Six-Pulse RIDME Sequence to Avoid Background Artifacts

Relaxation induced dipolar modulation enhancement (RIDME) is a valuable method for measuring nanometer-scale distances between electron spin centers. Such distances are widely used in structural biology to study biomolecular structures and track their conformational changes. Despite significant improvements of RIDME in recent years, the background analysis of primary RIDME signals remains to be challenging. In particular, it was recently shown that the five-pulse RIDME signals contain an artifact which can hinder the accurate extraction of distance distributions from RIDME time traces [as reported by Ritsch et al. (Phys Chem Chem Phys 21: 9810, 2019)]. Here, this artifact, as well as one additionally identified artifact, are systematically studied on several model compounds and the possible origins of both artifacts are discussed. In addition, a new six-pulse RIDME sequence is proposed that eliminates the artifact with the biggest impact on the extracted distance distributions. The efficiency of this pulse sequence is confirmed on several examples.