Nanomolar pulse dipolar EPR spectroscopy in proteins: Cu^II-Cu^II and nitroxide-nitroxide cases

The study of ever more complex biomolecular assemblies implicated in human health and disease is facilitated by a suite of complementary biophysical methods. Pulse Dipolar electron paramagnetic resonance Spectroscopy (PDS) is a powerful tool that provides highly precise geometric constraints in frozen solution, however the drive towards PDS at physiologically relevant sub-μM concentrations is limited by the currently achievable concentration sensitivity. Recently, PDS using a combination of nitroxide and Cu^II based spin labels allowed measuring 500 nM concentration of a model protein. Using commercial instrumentation and spin labels we demonstrate Cu^II-Cu^II and nitroxide-nitroxide PDS measurements at protein concentrations below previous examples reaching 500 and 100 nM, respectively. These results demonstrate the general feasibility of sub-μM PDS measurements at short to intermediate distances (~1.5 - 3.5 nm), and are of particular relevance for applications where the achievable concentration is limiting.