A General Model to Optimise Copper(II) Labelling Efficiency of Double-Histidine Motifs for Pulse Dipolar EPR Applications

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 Cu^II spin 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 Cu^II-nitrilotriacetic acid were previously investigated via relaxation induced dipolar modulation enhancement (RIDME), and demonstrated the feasibility of exploiting the double histidine motif for EPR applications at sub-μM protein concentrations. Herein, the feasibility of using modulation depth quantitation in Cu^II-Cu^II RIDME to simultaneously estimate a pair of non-identical independent K_D values in such a tetra-histidine model protein is addressed. Furthermore, we develop a general speciation model to optimise Cu^II labelling efficiency, in dependence of pairs of identical or disparate K_D values and total Cu^II label concentration. We find the dissociation constant estimates are in excellent agreement with previously determined values, and empirical modulation depths support the proposed model.