Determination paramagnetic effects

Paramagnetic effects of metals such as those observed here with Gd + on the Ca +-ATPase can be used to determine distances, r, between the metal and pertinent nuclei or, if r is known, to determine g, the number of nuclei involved in the interaction with the metal (11). Thus, the water relaxation data described here can be used to determine the number of exchangeable water protons on Gd3+ at the two Ca + sites on the ATPase. As shown in Table II, the value of q appears to be distinctly different at the two Gd3+ sites. Calculations yield three exchangeable water protons on Gd3+ at site 1 and two exchangeable protons at site 2. 

Distances between the nitroxide of bound steroid and several assigned protons in or near the active site could be determined on the basis of the paramagnetic effects of the nitroxide on the relaxation rates of the active site protons. These distances could then be used to dock the structure of the spin-labeled steroid into a partially refined 2.5-A resolution X-ray structure of the isomerase 128). However, in order to rationalize the chemical evidence that Asp-38 is involved in the catalytic mechanism, it was necessary to assume that steroid substrates and the spin-labeled steroid (16) are bound not only with reversal of the C-3 and C-17 positions, but also with reversal of the planes of the steroid ring systems ( up-sidedown binding), positioning Asp-38 above the C-4j8 and C-6j3 protons. 

Relaxation Times, Paramagnetic Effects, and N.Q.R. Studies.—A study of the relaxation times of phosphoryl compounds at two magnetic fields, and of the dependence of spin rotation and dipolar interactions upon viscosity and temperature, led to the approximate separation of dipole-dipole, anisotropy, and spin-rotation interactions, and indicated that second-order paramagnetic shielding was dominant. The P relaxation times 7i and Tz were determined for several lipid-water phases. Comparisons of changes of Tg which occur at the transition temperature for dipalmitoyl-lecithin indicated that the relaxation times reflect the mobility of the lipid head-group. ... 

Phonon mode changes, determination of Debye temperature, determination of effective mass, determination of anisotropy of lattice vibration, electron hopping in spinels, diffusion, determination of jump frequency and diffusion coefficient, paramagnetic spin relaxation, spin-spin relaxation, spin-lattice relaxation, superparamagnetism, determination of grain size... 

Here /3 = ehUm and Hmag gives the orbital paramagnetism of free atoms in states with non-zero angular momentum, but in second order also contributes to diamagnetism, reducing the main contribution which arises from H mag it also determines other effects, as will be evident shortly. 

Finally, paramagnetic relaxation was used in order to determine the effects of metal ion association with amyloid fibrils. Several specific sites of Cu were detected in the amyloid protein AjS(l O) using PRE and shift perturbation data firom solid-state NMR, and corroborated using detailed molecular dynamics (MD) models. It was also found by this means that the amyloid fibril structure is not significantly altered by Cu binding. This smdy is particularly interesting on account of its medical pertinence, as Cu° is often found at elevated concentration in cells containing Alzheimers plaques [103]. 

To b in with, the major criterion that has to be met is that there be a believable paramagnetic effect on T, and 7 2. With modem Fourier-trans-form NMR spectrometers, T, can usually be measured to better than 10% accuracy. Honesty is required of the experimentalist because the quality of the data depends on the signal-to-noise ratio of the individual NMR p iks and how many data points (Mildvan and Gupta, 1977) are taken to determine T,. Realistically, a 50% difference in T, is a believable change, whereas larger differences are earily measured. 

The chapter Electron Spin Resonance in Catalysis by Lunsford was prompted by the extensive activity in this field since the publication of an article on a similar subject in Volume 12 of this serial publication. This chapter is limited to paramagnetic species that are reasonably well defined by means of their spectra. It contains applications of ESR technique to the study of adsorbed atoms and molecules, and also to the evaluation of surface effects. The application of ESR to the determination of the state of transition metal ions in catalytic reactions is also discussed. 

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