Table 3.

Amino acid side chain and analog ionization data

Amino acid/analog	pKa	ΔH (kJ/mole)	ΔS (J/deg/mole)
Aspartic acid	3.87	4.0	−60.6
Glutamic acid	4.27	1.6	−76.5
Cysteine	8.39	36.0	−39.7
Tyrosine	10.05	25.1	−108.7
Arginine	12.48	51.8	−64.8
Lysine	10.53	48.5	−38.9
Histidine	6.00	29.9	13.0
Propanoic acid	4.87	−0.6	−95.3
Phenol	9.98	23.6	−111.6
phenyl-arginine	12.40	50.0	−20.9
Monomethylamine	10.63	54.7	−19.7
Imidazole	6.99	36.7	−10.5

Measurements at 25°C and ionic strength between 0 and 0.15 M for the proton dissociations, AH <=> A⁻ + H⁺ (acid) and BH⁺ <=> B + H⁺ (base) (Izatt and Christensen 1976). Only side-chain ionization pK_a shown for amino acids.