Structures of Amino Acids TABLE 19.1 The 20 Common Amino Acids in Proteins continued pKa pKa pKa side Name Abbreviations MW Stucture a-CO2H a-NH3+ chain Neutral Amino Acids continued Serine Ser 105 2.21 9.15 5.68 HO H HO H Threonine Thr 119 2.09 9.10 5.60 H3C H3 Tryptophan Trp 2 204 2.83 9.39 5.89 Tyrosine Tyr 181 2.20 9.11 10.07 5.66 HO CH3 Valine Val 117 2.32 9.62 5.96 H3C 0 Structures of Amino Acids