Table 1. Various affinity moieties available for protein purification and their characteristics

Affinity moiety	Amino acid length (molecular mass)	Affinity (K_D)	Precipitation reagent	Elution method
His₆	6 (0.8 kDa)	1 mM	Nickel	Imidazole
FLAG peptide	8 (1 kDa)	0.1-100 nM^a	Antibody	FLAG peptide
HA peptide	9 (1.1 kDa)	0.1-100 nM^a	Antibody	HA peptide
Myc peptide	10 (1.2 kDa)	0.1-100 nM^a	Antibody	Myc peptide
Biotin “mimicking” peptides (SBP, Strep-tag, etc.)	8-35 (1-4 kDa)	2-40 nM	Streptavidin	Biotin
Calmodulin binding peptide of MLCK2	26 (3 kDa)	2 nM	Calmodulin	Calcium chelation
Biotinylation target peptide	75-90 (10 kDa)	1 ftM	Streptavidin	Protease cleavage
Tandem IgG binding domains of Protein A or Protein G	~125 (14 kDa)	10 nM	Human IgG	Protease cleavage
Glutathione S-transferase (GST)	224 (26 kDa)	80 nM	Glutathione	Glutathione

^aReported range for the K_D for antibody-antigen interactions. The actual K_D for antibodies against these peptides has not been published.