Pulsed ENDOR

Advances in the methodology have been reviewed [48]. Commercial equipment is available up to W-band, while pulsed ENDOR spectrometers at higher microwave frequencies have been constructed in specialized laboratories. Microwave frequencies higher than X-band are of advantage in applications where the amount of sample is limited. The high sensitivity at W-band (microwave frequency 95 GHz) 

Pulsed ENDOR complements ESEEM and conventional ENDOR in several respects  

---

Pulsed ENDOR offers several distinct advantages over conventional CW ENDOR spectroscopy. Since there is no MW power during the observation of the ESE, klystron noise is largely eliminated. Furthemiore, there is an additional advantage in that, unlike the case in conventional CW ENDOR spectroscopy, the detection of ENDOR spin echoes does not depend on a critical balance of the RE and MW powers and the various relaxation times. Consequently, the temperature is not such a critical parameter in pulsed ENDOR spectroscopy. Additionally the pulsed teclmique pemiits a study of transient radicals. 

Figure Bl.15.13. Pulsed ENDOR spectroscopy. (A) Top energy level diagram of an. S-/=i spin system (see also figure Bl,15,8(A)). The size of the filled circles represents the relative population of the four levels at different times during the (3+1) Davies ENDOR sequence (bottom). (B) The Mims ENDOR sequence.

More advanced pulsed teclmiques have also been developed. For a review of pulsed ENDOR techniques the reader is referred to [43, 44 and 45]. 

Davies E R 1974 A new pulse ENDOR technique Phys. Lett. 47 1-2... 

Grupp A and Mehring M 1990 Pulsed ENDOR spectroscopy in solids Modern Pulsed and Continuous-Wave Electron Spin Resonance ed L Kevan and M K Bowman (New York Wiley) ch 4, pp 195-229... 

Dinse K P 1989 Pulsed ENDOR Advanced EPR in Biology and Biochemistry ed A J Hoff (Amsterdam Elsevier) ch 17, pp 615-30... 

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

The high-resolution EPR spectra of the 17-electron complex (446) have been studied in combination with pulse ENDOR and ESEEM techniques.750... 

Use of CW ENDOR techniques to detect P-proton hyperfine couplings and matrix nuclei Pulsed ENDOR techniques to detect P-proton hyperfine couplings and matrix nuclei HYSCORE techniques to detect a-proton anisotropic coupling tensors... 

Advanced EPR techniques such as CW and pulsed ENDOR, electron spin-echo envelope modulation (ESEEM), and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) have been successfully used to examine complexation and electron transfer between carotenoids and the surrounding media in which the carotenoid is located. 

The Davies pulsed ENDOR spectrum of canthaxanthin oxidized on silica-alumina measured in the temperature range of 3.3-80K showed no lineshape changes, which is in agreement with previous 330 GHz EPR studies of canthaxanthin radical cations (Konovalova et al. 1999). This implies very rapid rotation of the methyl groups down to 3.3 K. 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

Konovalova, T. A., S. A. Dikanov et al. (2001a). Detection of anisotropic hyperfine components of chemically prepared carotenoid radical cations ID and 2D ESEEM and pulsed ENDOR study.. /. Phys. Chem. B 105 8361-8368. 

Lawrence, J., A. L. Focsan et al. (2008). Pulsed ENDOR studies of carotenoid oxidation in Cu(II)-substituted MCM-41 molecular sieves. J. Phys. Chem. B 112 1806-1819. 

A major limitation of CW double resonance methods is the sensitivity of the intensities of the transitions to the relative rates of spin relaxation processes. For that reason the peak intensities often convey little quantitative information about the numbers of spins involved and, in extreme cases, may be undetectable. This limitation can be especially severe for liquid samples where several relaxation pathways may have about the same rates. The situation is somewhat better in solids, especially at low temperatures, where some pathways are effectively frozen out. Fortunately, fewer limitations occur when pulsed radio and microwave fields are employed. In that case one can better adapt the excitation and detection timing to the rates of relaxation that are intrinsic to the sample.50 There are now several versions of pulsed ENDOR and other double resonance methods. Some of these methods also make it possible to separate in the time domain overlapping transitions that have different relaxation behavior, thereby improving the resolution of the spectrum. 

Fig. 9 Pulse ENDOR spectra of 1,3-dianthrylpropane [6b] at different temperatures.

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

Brecht, M., Stein, M., Trofanchuk, O., Lendzian, E, Bittl, R., Higuchi, Y. and Lubitz, W. (1998) Catalytic center of the [NiFe] hydrogenase A pulse ENDOR and ESEEM study. In D. Ziessow, W. Lubitz and E. Lendzian, (eds). Magnetic Resonance and Related Phenomena, TU Berlin, pp. 818-19. 

Figure 2A shows a pulsed ENDOR spectrum of an oxo-Cr(V) complex where the unpaired electron is interacting with a 1H nucleus with principal hyperfine values Hax = Hay = — 2 MHz and 1 a- 5 MHz. In this case, the isotropic hyperfine value is Haiso = (%+ % + V)/3 = 0.33 MHz ( 0.11x10 4 cm ), a value that is not resolved in the CW-EPR spectrum. The hyperfine contribution of this proton is, however, clearly resolved in the ENDOR spectrum. The ENDOR spectrum is centered around the proton Larmor frequency (vH), identifying the contribution as stemming from an interaction with a H nucleus. The principal values can be read directly from the spectrum, as indicated in Fig. 2A. 

The application of time-resolved pulse ENDOR to study the spin-polarized short-lived RP has been an important advancement. The feasibility of such... 

ESEEM experiments on Yp.398,399 Mino and Ono400,401 used a new technique, ELDOR-detected NMR, to measure the hyperfine structure of Yp in PS II. Except for slightly higher linewidths the spectrum was found to correspond with the one from pulse ENDOR. 

Lee et al used pulsed ENDOR (both Davies and Mims protocols) and ESEEM studies, at both X- and Q-band frequencies, of wild type and altered... 

---