Unambiguous Determination of Protein Arginine Ionization States in Solution by NMR Spectroscopy

Because arginine residues in proteins are expected to be in their protonated form almost without exception, reports demonstrating that a protein arginine residue is charge‐neutral are rare and potentially controversial. Herein, we present a ¹³C‐detected NMR experiment for probing individual arginine residues in proteins notwithstanding the presence of chemical and conformational exchange effects. In the experiment, the ¹⁵N^η and ¹⁵N^ϵ chemical shifts of an arginine head group are correlated with that of the directly attached ¹³C^ζ. In the resulting spectrum, the number of protons in the arginine head group can be obtained directly from the ¹⁵N–¹H scalar coupling splitting pattern. We applied this method to unambiguously determine the ionization state of the R52 side chain in the photoactive yellow protein from Halorhodospira halophila . Although only three H^η atoms were previously identified by neutron crystallography, we show that R52 is predominantly protonated in solution.