Simulation of ENDOR spectra

Several factors unique for ENDOR affect the intensities, i.e. magnetic relaxation, hyperfine enhancement, and angular selection. The two first effects also affect spectra of liquid and crystalline samples, while the third is typical for powder spectra of species with anisotropic g-values. Methods that take the two latter effects into account have been developed and are usually incorporated in software developed for the simulation of ENDOR spectra in the solid state. Simulations that take magnetic relaxation effects into account have been employed only to analyse ENDOR spectra in the liquid state [2]. It is possible that the commonly observed poor agreement between experimental and simulated intensities in the solid state is at least in part due to relaxation effects that are not taken into account in any software we are aware of. 

Experimental remedies to obtain these numbers are the TRIPLE CW and pulsed ENDOR methods described in this chapter. In some cases the number of equivalent nuclei belonging to a particular hyperfine coupling constant can be determined visually from a sufficiently resolved ESR spectrum. Thus, the main structure with or = 22.18 MHz in the ESR spectrum of the 9,10-dimethylanthracene cation radical in Fig. 2.3 is a septet due to the six equivalent of the methyl groups. Assignment of the couplings of the ring protons is often made, experimentally by selective deutera-tion, and theoretically by quantum chemistry. Simulations are particularly employed for the interpretation of ENDOR spectra of disordered solids discussed later in this chapter and in Chapter 3. 

Technical procedures to simulate powder ENDOR spectra are given in Appendix A3.3. Simulation is employed in several applications, of which some are exemplified below. 

F. 3.27 (a) First-derivative X-band ESR spectrum from a polycrystaUine sample of tiltmine. X-irradiated at 295 K and measured at 221 K. The arrows indicate field positions of ENDOR spectra in (b) and (c). (b) Experimental (top) and simulated powder ENDOR spectrum due to radical R1 at 221 K. The experimental spectrum was obtained by saturating the central ESR line at arrow b in (a), (c) Experimental (top) and simulated powder ENDOR spectrum due to radical R2 at 221 K. The experimental spectrum was obteiined by saturating the ESR line at arrow c in (a). The figure is reproduced from [12] vidth permission from Springer... 

Fig. 2. The simulated line shape of ENDOR spectra of the multiline in the samples, EG-H(a), EG-D(b) and GS-H(c) by using the best fitted values of distances and intensities given in Table 1 and the line width of about 110 kHz. Predicted ENDOR separations 79.0/r(A) MHz using the distances in Table 1 are indicated by symbols, a-a to f-f ...

Figure 10. The conventional multi-frequency approach to ENDOR/ESEEM by recording spectra at discrete spectrometer operating frequencies in two or more microwave ovtaves. These data represent simulated ESEEM/ENDOR spectra of an S=l/2,1=1 system using the hyperrfine parameters e Qq=1.6 MHz, t =0.45, and Ais = 4.0 MHz. Top and bottom spectra correspond to nuclear Zeeman energies above and below the ideal exact cancellation condition (center spectrum). The simphfied exact cancellation spectrum makes it easy to assign peaks to transitions (cf. Mims Peisach, 1978), but peak mobihty makes it difficult to assign numerical values to the hyperfine parameters based on a single spectrum.

Figure 6. Solid traces 1 and 2, experimental two-pulse ESE field sweep speetra of MRI contrast agent MS-325 and Gd aqua complex, respectively. Experimental cmiditions v = 130.2 GHz time interval r between the mw pulses, 300 ns durations of the mw pulses, 2 x 100 ns temperature, 8 K. Dashed traces 1 and 2, spectra simulated with the parameters Dm and AD given in Table 2 for MS-325 and Gd aqua complex, respectively. Arrows show field positions (A and B) typical for acquisition of ENDOR spectra.

The ESR spectrum of C6H6 " trapped in CFCI3 at 15 K is shown in Figure la and agrees with that reported previously [18]. The principal values of the hyperfine coupling were obtained from previous ESR and ENDOR measurements [17, 18]. The best agreement with experiment was obtained with the axes oriented as in Table 4. In the latter study, the simulated ENDOR spectra were insensitive to the orientation of the tensor axes, however, and the assignment was made on the basis of molecular orbital calculations [9]. The tensor data are reproduced here for convenience (see Table 4). 

Carotenoid neutral radicals are also formed under irradiation of carotenoids inside molecular sieves. Davies and Mims ENDOR spectra of lutein (Lut) radicals in Cu-MCM-41 were recorded and then compared with the simulated spectra using the isotropic and anisotropic hfcs predicted by DFT. The simulation of lutein radical cation, Lut +, generated the Mims ENDOR spectrum in Figure 9.7a. Its features at B through E could not account for the experimental spectrum by themselves, so contribution from different neutral radicals whose features coincided with those of the experimental... 

This review has looked at the direct effects of ionizing radiation on nucleic acids. The first step was to review detailed EPR/ENDOR experiments on irradiated model compounds at low temperatures in order to study the primary radiation-induced defects. Next, it was shown how these EPR spectra are used to simulate the EPR spectra of the DNA polymer. 

The discussion in Section 18.5 on simulations of the EPR spectra of DNA mentioned room for improvements. It would be very interesting to add new structures discussed herein to the simulations. New simulations should include sugar radicals, accurate hyperfine couplings from the EPR/ENDOR studies, perhaps an adenine oxidation product, and the oxidation product in 5-MeCytosine. Some of the DNA simulations Huttermann and co-workers performed included the thymine allyl radical [23], This assignment seemed improbable at the time since oxidation of thymine is not expected in DNA. It would be interesting to know if this allyl component used in the simulations might actually be from an oxidized 5-MeCytosine. 

Fig. 8a-d. 14N ENDOR spectra of VOTPP in ZnTPP recorded at 100 K. Perpendicular (a) and parallel (b) experimental spectra. Spectra (c) and (d) are computer simulations. Reproduced with permission from Ref. 59... 

Figure 13 35 GHz CW ENDOR (upper set of spectra) and ReMims (Doan) ENDOR (lower set of spectra) of a-V70A-FeMo-cofactor under turnover conditions with propar-gyl alcohol (PA) labeled at C3 H 3C=C-CH20H. The ENDOR spectra were recorded at field positions (gobs values as indicated) across the EPR envelope of the PA intermediate, which has g = [2.123, 1.998, 1.986]. Spectra are centered at the Lar-mor frequency = 13.4 MHz at g = 2.0). Simulations are also shown as dashed lines for features of interest. (Adapted from Figure 4 in Lee, Igarashi, Laryukhin, Doan, Dos Santos, Dean, Seefeldt and Hoffman. Reprinted with permission, 2004 American Chemical Society)...

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