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

Abstract

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.