Confronting the Invisible: Assignment of Protein 1HN Chemical Shifts in Cases of Extreme Broadening.

NMR studies of intrinsically disordered proteins (IDPs) at neutral pH values are hampered by the rapid exchange of backbone amide protons with solvent. Although exchange rates can be modulated by changes in pH, interactions between IDPs that lead to phase separation sometimes only occur at neutral pH values or higher, where backbone amide-based experiments fail. Here we describe a simple NMR experiment for measuring amide proton chemical shifts in cases where 1HN spectra cannot be obtained. The approach uses a weak 1H B1 field, searching for elusive 1HN resonance frequencies that become encoded in the intensities of cross-peaks in three-dimensional 1Hα-detect spectra. Applications to the CAPRIN1 protein in both dilute- and phase-separated states highlight the utility of the method, establishing that accurate 1HN chemical shifts can be obtained even in cases where solvent hydrogen exchange rates are on the order of 1500 s-1.