W-band EPR spectra

FIGURE 3.8 W-band EPR spectra of aligned single crystals of [Fe4(OCH3)6(dpm)6] at 5 K (only the S = 5 ground state is populated). Inset coupling scheme of the four Fe(III) (s = 5/2) centers. Additional smaller effects and structural isomers cause the spectrum to be more complex than anticipated.17... 

Observation of the stereoselective manner of chiral substrates binding to these asymmetric metal-salen complexes was not confined to [VO(l,3)] or chiral epoxides. Recently we showed how asymmetric copper salen complexes, [Cu(l)] and [Cu(4)] (Fig. 1), could also discriminate between chiral amines (R-IS-methylbenzylamine, MBA) as evidenced by multi-frequency CW and pulsed EPR, ENDOR, HYSCORE and DPT [45]. The discrimination of the MBA enantiomers was directly observed by W-band EPR. By simulating the W-band EPR spectra of the individual diastereomeric adduct pairs (i.e. R,R -[Cu(4)]+R-MBA and R,/ -[Cu(4)]-l-5-MBA), accurate spin-Hamiltonian parameters could be extracted for each adduct. The EPR spectmm of the racemic combinations (i.e. ra -[Cu(4)]+rac-MBA) was then simulated using a linear combination of the g/A parameters for the homochiral (R,R -[Cu(4)]+R-MBA) and heterochiral (R,R -[Cu... 

Fig. 4.10 X-band epr spectra of Cu(mnt)2 (A) and Cu(mnt)(Et2dsc) (B) at 25 °C. The reaction is monitored at the field strengths indicated by the arrows. Full details for the mixing and accumulation of epr signals are given in Ref. 163. Reproduced with permission from J. Stach, R. Kironse, W. Dietzsch,...

The X band EPR spectra show a very complicated signal from 500 G to 6000 G. The spectra are fairly comparable to these observed for the [(Febpy)20](S04)2.5H20 and [(Fephen)20](S04)2.5H20 complexes [3]. Interpretation of such coupled Fe d system remains complex. Therefor a W-band EPR study of the complexes was performed. X- and W-band measurements were recorded at 293K. Below 80K, only the S =0 state is significantly populated, the complexes are EPR inactive and create a diamagnetic matrix. 

Figure 1. X-band EPR spectra (77 K) of 0.05 M solution of complex 1 in CHiCK (a) in CH2CI2 containing IM of NMO (b, c) spectrum of Mn"(saten) precursor of complex 2 in DMSO (d) spectrum of (salcn)Mn 0 complex recorded 1 min after reading complex I with one equivalent of/w-CPBA at 0 C(c) [49]. Spectrometer frequency 9.3 GHz, microwave power 40 mW, modulation frequency 100 kHz, modulation amplitude 20 G, gain 2.5xl0 (a-c), 1.0x10 (d), 2.5x10 (c).

Figure 7. (Left) X-band EPR spectra of the Rhus vernicifera tree laccase (a) the resting oxidized form at 2.02 mW and 77 K, (b) the native intermediate at 2.02 mW and 77 K, and (c) the native intermediate taken at 0.5-25 W at 10 K. (Right) Energy diagram of the ground and low-lying doublet states of the native intermediate, with depiction of the origin of the low g-value observed in (c) see text for details. Spectra adapted from [22],...

Figure 3 Comparison of 9.5 and 94.4 GHz EPR spectra of iso-1-cythochrome c from Saccharomyces cerevisiae labeled at native cysteine 102 with MTSSL illustrates an enhanced sensitivity of W-band EPR line shape to changes in local protein dynamics upon protein denaturing. The spectra at each of the frequencies were taken at room temperature and are normalized by the value of the double integral. Protein was denatured by addition of 2 M of guanidine hydrochloride (Pierce, Illinois). Protein concentration was 0.2 mM in a HEPES buffer at pH 6.5 (Smirnov et al, in preparation)...

EPR spectra were recorded with a Varian E9 X-band spectrometer using field (100 kHz) and light (13 or 83 Hz) modulation with phase-sensitive detection at the modulation frequencies (19). Typically, the field modulation amplitude employed ranged from 20 to 40 gauss, the microwave power from 0.1 to 0.5 mW. Measurements were performed on frozen solutions of the porphyrins at about 100 K using the standard Varian variable temperature accessory or at about 10 R with an Oxford Instruments helium gas cryostat. Light sources used for photoexcitation were a 1000 W Xe arc source powered by a Photochemical Research Associates Supply with electronic modulation... 

Fig. 4 Schematic illustration of the coordinated Co -phenoxyl radical, bearing coordinated acetate groups, derived from [Co(l)] after addition of acetic acid under aerobic conditions. Top (a, c) the X- and W-band CW EPR spectra of [Co°(r)(OAc) ](OAc) (n = m = 1 or n = 2, m = G) and (b) the X-band CW-EPR spectrum of [Co (l )(Py)2]. Bottom the DFT-computed spin densities of [Co(l )(OAc)] shown from the side and top elevation. Blue is positive spin density, green represents negative spin density. Adapted and reprinted with permission from [85]. Copyright 2011 American Chemical Society...

Equation (1) also nicely shows that the major advantage of performing high-field/high-frequency EPR, e.g., going to W-band ( 94 GHz) frequencies, is the improved g-resolution, while the hyperfine resolution remains unaltered. It thus becomes possible to separate contributions to the spectra from electron-Zeeman and hyperfine anisotropies. 

A detailed polarity analysis can be performed on membrane proteins at cryogenic temperatures extracting the and parameters from low temperature W-band CW EPR spectra. In the temperature regime below 200 K the reorientational correlation time of an otherwise unrestricted spin label side chain exceeds 100 ns,... 

Historically, the vast majority of EPR experiments have been performed at a microwave frequency between about 9 and 9.5 GHz, which falls in the range that is called X-band. At this microwave frequency the free electron g value corresponds to a resonant field of about 3,300 G (330 mT). Relatively recently, commercially available spectrometers have become available over a widening range of frequencies currently about 1 GHz (L-band) to 95 GHz (W-band). It now becomes important to consider what EPR frequency is optimum to answer a particular question. For some questions, the clearest answers are obtained by comparing spectra as a function of microwave frequency. ... 

To try to understand the free radical line, Arabian crude oil (Arabian Light Crude Oil) and Colombian crude oil (Cusiana Crude oil) were studied by EPR in X- (9 GHz), Q- (34 GHz), and W- bands (34 GHz). The spectra obtained at different frequencies are shown in Fig. 6 and Fig. 7. 

Fig. 6. Free radical EPR spectra of Arabian crude oil at room temperature obtained in a X-band b Q- band c W- band Af/j /2 is the half height separation of the EPR derivative peak and Af/pp is the separation of the EPR derivative peak (Di Mauro et al., 2005).

Asymmetrical Hnes of the free radical were observed in all EPR spectra (Figs. 6 and 7). However, asymmetry was more pronormced in the spectra obtained in the W- band (Figs. 6c and 7c). The as3rmmetry in the line is due to the superposition of all the possible orientations of the paramagnetic species in the system and to the contributions of different chemical species that interact with the unpaired electron. 

Fig. 8. Line width AH of the free radical signal versus microwave frequency of EPR spectra recorded in the X-, Q- and W- bands at room temperature. , AHpp (Arabian petroleum) , AHpp (Colombian petroleum) A, AHy2 (Arabian petroleum) T, AH /2 (Colombian petroleum).

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