Protonation-state determination in proteins using high-resolution X-ray crystallography: effects of resolution and completeness

A bond-distance analysis to determine the protonation states of ionizable amino acids has been made for trypsin at 1.2 Å resolution, subtilisin at 1.26 Å resolution and lysozyme at 0.65 Å resolution. This was effective for Asp and Glu but not for His.