754 AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS (d) On further treatment with base, both the monoanions in part (c) yield the same dianion CHCHCO 27.4 At pH I the carboxylate oxygen and both nitrogens of lysine are protonated H3NCH, CH,CH,CH,CHCO,H Principal form at pH As the ph is raised, the carboxyl proton is removed first. H3 NCH,CH,CH CH, CHCO H HO HNCH..CHCO+ HO The pk value for the first ionization of lysine is 2. 18(from Table 27.3), and so this process is virtually complete when the pH is greater than this value The second pKa value for lysine is 8.95. This is a fairly typical value for the second pKa of amino acids and likely corresponds to proton removal from the nitrogen on the a carbon. The species that results is the predominant one at pH 9 H3NCH, CH, CH, CH,CHCO, HO H3NCH, CH,CH, CH- CHCO, H,O Principal form at pH 9) The pKa value for the third ionization of lysine is 10.53. This value is fairly high compared with those of most of the amino acids in Tables 27. 1 to 27.3 and suggests that this proton is removed from the nitrogen of the side chain. The species that results is the major species present at pH values greater than 10.53 H3NCH,CH,CH, CH,CHCO, HO H,NCH, CH,CH,CH,CHCO, N (Principal form at pH 13) 27.5 To convert 3-methylbutanoic acid to valine, a leaving group must be introduced at the a carbon prior to displacement by ammonia. This is best accomplished by bromination under the conditions of the Hell-Volhard-Zelinsky reaction Br2- P H3 (CH3)2CHCH2 CO2H or Br. PC(CH3)2CHCHCO2H (CH3)2CHCHCO 3-Methylbutanoi 2-Bromo-3-methylbutanoic Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website754 AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS (d) On further treatment with base, both the monoanions in part (c) yield the same dianion. 27.4 At pH 1 the carboxylate oxygen and both nitrogens of lysine are protonated. As the pH is raised, the carboxyl proton is removed first. The pKa value for the first ionization of lysine is 2.18 (from Table 27.3), and so this process is virtually complete when the pH is greater than this value. The second pKa value for lysine is 8.95. This is a fairly typical value for the second pKa of amino acids and likely corresponds to proton removal from the nitrogen on the carbon. The species that results is the predominant one at pH 9. The pKa value for the third ionization of lysine is 10.53. This value is fairly high compared with those of most of the amino acids in Tables 27.1 to 27.3 and suggests that this proton is removed from the nitrogen of the side chain. The species that results is the major species present at pH values greater than 10.53. 27.5 To convert 3-methylbutanoic acid to valine, a leaving group must be introduced at the carbon prior to displacement by ammonia. This is best accomplished by bromination under the conditions of the Hell–Volhard–Zelinsky reaction. (CH3)2CHCH2CO2H Br2, P or Br2, PCl3 NH3 (CH3)2CHCHCO2H Br (CH3)2CHCHCO2 NH3 3-Methylbutanoic acid 2-Bromo-3-methylbutanoic acid Valine H3NCH2CH2CH2CH2CHCO2 HO NH2 H2NCH2CH2CH2CH2CHCO2 NH2 (Principal form at pH 13) H3NCH2CH2CH2CH2CHCO2 HO NH3 H3NCH2CH2CH2CH2CHCO2 H2O NH2 (Principal form at pH 9) H3NCH2CH2CH2CH2CHCO2H HO NH3 H3NCH2CH2CH2CH2CHCO2 H2O NH3 H3NCH2CH2CH2CH2CHCO2H NH3 (Principal form at pH 1) O CH2CHCO2 NH2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website