EPR absorption derivative

Figure 1. EPR absorption derivative spectrum of aconitase (as isolated from beef heart mitochondria) in 100 mM potassium phosphate, pH 7.0. Experimental conditions for obtaining the EPR spectrum were 10 K, 100 microwatts power, 0.8 milliTesla (mT) modulation amplitude, and 9.42 GHz microwave frequency.

Figure 7. EPR absorption derivative spectra of photoreduced active aconitase. Enzyme ( 5 mg/ml) in 100 mM Hepes, pH 7.5, plus 5 pM deazaflavin and 10 mM potassium oxalate was photoreduced in the presence of either A) 10 mM tricarb-allylate, B) 1 mM citrate, or C) 10 mM rrans-aconitate. The numbers above each spectrum are the g-values of prominent features. Experimental conditions for obtaining EPR spectra were 13 K, 1 milliwatt microwave power, 0.8 mT modulation amplitude, and 9.24 GHz microwave frequency.

Figure 10. Inset, EPR absorption derivative spectnim of photoreduced active aconitase in the presence of citrate. Field positions at which ENDOR spectra were recorded are uvhcated by arrows. Numbm at the top are from the g-vdue scale. Main figure, 1 0 ENDOR spectrum at g = 1.88 for photoreduced active aconitase in the presence of citrate and 38% enriched (Reproduced with permission...

Figure 11. The reduction of dihistidine hemichrome of hemoglobin H as studied by EPR The features of the EPR absorption derivative, labelled gx, gy, and gt, arise from the hemichrome. The feature at g = 2 is the high field end of the EPR spectrum of the high-spin ferric protein not yet converted to hemichrome.

Figure Bl.15.6. The EPR spectrum of tire perinaphthenyl radical in mineral oil taken at room temperature. (A) First derivative of the EPR absorption x with respect to the external magnetic field, Bq. (B) Integrated EPR spectrum.

FIGURE 5.4 Anisotropy in absorption and derivative powder-type ERP spectra. (A) axial intensity pattern (B) axial EPR absorption (C) axial EPR derivative (D) rhombic EPR absorption (E) rhombic EPR derivative (F) the spectrum of horse heart cytochrome c, a rhombic EPR derivative with anisotropic broadening (Hagen 2006). (Reproduced by permission of The Royal Society of Chemistry.)... 

FIGURE 6.1 Integration of an EPR spectrum. The EPR derivative spectrum of the hydrated copper ion (trace A) is numerically integrated to its EPR absorption spectrum (trace B) and a second time integrated (trace C) to obtain the area under the absorption spectrum. Note that both the derivative and the absorption spectrum start and end at zero, while the doubly integrated spectrum levels off to a constant value the second-integral value. 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

Figure 3.11 (A) Typical EPR absorption curve. (B) First derivative EPR absorption curve.

The second thing that EPR can tell us is concentration. As with all forms of spectroscopy, the intensity of an EPR absorption peak is proportional to the concentration of the analyte giving rise to this absorption. (Care - because we record an EPR spectrum as a first derivative, the concentration of analyte is in fact proportional to the integral of the EPR peak height.) However, EPR is so powerful a technique that to determine a concentration alone would be wasteful. 

The size of this iris determines the refiection coefficient of the cavity. The iris size needed for zero reflection coefficient, i.e., a matched cavity, depends on the dielectric and conductivity losses of the sample in the cavity and is adjusted by means of a post which partially extends across the iris. By adjustment of the position of this post, the cavity can generally be matched to the bridge. By means of the slide screw tuner of Fig. 21, the bridge is slightly unbalanced as in the rf circuitry of NMR. EPR absorption is then observed in ways similar to NMR by display of the EPR line on a CRO or by rectification of the AC components and display of the first derivative of the EPR signal on a graphic recorder. 

Fig. 29. First derivative of EPR absorption of 0.55 wt. % chromium on alumina reduced in hydrogen at 500°. y = 9.47 kmc./second (,173).

Figure 16.1. (a) Simplified scheme of EPR phenomenon, showing the energy-level splitting (Zeeman effect) for the electron spin S = 1/2 (Ms = +1/2) as a function of applied magnetic field (H), (b) the EPR absorption line, and (c) first derivative of absorption line, indicating the g value and line width (AH), normally detected in the EPR spectra. 

The conducting phase of TMQ /16/. Microwave conductivity experiments, performed at low temperature on the samples used for the pressure experiment, have succeeded in showing an increase by a factor 10 from 300 K down to loo K, /41/. The possibility of susceptibility measurement via low field method is limited by the EPR line broadening occurring at low temperature and under pressure. The spin susceptibility was derived from a fit of the EPR absorption line shape with a Lorentzian curve. The proton relaxation time was measured under pressure at low field lOe- with a pulse spectrometer. Figures... 

Let us now examine some real EPR spectra taken from porphyrin samples. Hemin chloride dissolved in N,N-dimethylformamide gives the EPR spectra shown in Figure 2. The lower curve (absorption) extends from g = 6-2 and the upper curve (absorption derivative) excursion is very large near g = 6 but barely discemable at g = 2. As far as one can tell from such a spectrum, the system is axial that is, gp and gy are both equal to 6, while gz is equal to 2. One can easily convert the hemin to a low-spin compound by adding ligands which will replace chloride. One such hgand is mercaptoethanol. By adding mercaptoethanol to this... 

In this technique, the ENDOR response is observed as changes in the EPR absorption-mode (in-phase) signal as an rf source is swept. Commonly the EPR signal is detected without field modulation, but the rf frequency is modulated at frequencies around 10 kHz. The signal is decoded at this modulation frequency by a phase-sensitive detector. As a consequence of this scheme, the ENDOR signal appears as the derivative display. The standard commercially available spectrometer is set up in this fashion. In this type of ENDOR experiment, the degree of desaturation of the EPR signal depends on the complicated interplay of all relaxation... 

Fig. 1. Angular dependence (a) of the resonance field and (b) of the linewidth Af/ in La,923Sro(,5Cu04 at 70K by rotating the c-axis with respect to the magnetic field. A purely axial symmetric behavior is indicated by the solid lines. The inset shows the absorption derivative indicating a Lorentzian lineshape of the EPR spectrum for the crystal orientation c H. From Kochdaev et al. (1997).

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

---