Table 3-1 - Amino Acid pKa Values

Nonpolar , Aliphatic R Groups

Amino Acid	Abbreviation / Symbol	$M_r$	$pK_{a_1} : \ce{-COOH}$	$pK_{a_2} : \ce{-NH3+}$	$pK_{a_3} : \ce{-R}$	$pI$	Hydropathy Index	% Occurrence in Proteins
Glycine	Gly G	75	2.34	9.60		5.97	−0.4	7.2
Alanine	Ala A	89	2.34	9.69		6.01	1.8	7.8
Proline	Pro P	115	1.99	10.96		6.48	−1.6	5.2
Valine	Val V	117	2.32	9.62		5.97	4.2	6.6
Leucine	Leu L	131	2.36	9.60		5.98	3.8	9.1
Isoleucine	Ile I	131	2.36	9.68		6.02	4.5	5.3
Methionine	Met M	149	2.28	9.21		5.74	1.9	2.3

Aromatic R Groups

Amino Acid	Abbreviation / Symbol	$M_r$	$pK_{a_1} : \ce{-COOH}$	$pK_{a_2} : \ce{-NH3+}$	$pK_{a_3} : \ce{-R}$	$pI$	Hydropathy Index	% Occurrence in Proteins
Phenylalanine	Phe F	165	1.83	9.13		5.48	2.8	3.9
Tyrosine	Tyr Y	181	2.20	9.11	10.07	5.66	−1.3	3.2
Tryptophan	Trp W	204	2.38	9.39		5.89	−0.9	1.4

Polar , Uncharged R Groups

Amino Acid	Abbreviation / Symbol	$M_r$	$pK_{a_1} : \ce{-COOH}$	$pK_{a_2} : \ce{-NH3+}$	$pK_{a_3} : \ce{-R}$	$pI$	Hydropathy Index	% Occurrence in Proteins
Serine	Ser S	105	2.21	9.15		5.68	−0.8	6.8
Threonine	Thr T	119	2.11	9.62		5.87	−0.7	5.9
Cysteinee	Cys C	121	1.96	10.28	8.18	5.07	2.5	1.9
Asparagine	Asn N	132	2.02	8.80		5.41	−3.5	4.3
Glutamine	Gln Q	146	2.17	9.13		5.65	−3.5	4.2

Positively Charged R Groups

Amino Acid	Abbreviation / Symbol	$M_r$	$pK_{a_1} : \ce{-COOH}$	$pK_{a_2} : \ce{-NH3+}$	$pK_{a_3} : \ce{-R}$	$pI$	Hydropathy Index	% Occurrence in Proteins
Lysine	Lys K	146	2.18	8.95	10.53	9.74	−3.9	5.9
Histidine	His H	155	1.82	9.17	6.00	7.59	−3.2	2.3
Arginine	Arg R	174	2.17	9.04	12.48	10.76	−4.5	5.1

Negatively Charged R Groups

Amino Acid	Abbreviation / Symbol	$M_r$	$pK_{a_1} : \ce{-COOH}$	$pK_{a_2} : \ce{-NH3+}$	$pK_{a_3} : \ce{-R}$	$pI$	Hydropathy Index	% Occurrence in Proteins
Aspartate	Asp D	133	1.88	9.60	3.65	2.77	−3.5	5.3
Glutamate	Glu E	147	2.19	9.67	4.25	3.22	−3.5	6.3