Deconvoluting Monomer- and Dimer-Specific Distance Distributions between Spin Labels in a Monomer/Dimer Mixture Using T 1 -Edited DEER EPR Spectroscopy.

Double electron-electron resonance (DEER) EPR is a powerful tool in structural biology, providing distances between pairs of spin labels. When the sample consists of a mixture of oligomeric species (e.g., monomer and dimer), the question arises as to how to assign the peaks in the DEER-derived probability distance distribution to the individual species. Here, we propose incorporating an EPR longitudinal electron relaxation ( T 1 ) inversion recovery experiment within a DEER pulse sequence to resolve this problem. The apparent T 1 between dipolar coupled electron spins measured from the inversion recovery time (τ inv ) dependence of the peak intensities in the T 1 -edited DEER-derived probability P ( r ) distance distribution will be affected by the number of nitroxide labels attached to the biomolecule of interest, for example, two for a monomer and four for a dimer. We show that global fitting of all the T 1 -edited DEER echo curves, recorded over a range of τ inv values, permits the deconvolution of distances between spin labels originating from monomeric (longer T 1 ) and dimeric (shorter T 1 ) species. This is especially useful when the trapping of spin labels in different conformational states during freezing gives rise to complex P ( r ) distance distributions. The utility of this approach is demonstrated for two systems, the β 1 adrenergic receptor and a construct of the huntingtin exon-1 protein fused to the immunoglobulin domain of protein G, both of which exist in a monomer-dimer equilibrium.