DEER Studies of Membrane Proteins

Abstract

Pulsed Electron Paramagnetic Resonance (EPR) spectroscopic techniques such as Double Electron-Electron Resonance (DEER) can provide pertinent structural information on a wide variety of biological systems that have been spin-labeled. This powerful technique can be used to measure distances between 2 spin labels out to about 70 A. However, the application of DEER spectroscopy to study membrane proteins can be difficult due to short phase memory times (Tm) and weak DEER modulation in more biologically relevant proteoliposomes when compared to water soluble proteins or membrane proteins in detergent micelles. The combination of these factors often leads to broad distance distributions, poor signal to noise, and limitations in the determination of longer distances. The short phase memory times are typically due to uneven distributions of spin-labeled protein within the lipid bilayer, which creates local inhomogeneous pockets of high spin concentrations. Approaches to overcome these limitations and improve the quality of DEER measurements for membrane proteins will be discussed: lipodisq nanoparticles, bi-functional spin labels (BSL), and Q-band pulsed EPR spectroscopy.