Electron spin resonance studies of anisotropic rotational reorientation and slow tumbling in liquid and frozen media. III. Perdeuterated 2,2,6,6-tetramethyl-4-piperidone N-oxide and an analysis of fluctuating torques

Abstract

Publication costs assisted by the Petroleum Research Fund A detailed analysis of line shapes and relaxation is made for the perdeuterated 2,2,6,6-tetramethyl-4-piperidone AT-oxide (PD-Tempone) nitroxide radical covering the whole range from fast motional narrowing (tr ~10-12 sec) to the rigid limit (tr ~10-6 sec) in several deuterated solvents. The slow tumbling resultsshow particularly good agreement with the slow tumbling theory of Freed, et ai, for a reorientational model of moderate jumps (ca. 50°r ms), and essentially isotropic reorientation. It is shown that while such a model may not be readily distinguished, purely from its slow tumbling spectral predictions, from a free diffusion model including inertial effects, the latter model is incompatible with most other considerations. However, a simple analysis explicitly including the fluctuating (or random) torques indicates that more fundamental analysis of the dynamics may explain the slow tumbling results without necessarily involving substantial jumps. The spectral analysis is considerably enhanced by the increased resolution obtained from the use of the perdeuterated spin probe and deuterated solvents. Careful analysis of results at X-band and 35 GHz has shown that the nonsecular spectral densities exhibit significant deviations from a Debyelike spectral density yielding results very similar to those recently reported for the peroxylaminedisulfonate (PADS) radical. Related observations are discussed of apparent non-Debye-like spectral densities in the incipient slow tumbling spectra. These anomalies are also found to be amenable to a unified explanation in terms of the fluctuating torques. The analysis of much of the results in terms of a simple model yields an rms value for the fluctuating torques of ca, V6kT and tm ~tr where tm is the relaxation time of the torques. The high-temperature electron-spin flip processes are consistent with a Hubbard-type spinrotational mechanism, but the low-temperature results may be due to spin-rotational relaxation from intramolecular motions. The simple analysis of spin-rotational relaxation in terms of relaxation of the fluctuating torques is found to yield equivalent predictions to conventional treatments when the estimated values of rms torque and tm are used.

Structural and motional changes in glyceraldehyde-3-phosphate dehydrogenase upon binding to the band-3 protein of the erythrocyte membrane examined with [15N,2H]maleimide spin label and electron paramagnetic resonance.

“…In this approach, the stochastic Liouville equation for the spindensity matrix was modified to consider the effects of Brownian anisotropic rotational diffusion upon EPR and ST-EPR lineshapes. This treatment was similar to that reported by Freed et al (17), who also found that meaningful simulation of experimental spectra was greatly facilitated by perdeuteration of the spin label 2,2,6,6-tetramethyl-4-piperidone-l-oxyl (PDtempone) (18 All incubations were at 2°C.…”

Exchangeable Cation and Solvent Effects Upon the Interlamellar Environment of Smectites: Esr Spin Probe Studies

“…The A and z values determined for the II and I orientations for the hydrated systems ( Table 1) are indicative that the probe is tumbling more rapidly about one molecular axis than about others. In isotropic media, asymmetry in tumbling is expected because of the axially symmetric ellipsoid shape of the probe (Hwang et al, 1975); however, * Broad X-ray diffraction peak representing random interstratification.…”

“…There is some evidence from X-ray powder diffraction data (Table 2) that the more weakly hydrating exchange cations permit smaller interlamellar spacings, thereby reducing the volume available for molecular rotation. The effective rotational diameter of the spin probe is about 6.5 A (Hwang et al, 1975), so that tumbling in a ~ 10• wide interlayer might be expected to be somewhat restricted.…”

A paramagnetic molecular voltmeter

“…Adding power P as a new dimension provides enough information to determine R 1 , in addition to R 2 , and Γ G (73,(75)(76)(77)(78)(79)(80)(81), thus achieving our primary goal of accurately determining relaxivities χ 1 . Small nitroxides in deoxygenated solution exhibit discrete superhyperfine splitting.…”