Effect of Hydrostatic Pressure on the Tensor and Hyperfine Coupling Constants of the Nitroxide Radical Characterized by Calculations.

We present a computational study characterizing the effect of hydrostatic pressure on magnetic spin parameters that are used to analyze the electron paramagnetic resonance (EPR) spectra. Site-directed spin labeling (SDSL) in combination with EPR spectroscopy is a powerful tool for investigating the structure and dynamics of biological molecules. In studies using SDSL-based EPR spectroscopy, it is essential to know the spin parameters, such as the factor and the hyperfine constants, precisely. However, the experimental characterization of these spin parameters under extreme conditions is often challenging. We report quantum chemistry calculations of tensors and hyperfine coupling tensors ( tensors) for the nitroxide radical spin label in the pressure range of 0-15 GPa. The hydrostatic pressure causes structural changes, which, in turn, result in linear changes of the and tensors. The observed linear dependence of the and tensors suggests that these quantities can serve as reporters of local pressure in complex environments. The corresponding simulated EPR spectra at 9 and 230 GHz reveal that the changes of the EPR spectrum are more pronounced in the former. Our results indicate that the computational approach can address the challenge of determining magnetic spin parameters under extreme conditions, such as under high hydrostatic pressure.