Spin Concentrations

A second unusual EPR spectrum was observed in the oxidized (as-isolated) protein (Fig. 3). This spectrum, which was assigned to an S = z system, was not reminiscent of any Fe-S cluster. Indeed, with g-values of 1.968, 1.953, and 1.903, it looked more like a molybdenum or tungsten spectrum. However, chemical analysis ruled out the possibility that this EPR spectrum arose from Mo or W, and the spectrum was assigned to an Fe-S center instead. The spin concentration, however, was sub stoichiometric and sample-dependent. Furthermore, when the as-isolated protein was oxidized with ferricyanide, it became EPR silent. This, together with the iron determination and the fingerprint of the reduced protein, led Hagen and colleagues to the... 

Yang, A.-S. and B. J. Gaffney (1987). Determination of relative spin concentration in some high-spin ferric proteins using E/D distribution in electron paramagnetic resonance simulations. Biophys. J. 51 55-67. 

Experiments on catalysts often require the measurement of an absolute spin concentration. Although this is a somewhat tedious task, it can be carried out to a moderate degree of accuracy. The number of spins per gram of unknown n is given by... 

Although the error in the absolute spin concentration will vary from one type of study to another, a reasonable value is about 20%. The error in relative concentrations between samples having identical line shapes is probably closer to 2%. In the latter case the relative amplitude of the derivative spectrum may be used hence, the troublesome double integration may be avoided. 

Purify the modified oligo from unreacted crosslinker and reaction byproducts using a micro-spin concentrator with a membrane having a molecular weight cutoff able to retain the size of the DNA being modified. 

ESR Studies. Jackson and Wynne-Jones (9) studied the ESR spectra from the chars of a number of polymers and found a correlation between d.c. resistivity and the free electron spin concentration but no correlation with C/H ratio. The g-values they measured were observed to be quite close to that of the free electron, i.e. 2.0026 for cellulosic char vs 2.0023 for the free electron. They concluded that it was not possible to determine whether the observed free spin was due to a or -n electrons. 

In this work, only a brief study was made. The ESR spectra from BPA-PC and BPC-PC chars consisted of a single line and were quite similar to the signal reported for cellulosic char and to each other (Table IV), except that free spin concentration in the BPA-PC burn char was 15X greater than that from BPC-PC and 2.5X greater than observed in the BPA-PC 600° N2 pyrolysis char. It is too early in this study to make any conclusions about these results other than these chars contain significant concentrations of free radicals. 

Chemisorption of oxygen on char has often been discussed previously in terms of free radical concentration in the char (1.5,6). For cellulose chars Bradbury and Shafizadeh (1) found that free spin concentration reached a sharp maximum at HTT 550°C, coinciding with maximum CSA and drew the obvious conclusion that the extent of CSA was at least partly related to free radical content of the char. However, in subsequent work on cellulose char, DeGroot and Shafizadeh (3) have found that unpaired spin concentration continues to increase up to HTT 700"C. Ihe CSA of the char must therefore depend on factors other than free radical concentration. 

In [16e-h] it appears that, since the spin delocalization is considerable, the radicals must be kinetically less stable at sites other than the original radical centre where there is no steric protection. Therefore, it was difficult to obtain samples that contain satisfactory spin concentrations. In [16j] and [16k], by contrast, the spin distribution seems not to be extensive and so the kinetic stability of the radical centres must be well protected. 

Nakahara et al. (2) prepared radical polymeric agents, (III) and (IV), for use as active materials in anode layers to provide a stable secondary battery with a higher energy density and a large capacity. Radical agents prepared had a spin concentration of 1.0 x 1021 spins/g or more. 

Natural radiation, a-, (3- and y-rays from radioactive elements in the environment or intrinsic to materials, ionizes the materia] and produces paramagnetic defects or radicals. They are often quite stable and accumulate with time. The ESR signal intensity is proportional to the total dose of natural radiation, i.e., to the product between the annual radiation dose rate and the time elapsed after their formation or an event which zeroed the spin concentration. 

The purified Pseudomonad enzyme shows the ESR signal at g = 6, characteristic of a high-spin protohemoprotein. Upon the addition of the organic substrate tryptophan to the enzyme, the signal becomes narrow and sharp, indicating that the axial symmetry of the heme increases while the total spin concentration due to the heme iron is unchanged177. These results indicate that the organic substrate binds with... 

Spectra. All EPR measurements were made with a Varian 100 kc. modulation spectrometer. Spin concentrations were estimated by comparison with solid diphenylpicrylhvcurazyl. Numbers of radicals were assumed to be proportional to signal height times width squared. Temperature dependence plots were obtained by controlling the temperature of the resonance cavity with cooled or heated nitrogen. Signal intensities of the temperature plot were estimated by comparing with the intensity of the sample at 25°C. 

Samples made under air and under nitrogen showed little difference in spin concentration. 

Samples show high spin concentration when placed in aqueous base. 

The magnitude of the spin concentration would rule out trace oxygen effects. 

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