Nitrogen hyperfine coupling

The electron spin resonance of the nitroxalkylcorrinoids can be readily observed in aqueous solution at room temperature. Both the cobalamin and cobinamide show nitrogen hyperfine coupling constants of 17.2 gauss. A typical spectrum is shown in Fig. 20. The line widths for the low, intermediate, and high field peaks are 1.87, 1.87, and 2.20... 

Purines. - A study of anhydrous deoxyadenosine (compound iv) single crystals x-irradiated (70 kGy) and investigated at 10 K found four base radicals (21-24) and one deoxyribose radical (25) (see p. 251).20 Radical 21, a de-protonated electron loss product, was stable to 100 K and readily photobleached at 10 K. Its nitrogen hyperfine coupling tensors were estimated by using values that resulted in acceptable EPR spectral simulations these were A(N10) = (1.76, 0, 0) mT, A(N3) = (0.99, 0, 0) mT and A(N1) = (0.34, 0, 0) mT. 

E° vs. NHE for the couple Ni /Ni measured by cyclic voltammetry carbon paste electrode, scan rate 100 mV s, pH — 9.6 in aqueous 0.1 M NaCI04. Values in italics are estimated values. Nitrogen hyperfine coupling constant in gauss. c E° measured with [NH3] = 1M. 

The importance of the different nodal properties of the two types of SOMO shown in Fig. 16 lies in their influence upon the nitrogen hyperfine coupling A(14N) in the electron spin resonance spectra in... 

To complete this section, mention will be made of the interesting radical H2CN-, recently identified in an argon matrix at 4-2°K (Cochran et al., 1962). This was formed by photolysing HI in the presence of HCN. The two protons have an isotropic hyperfine coupling of 87-4 G. The nitrogen hyperfine coupling was not completely documented. 

Fig. 3. First derivative electron spin resonance spectra. (A) ESR spectrum of an unpaired electron. (B) ESR spectrum of an unpaired electron interacting with a nitroxide resulting in a nitrogen hyperfine coupling constant aN. (C) ESR spectrum of an unpaired electron interacting with a H nucleus and a l4N nucleus as is typical for PBN radical adducts. (D) ESR spectrum of an unpaired electron interacting with the l3C nucleus, the H nucleus and the 14N nucleus of the trichloromethyl radical adduct of PBN, where the carbon tetrachloride was labeled with 13C.

In the actual experiments, EPR spectra are also recorded at every orientation, from which one can make good estimates of anisotropic nitrogen hyperfine couplings which are not normally detected in the ENDOR experiments. In most cases complicated single crystal (and even powder) EPR spectra can be faithfully reproduced with the accurate proton couplings obtained from the ENDOR experiments and the nitrogen hyperfine couplings obtained from the EPR spectra. Examples of these combined results will be presented. 

We have already discussed the effect of polarity on the nitrogen hyperfine coupling. Labels can be used to make statements about the hydrophobicity of a binding site. 

Nitrogen hyperfine coupling constants (xlO4, cm-1), n.o. not observed. Zero-held Mossbauer parameters, 80K. 

A corresponding solid-state study for a sulphonamidyl radical would be appropriate, but it seems reasonable to assume that such radicals adopt a structure not too dissimilar from 03SNH. Certainly, the ab initio calculations for MeS02NHMe indicate that a nN configuration in which the N—C bond subtends an angle of ca 120° to the S—N bond is the most stable. This corresponds nicely with the structure 11. Experimentally, the question as to whether sulphonamidyl radicals adopt the bent structure 12 or the linear structure 13 and 14 is resolved by comparison of the data for acyclic and cyclic radicals. The nitrogen hyperfine couplings for these types of radical are so similar that it can be concluded that both adopt the bent structure 12. 

TABLE 7. Isotropic and anisotropic components of the nitrogen hyperfine coupling, and the derived 2s and 2p orbital spin densities for 03SNH2 + a,ft... 

Hyperfine spectroscopy methods, such as ID ESEEM, 2D HYSCORE, and ENDOR, have been employed to determine the coordination sphere of Mn " ions bound to the HHRz or to the Diels-Alder ribozyme. The HHRz was first investigated by X-band stimulated echo ESEEM by Britt and coworkers [102]. The ESEEM data revealed nitrogen hyperfine coupling of 2.3 MHz,... 

Spin probes I and II are studied in the system CTAB-hexanol-water [3,21]. Spin probe I, with its nitroxyl radical quite distant from the polar headgroup, probes the nonpolar region of the interface whereas spin probe II, with the nitroxyl radical close to the polar headgroup, explores the polar region of the interface. From the heights (/i) and the linewidths (A//) of the different multiplets (due to the electron-nitrogen hyperfine coupling), one can compute the rotational correlation times (t ) of these spin labels [84] from the relationship... 

Tables Electron-Nitrogen Hyperfine Coupling Constants for Spin Probes I and II Dissolved in CTAB-Hexanol-Water Microemulsion...

Finally, the nitrogen hyperfine coupling (4, in gauss) is highly dependent on the medium polarity. The A values can therefore be used to characterize the local polarity in the microemulsions [66,68]. The A values of spin probes I and II (Table 8) show clearly that they are close to the hexanol value and are quite different from that obtained in water. The interface can therefore be assumed to be nonpolar, and no water penetration can be detected in our system. Note that even the nitroxyl group of spin probe II, although close to the polar headgroup, is not in direct interaction with the more polar interface. 

Spin Probes. There have been many ESR studies on spin probes in colloidal and biological systems. One of the stable spin probes often used is 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPOL Fig. 20-9). The spin probe has two ofnoteworthy advantages (1) nitrogen hyperfine coupling constant An is sensitive to solvent polarity, and (2) the rotational correlation times (tc) are functions of the solvent viscosity. Therefore, the ESR spectrum of TEMPOL provides information on the local polarity and viscosity of the microenvironment surrounding the TEMPOL probe. 

---