A general model to optimise CuIIlabelling efficiency of double-histidine motifs for pulse dipolar EPR applications

Abstract

Electron paramagnetic resonance (EPR) distance measurements are making increasingly important contributions to studies of biomolecules underpinning health and disease by providing highly accurate and precise geometric constraints. Combining double-histidine (dH) motifs with CuIIspin labels shows promise for further increasing the precision of distance measurements, and for investigating subtle conformational changes. However, non-covalent coordination-based spin labelling is vulnerable to low binding affinity. Dissociation constants of dH motifs for CuII-nitrilotriacetic acid were previously investigatedviarelaxation induced dipolar modulation enhancement (RIDME), and demonstrated the feasibility of exploiting the dH motif for EPR applications at sub-μM protein concentrations. Herein, the feasibility of using modulation depth quantitation in CuII-CuIIRIDME to simultaneously estimate a pair of non-identical independentKDvalues in such a tetra-histidine model protein is addressed. Furthermore, we develop a general speciation model to optimise CuIIlabelling efficiency, depending upon pairs of identical or disparateKDvalues and total CuIIlabel concentration. We find the dissociation constant estimates are in excellent agreement with previously determined values, and empirical modulation depths support the proposed model.