Amino acid pK (or pKa) values describe the ionization behavior of ionizable groups found in proteins. These groups are the carboxylic acids of the carboxyl terminus, aspartic acid and glutamic acid, the imidazole ring of histidine, the α-amino group at the amino terminus, the sulfhydryl group of cysteine, the phenolic group of tyrosine, the amino group of lysine and the guanidino group of arginine. Any study of the pH dependence of protein behavior (binding, activity or stability, for example) depends on understanding the pK values of the amino acids. With such importance on the amino acid pK values, many authors writing in the protein chemistry literature have compiled tables of intrinsic pK values over the last ≈70 years. These tables are meant to represent the pK values in the absence of any outside electrostatic influence. These tables are built from the study of free amino acids, small model compounds similar to peptides, short peptides or measurements made in folded proteins. While none of these tables of pK values provide specific information about a specific ionizable group in a specific protein, they do serve as a good starting place if you have no other information or are trying to understand the local environment of an ionizable group in a particular protein or peptide. Many of these tables of amino acid pK values are collected here for use and study.

In order to facilitate the use of these amino acid pK tables, they are collected here in both human and machine (dsv, json & xml) readable forms. The selection criteria used is that data presented by an author is a complete set of pK values (pK values for the seven ionizable side chains along with the α carboxyl and α amino groups) or there is enough data given that one or more of these pK values can be derived from their data. All derived values are clearly distinguished in the human readable presentation and the method used to derive that value is detailed in the footnote for that value. Along with the data for each table of pK values are complete notes describing the model data and methods along with complete citations for each reference.