On the Use of Q-Band Double Electron-Electron Resonance To Resolve the Relative Orientations of Two Double Histidine-Bound Cu2+ Ions in a Protein.

In this work, we explore the potential of a rigid Cu2+ spin-labeling technique, the double histidine (dHis) motif, along with Q-band electron paramagnetic resonance to report on the relative orientations of the spin labels. We show that the precision of the dHis motif, coupled with the sensitivity and resolution of Q-band frequencies, may allow for the straightforward determination of the relative orientation of the dHis-Cu2+ labels using double electron-electron resonance (DEER). We performed Q-band DEER measurements at different magnetic fields on a protein containing two dHis Cu2+ sites. These measurements exhibited orientational selectivity such that each discrete magnetic field yielded a unique DEER signal. We determined the relative orientation of the two metal centers by simulating the orientationally selective DEER data. These relative orientations were validated by visual analysis of the protein crystal structure modified with dHis sites. The simple visual analysis was shown to agree well with the angular values determined via simulation of the experimental data. The combination of the dHis-Cu2+ motif along with the advantages of the Q-band can aid in the accurate measurement of protein structural and conformational dynamics.