Mapping the Structure of Metalloproteins with RIDME.

Distance measurements in biological macromolecules represent a very active field of application of pulsed electron paramagnetic resonance (EPR) spectroscopy. The relatively recently introduced pulsed EPR method of relaxation-induced dipolar modulation enhancement (RIDME) is conceptually similar to the popular double electron-electron resonance (DEER), but is much more suitable for studying the structures of metalloproteins while using their native paramagnetic metal centers as structural reference points. In particular, RIDME can largely alleviate the sensitivity and orientational selectivity problems that limit the application of DEER to such systems. In this contribution, the theoretical principles, implementation, optimization, and available experimental examples of RIDME are described with the purpose of enhancing the familiarity with this technique and promoting its application.