A simple double quantum coherence ESR sequence that minimizes nuclear modulations in Cu²⁺-ion based distance measurements

Double quantum coherence (DQC) ESR is a sensitive method to measure magnetic dipolar interactions between spin labels. However, the DQC experiment on Cu²⁺ centers presents a challenge at X-band. The Cu²⁺ centers are usually coordinated to histidine residues in proteins. The electron-nuclear interaction between the Cu²⁺ ion and the remote nitrogen in the imidazole ring can interfere with the electron-electron dipolar interaction. Herein, we report on a modified DQC experiment that has the advantage of reduced contributions from electron-nuclear interactions, which enhances the resolution of the DQC signal to the electron-electron dipolar modulations. The modified pulse-sequence is verified on Cu²⁺-NO system in a polyalanine-based peptide and on a coupled Cu²⁺ system in a polyproline-based peptide. The modified DQC data were compared with the DEER data and good agreement was found.