ESEEM spectra

Identification of nitrogen donor ligands was possible from the pulsed ENDOR and ESEEM spectra of Hred from M. capsulatus (Bath)... 

FIGURE 9.3 Spectra of the mixture of canthaxanthin (2mM) and A1C13 (2mM) in CH2C12 measured at 60 K at the field B0=3349G and microwave frequency 9.3757 GHz (a) superimposed plot of a set of three-pulse ESEEM spectra as the modulus Fourier transform and (b) HYSCORE spectrum measured with a x=152ns. (From Konovalova, T.A., J. Phys. Chem. B, 105, 8361, 2001. With permission.)... 

Dikanov, S. A. and M. K. Bowman (1995). Cross-peak lineshape of two-dimensional ESEEM spectra in disordered S = l/2,1=1/2 spin systems. J. Magn. Reson. A 116 125-128. 

Fig. 6. ESEEM spectra of Ni in the F4j0-reducing and methyl viologen (MV)-reducing hydrogenases from M. thermoautotrophicum (A/f strain). Spectra (a) were obtained using a three-pulse stimulated-echo sequence, the time T between the second and third pulses being varied, (b) Fourier transform of FH2ase data (c) simulated spectra. Spectra are the average of recordings from g = 2.0 to 2.34. Reproduced, with permission, from Ref. 56.

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. 

Fig. 10. ESEEM spectra for TID and T2D Fet3p. The upper spectrum of the type 2 Cu(II) site (TID mutant) was collected at 2690 G with t = 260 ns. The lower spectrum of the type 1 Cu(II) site (T2D mutant) was collected at 2825 G with t = 250 ns. The measurement conditions were as follows microwave frequency, 8.80 GHz microwave power, 43 dB sample temperature, 4.2 K. The sample was 0.5 mM Fet3p in MES buffer, pH 6.0, containing 25% ethylene glycol (Aznar et al., 2002).

Fig. 11. Simulated and experimental two-pulse H20 (solvent water) ESEEM spectra. Theoretical ESEEM spectra for equatorial, axial, and ambient water are calculated as indicated. These can be compared to the experimental envelopes for the EetSp type 1 and type 2 Cu(II) sites (solid lines) and the simulations for these envelopes assuming for the type 1 copper, only ambient water, and for the type 2 copper, a combination of one equatorial, one axial, and ambient water (dotted lines) (Aznar et aL, 2002).

To determine whether PLP was actually associated with the lysine radical, [4 - H]PLP was synthesized and exchanged into the enzyme, and the [4 - H]PLP-enzyme was used to prepare a sample of the putative product radical 3. The EPR spectrum of the sample containing [4 - H]PLP proved to be identical with that of a matched sample containing PLP. The two samples were submitted to electron spin echo envelope modulation spectroscopy (ESEEM). The ESEEM spectra revealed a signal corresponding to the Larmor frequency for deuterium in the sample containing [4 - H]PLP (Fig. 5) and no signal in the PLP sample. This meant that the deuterium in [4 - H]PLP must be... 

The analysis of ESEEM data usually involves simulation of the ESEEM spectrum from a spin Hamiltonian that models the nuclear transition frequencies. Therefore it is important that ESEEM spectra accurately report on the... 

Figure 8 Three-pulse ESEEM data (a) and (c) and corresponding ESEEM spectra (b) and (d) for Fe(II)NO-TauD treated with aKG and taurine. ESEEM data were collected under the following conditions microwave frequency, 9.723 GHz magnetic field strength, (a) 171.0 mT and (c) 346mT 90° — r — 90° — T — 90° sequence with 16ns pulses r value, 136ns T increment, 16ns repetition rate, 1 kHz events averaged/time point, 100 scans, 4 and sample temperamre, 4.2 K...

Figme 10 shows the H-ESEEM spectra that result from repeating the measmement and processing procedme... 

Figure 10 The magnetic field dependence of H-ESEEM spectra (solid lines) obtained for Fe(II)NO-TauD samples treated with aKG and Ci- H-taurine using the ratio method described for Figure 9. The field positions displayed are (a) 171.0mT (b) 190.0 mT (c) 290.0 mT and (d) 346.0 mT. Simulations of these H-ESEEM spectra (dashed lines) are plotted along with the data. Hamiltonian parameters used for the simulations were principal g-values, 4.0, 4.0, 2.0 principal deuterium hyperfine values, —0.25, —0.25, 0.50 MHz Euler angles forhyperfine tensor, 0, 17°, 0 Q, 0.20 MHz ], 0 and Euler angles relating nqi to hyperfine, 0, 23°, 0...

For example, the one-electron models incorrectly predict (even at a qualitative level) the Knight shifts in and NMR spectra of ammoniated electron, and solvated electrons in amines (Sec. 4.1). The same problem arises in the explanation of magnetic (hyperfine) parameters obtained from ESEEM spectra of trapped (hydrated) electrons in low-temperature alkaline ices. The recent resonance Raman spectra of also appear to be incompatible with the one-... 

Using DFT calculations, it is possible to calculate hyperfine constants and then simulate H EPR and H ESEEM spectra of the The correspondence between such simulated and experimental spectra is very good, with all of the salient features discussed above reproduced. The residual discrepancy is for nuclei the calculated second moment M ) of the EPR for the 37% oxygen-17 enriched sample studied by Schlick et is 2250 vs. the reported experimental estimate of 134 G. This is not a failure of the particular DPT model all ab initio and DFT models of the give large estimates for... 

ESR-related spectroscopies that hold the potential to overcome some resolution limitations and yield more information than the classical ESR approach about the chemical environment of paramagnetic metal ions are the electron—nuclear double resonance (ENDOR) (Kevan and Kispert, 1976) and electron-spin echo envelope modulation (ESEEM) (Kevan and Schwartz, 1979) spectroscopies. Either ENDOR or ESEEM represents by principle a useful tool in extending resolution of the ESR experiment. However, the sensitivity of ENDOR and ESEEM is much lower than that of ESR, and interpretation of ENDOR and ESEEM spectra is not a simple matter, especially if ligands are not well characterized, as is the case for HSs. Both ENDOR and ESEEM techniques have not yet been applied to strictly metal-HS complexes, but the sensitivity and ease of carrying out experiments are improving rapidly, so major scientific activity may be anticipated to occur in this area of ESR spectroscopy. 

ENDOR studies on some anion adducts of SOD are available at low temperature. H3rperfine interactions of H and N with the Cu(II) site have been measured 248, 276). From these data it appears that at 4.2 K CN loosens the coupling with His-48, whereas little change occurs on N3 binding 248). ESEEM spectra of SOD with cyanide 250) indicate that the inhibitor substitutes one of the histidines with the larger hyperfine constant (His-46 or His-48), producing a square planar configuration with the other three histidines. 

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