Interaction of spin labels with transition metals: Part 2

Abstract During the years since our previous review there has been an increasing tendency for researchers to recognize the existence of electron-electron spin-spin interactions, and to consider the possibility of both dipolar and exchange contribution to the spin-spin interaction. Despite the well-documented need, few of the studies of spin-spin interaction have included quantitation of the EPR signals or demonstration that the signal examined was due to the species of interest, few systematic studies of the factors that determine the magnitude of the exchange interaction have been carried out, and the equations most frequently used to obtain interspin distances have not been calibrated for the spin systems to which they have been applied. Paramagnetic metals have been coordinated to about 130 spin-labeled ligands: the majority of these studies have used Cu(II). Some studies have examined other metals with relatively slow electron spin relaxation rates including Ag(II), VO(IV), low spin Co(II), and Cr(III). Unfortunately in some papers it is not stated whether the EPR spectra were obtained on dilute solutions or magnetically concentrated solids. Most of the work on discrete complexes, other than from our laboratory, has been on systems in which the exchange interaction is large relative to the nuclear hyperfine splittings and the g value difference. The resulting averaged g value and reduced metal hyperfine splitting have been recognized for about 30 spin-labeled Cu(II) or vanadyl complexes. In some reports it was suggested that averaged g and reduced A values for spin-labeled copper complexes in fluid solution indicated that the nitroxyl oxygen was coordinated to the metal. Coordination of the nitroxyl oxygen gives ferromagnetic or antiferromagnetic interaction that is large enough to measure by magnetic susceptibility. In these complexes there is no signal from the singlet state and incomplete motional averaging of the large zero-field splitting of the triplet state causes severe broadening of the fluid solution spectra. Therefore it is probable that spectra in which average g and A values are observed are due to complexation of the ligand via atoms other than the nitroxyl oxygen. Studies of resolved AB splittings have examined the factors that contribute to the value of the exchange coupling constant, J . Further studies are needed to improve our understanding of the relationship of the bond pathway between the two paramagnetic centers to the magnitude of the exchange interaction. Complexes with more rapidly relaxing transition metals have not been studied extensively, but are likely to receive greater attention in the next few years as pulsed EPR techniques become accessible to more researchers. Diverse geometries have been observed for complexes in which the nitroxyl oxygen is coordinated to a transition metal. Further studies in this area are needed in order to define the relationship between geometry and the magnitude of the exchange interaction. Collision broadening of nitroxyl radical EPR spectra by paramagnetic metal ions is often stated to be due to either exchange or dipolar interactions without any indication of the basis for the judgement. Most of the efforts to understand the relative importance of the two contributions are in the Russian literature, in which it is shown that the importance of the dipolar contribution depends strongly on viscosity and metal ion. Collision interactions between paramagnetic transition metals and nitroxyl radicals have been widely used in biochemical studies to (a) distinguish between nitroxyl radicals in environments that are accessible to bulk solution and environments that are not accessible and (b) to broaden the signal from nitroxyl in bulk solution thereby facilitating study of the signal from nitroxyl that is not accessible to the bulk solution. Efforts are underway to estimate interspin distances from the effect of metal ions on nitroxyl relaxation rates. To date this work has used continuous wave EPR. Pulsed EPR techniques are likely to facilitate these studies. A disconcertingly large fraction of the applications of the Leigh theory to the measurement of metal-nitroxyl and metal-metal distances in biological systems do not pay sufficient attention to the fundamental assumptions of the model. The applications to copper-nitroxyl interactions are questionable since copper(II) relaxation rates are generally quite long. Greater attention needs to be paid to questions of chemical reversibility, quantitation of the EPR signal, and interference from small amounts of nitroxyl that are not interacting with the paramagnetic metal. More accurate measurements of electron spin relaxation rates are urgently needed to be included in these calculations. Some distance measurements have been made from changes in relaxation behavior based on continuous wave power saturation curves which yield a value for the product T 1 T 2 . Pulsed EPR measurements of relaxation time changes have not yet been applied to these estimates. Where compared, the several distance measurement techniques are in reasonable agreement, though there are some discrepancies. However, these comparisons may not reveal substantial inadequacies in the models. If the experimentally obtained parameter depends on r to the sixth power, an error by as much as a factor of 2 in an input only causes the value of r to change by 12%. Most of the distance measurements discussed in Section E rely upon the assumption that the spin-spin interaction is exclusively dipolar. The growing body of data discussed in Section B indicates that exchange interaction can be a significant factor in spin-spin interactions in a wide range of systems. This information should cause a greater use of distance measurement techniques that take into account both dipolar and exchange interactions, i.e., measurements of the intensity of half-field transitions and analysis of resolved splittings.