Spin-label NOmetry is defined as the use of nitroxide radical spin labels to monitor the diffusion-concentration product of nitric oxide ( NO) with spin labels. Bimolecular collisions of nitric oxide with spin labels alter the EPR spectral parameters of the spin labels resulting in line broadening. The Smoluchowski equation for the bimolecular collision rate of dissolved nitric oxide with spin labels yielded the value for the diffusion coefficient of nitric oxide in water that is in agreement with the published value obtained by conventional methods. In heterogenous systems such as membranes the collision product can be determined for depths at which the spin label is localized. Profiles of nitric oxide diffusion-concentration product across egg yolk phosphotidylcholine (EYPC) and dioleoylphosphatidylcholine (DOPC) membranes in the absence and presence of 30 mol% cholesterol were obtained. For a pure membrane, the collision rate in the hydrocarbon region of the bilayer is higher than in water, while in the polar headgroup region, it is about the same. Cholesterol decreases the product in the polar headgroup region, while significantly increasing it in the membrane center. Higher collision rates in the membrane center suggests that 1) nitric oxide is protected in membranes against reactions with water-soluble compounds, and that 2) extended membrane systems may provide routes for intracellular nitric oxide transport.
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