separation of interactions. From the multitude of such experiments, we select here combination-peak ESEEM and hyperfine sublevel correlation spectroscopy (HYSCORE), which can separate the anisotropic dipole-dipole part of the hyperfine coupling from the isotropic Fermi contact interaction. 

The pulse EPR methods discussed here for measuring nuclear transition frequencies can be classified into two categories. The first involves using electron nuclear double resonance (ENDOR) techniques where flie signal arises from the excitation of EPR and NMR transitions by microwave (m.w.) and radiofrequency (r.f) irradiation, respectively. In the second class of experiments, based on flic electron spin echo envelope modulation (ESEEM) effect, flic nuclear transition frequencies are indirectly measured by the creation and detection of electron or nuclear coherences using only m.w. pulses. No r.f irradiation is required. ENDOR and ESEEM spectra often give complementary information. ENDOR experiments are especially suited for measuring nuclear frequencies above approximately 5 MHz, and are often most sensitive when the hyperfine interaction in not very anisotropic. Conversely, anisotropic interactions are required for an ESEEM effect, and the technique can easily measure low nuclear frequencies. 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

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