Effects of Hydrostatic Pressure and Temperature on Spin Exchange between Free Radicals in Solution

Spin-label oximetry at Q- and W-band

Spin-lattice relaxation times (T₁s) of small water-soluble spin-labels in the aqueous phase as well as lipid-type spin-labels in membranes increase when the microwave frequency increases from 2 to 35 GHz (Hyde, et al., J. Phys. Chem. B 108 (2004) 9524-9529). The T₁s measured at W-band (94 GHz) for the water-soluble spin-labels CTPO and TEMPONE (Froncisz, et al., J. Magn. Reson. 193 (2008) 297-304) are, however, shorter than when measured at Q-band (35 GHz). In this paper, the decreasing trends at W-band have been confirmed for commonly used lipid-type spin-labels in model membranes. It is concluded that the longest values of T₁ will generally be found at Q-band, noting that long values are advantageous for measurement of bimolecular collisions with oxygen. The contribution of dissolved molecular oxygen to the relaxation rate was found to be independent of microwave frequency up to 94 GHz for lipid-type spin-labels in membranes. This contribution is expressed in terms of the oxygen transport parameter W=T₁⁻¹(Air)-T₁⁻¹(N₂), which is a function of both concentration and translational diffusion of oxygen in the local environment of a spin-label. The new capabilities in measurement of the oxygen transport parameter using saturation-recovery (SR) EPR at Q- and W-band have been demonstrated in saturated (DMPC) and unsaturated (POPC) lipid bilayer membranes with the use of stearic acid (n-SASL) and phosphatidylcholine (n-PC) spin-labels, and compared with results obtained earlier at X-band. SR EPR spin-label oximetry at Q- and W-band has the potential to be a powerful tool for studying samples of small volume, ~30 nL. These benefits, together with other factors such as a higher resonator efficiency parameter and a new technique for canceling free induction decay signals, are discussed.

The diffusion-solubility of oxygen in lipid bilayers

The product, Do alpha, of the oxygen diffusion coefficient, Do, and the oxygen solubility, alpha, is determined in phosphatidylcholine bilayers at temperatures above the lipid phase transitions from ESR spin-exchange measurements. The resulting values of Do alpha are in good agreement with those obtained from fluorescence-quenching experiments. The use of fatty acid spin labels makes it possible to measure Do alpha as a function of the coordinate perpendicular to the bilayer surface. The results indicate that do alpha is a strong function of this coordinate; it is greatest in the bilayer center and least near the bilayer head groups.