Orientational Selectivity in Pulsed-EPR Does Not Have to be Complicated

The development of rigid spin labels and high-frequency spectrometers have improved the capabilities of pulsed-EPR distance measurements in the field of structural biology. Rigid spin labels provide distance constraints that better report on the conformations of the protein or DNA backbone. Additionally, spectrometers at high frequencies improve the sensitivity of pulsed-EPR, even enabling experiments at concentrations close to cellular conditions. Unfortunately, these advents can come with a complication in that the microwave pulse cannot completely sample all orientations of the spins. Consequently, insufficient sampling biases the dipolar frequencies in a manner that depends on the relative orientations of the intramolecularly interacting spins. These relative orientations are generally unknown a priori in a bilabeled biomolecule. This biasing effect, dubbed ‘orientational selectivity,’ is a bottleneck to interpreting distance measurements from the dipolar signal. This review provides an overview of orientational selectivity in the context of distance measurements using Double Electron–Electron Resonance. First, we discuss the genesis of orientational selectivity and briefly overview the literature on spin labels that have manifested orientational selectivity. Second, we outline the various strategies to account for orientational selectivity effects for extracting the distance constraints. Finally, we showcase a new perspective on analyzing orientational selectivity and designing efficient experimental schemes for overcoming orientational selectivity.