Accessing Methyl Groups in Proteins via ¹H-detected MAS Solid-state NMR Spectroscopy Employing Random Protonation

We recently introduced RAP (reduced adjoining protonation) labelling as an easy to implement and cost-effective strategy to yield selectively methyl protonated protein samples. We show here that even though the amount of H₂O employed in the bacterial growth medium is rather low, the intensities obtained in MAS solid-state NMR ¹H,¹³C correlation spectra are comparable to spectra obtained for samples in which α-ketoisovalerate was employed as precursor. In addition to correlations for Leu and Val residues, RAP labelled samples yield also resonances for all methyl containing side chains. The labelling scheme has been employed to quantify order parameters, together with the respective asymmetry parameters. We obtain a very good correlation between the order parameters measured using a GlcRAP (glucose carbon source) and a α-ketoisovalerate labelled sample. The labelling scheme holds the potential to be very useful for the collection of long-range distance restraints among side chain atoms. Experiments are demonstrated using RAP and α-ketoisovalerate labelled samples of the α-spectrin SH3 domain, and are applied to fibrils formed from the Alzheimer’s disease Aβ_1-40 peptide.