Rapid Quantitative Measurements of Paramagnetic Relaxation Enhancements in Cu(II)-Tagged Proteins by Proton-Detected Solid State NMR Spectroscopy

We demonstrate rapid quantitative measurements of site-resolved paramagnetic relaxation enhancements (PREs), which are a source of valuable structural restraints corresponding to electron-nucleus distances in the similar to 10-20 angstrom regime, in solid-state nuclear magnetic resonance (NMR) spectra of proteins containing covalent Cu2+-binding tags. Specifically, using protein GB1 K28C-EDTA-Cu2+ mutant as a model, we show the determination of backbone amide N-15 longitudinal and H-1 transverse PREs within a few hours of experiment time based on proton-detected 2D or 3D correlation spectra recorded with magic-angle spinning frequencies >= similar to 60 kHz for samples containing similar to 10-50 nanomoles of H-2, C-13, N-15-labeled protein back-exchanged in H2O. Additionally, we show that the electron relaxation time for the Cu2+ center, needed to convert PREs into distances, can be estimated directly from the experimental data. Altogether, these results are important for establishing solid-state NMR based on paramagnetic-tagging as a routine tool for structure determination of natively diamagnetic proteins.