1H-15N HSQC spectra of (A) 15N-Arg-PMK with backbone residues labeled and (B) expansion of the arginine sidechain region, with assignment labels. Overlaid expansions in boxes show data for mutation-based assignments (mutated PMK in black). Spectra were of 500 µM PMK (20 mM potassium phosphate, 5 mM DTT, 10% D2O, 10% d6-glycerol), at 298K on a 600 MHz Varian NMR System.