Simulating Electron Paramagnetic Resonance Spectra of Slow-Motion Systems in the Time Domain

Abstract

Electron paramagnetic resonance (EPR) spectroscopy has proven to be an excellent tool for probing local structure and dynamics in biological systems. However, the corresponding spectra can be complex and difficult to interpret. In order to accurately interpret experimental data, there has been a corresponding active development of computational tools and user-friendly programs to simulate continuous-wave (CW) EPR spectra, especially those in the slow-motion regime (dynamical time scales of ≈10-100 ns for nitroxides at 9-10 GHz). Though the standard method has been to numerically solve the stochastic Liouville equation in the frequency domain, more recently, simulating CW EPR spectra in the time domain has become increasingly popular. Existing works have applied this method by simulating spin relaxation due to rotational diffusion, either by generating stochastic or molecular dynamics trajectories. Other significant spin relaxation mechanisms that affect CW EPR spectral lineshapes have been treated in a phenomenological manner. In order to bridge this gap in computational complexity and more accurately simulate EPR spectra obtained in experiments, we extend previous treatments by explicitly including other spin relaxation mechanisms and explore additional motional models. In addition, our programs are implemented in the widely used Matlab software environment.