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27.3 Acid-Base Behavior of Amino Acids

27.3 Acid-Base Behavior of Amino Acids

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27.3 Acid-Base Behavior of Amino Acids

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  1. 27.3Acid-Base Behavior of Amino Acids

  2. Recall • While their name implies that amino acids are compounds that contain an —NH2 group and a —CO2H group, these groups are actually present as —NH3+ and —CO2– respectively. • How do we know this?

  3. O O •• •• •• •• + – •• •• •• H3NCH2C O H2NCH2C OH •• •• •• Properties of Glycine • The properties of glycine: • high melting point (when heated to 233°C it decomposes before it melts)solubility: soluble in water; not soluble in nonpolar solvent more consistent with this than this

  4. O •• •• + – •• H3NCH2C O •• •• Properties of Glycine • The properties of glycine: • high melting point (when heated to 233°C it decomposes before it melts)solubility: soluble in water; not soluble in nonpolar solvent more consistent with this called a zwitterion or dipolar ion

  5. Acid-Base Properties of Glycine • The zwitterionic structure of glycine also follows from considering its acid-base properties. • A good way to think about this is to start with the structure of glycine in strongly acidic solution, say pH = 1. • At pH = 1, glycine exists in its protonated form (a monocation).

  6. O •• •• + •• H3NCH2C OH •• Acid-Base Properties of Glycine • The zwitterionic structure of glycine also follows from considering its acid-base properties. • A good way to think about this is to start with the structure of glycine in strongly acidic solution, say pH = 1. • At pH = 1, glycine exists in its protonated form (a monocation).

  7. O •• •• + •• H3NCH2C OH •• Acid-Base Properties of Glycine • Now ask yourself "As the pH is raised, which is the first proton to be removed? Is it the proton attached to the positively charged nitrogen, or is it the proton of the carboxyl group?" • You can choose between them by estimating their respective pKas.

  8. O •• •• + •• H3NCH2C OH •• Acid-Base Properties of Glycine • Now ask yourself "As the pH is raised, which is the first proton to be removed? Is it the proton attached to the positively charged nitrogen, or is it the proton of the carboxyl group?" • You can choose between them by estimating their respective pKas. typical ammonium ion: pKa ~9 typical carboxylic acid: pKa ~5

  9. O •• •• + •• H3NCH2C OH •• Acid-Base Properties of Glycine • The more acidic proton belongs to the CO2H group. It is the first one removed as the pH is raised. typical carboxylic acid: pKa ~5

  10. O •• •• + – •• H3NCH2C O •• •• O •• •• + •• H3NCH2C OH •• Acid-Base Properties of Glycine • Therefore, the more stable neutral form of glycine is the zwitterion. typical carboxylic acid: pKa ~5

  11. O •• •• + •• H3NCH2C OH •• Acid-Base Properties of Glycine • The measured pKa of glycine is 2.34. • Glycine is stronger than a typical carboxylic acid because the positively charged N acts as an electron-withdrawing, acid-strengthening substituent on the a carbon. typical carboxylic acid: pKa ~5

  12. O O •• •• •• •• – + HO – – •• •• •• H3NCH2C O H2NCH2C O •• •• •• •• Acid-Base Properties of Glycine A proton attached to N in the zwitterionic form of nitrogen can be removed as the pH is increased further. • The pKa for removal of this proton is 9.60.This value is about the same as that for NH4+ (9.3).

  13. O •• •• + •• H3NCH2C OH •• O •• •• + – •• H3NCH2C O •• •• O •• •• – •• •• H2NCH2C O •• •• Isoelectric Point pI • The pH at which the concentration of the zwitterion is a maximum is called the isoelectric point. Its numerical value is the average of the two pKas. • The pI of glycine is 5.97. pKa = 2.34 pKa = 9.60

  14. Acid-Base Properties of Amino Acids • One way in which amino acids differ is in respect to their acid-base properties. This is the basis for certain experimental methods for separating and identifying them. • Just as important, the difference in acid-base properties among various side chains affects the properties of the proteins that contain them. • Table 27.2 gives pKa and pI values for amino acids with neutral side chains.

  15. O H + – H3N O C C H Table 27.2Amino Acids with Neutral Side Chains pKa1 = 2.34pKa2 = 9.60pI = 5.97 Glycine

  16. O H + – H3N O C C CH3 Table 27.2Amino Acids with Neutral Side Chains pKa1 = 2.34pKa2 = 9.69pI = 6.00 Alanine

  17. O H + – H3N O C C CH(CH3)2 Table 27.2Amino Acids with Neutral Side Chains pKa1 = 2.32pKa2 = 9.62pI = 5.96 Valine

  18. O H + – H3N O C C CH2CH(CH3)2 Table 27.2Amino Acids with Neutral Side Chains pKa1 = 2.36pKa2 = 9.60pI = 5.98 Leucine

  19. Table 27.2Amino Acids with Neutral Side Chains O H pKa1 = 2.36pKa2 = 9.60pI = 5.98 + – H3N O C C Isoleucine CH3CHCH2CH3

  20. Table 27.2Amino Acids with Neutral Side Chains O H pKa1 = 2.28pKa2 = 9.21pI = 5.74 + – H3N O C C Methionine CH3SCH2CH2

  21. O H + – H2N O C C CH2 H2C CH2 Table 27.2Amino Acids with Neutral Side Chains pKa1 = 1.99pKa2 = 10.60pI = 6.30 Proline

  22. O H + – H3N O C C CH2 Table 27.2Amino Acids with Neutral Side Chains pKa1 = 1.83pKa2 = 9.13pI = 5.48 Phenylalanine

  23. O H + – H3N O C C CH2 N H Table 27.2Amino Acids with Neutral Side Chains pKa1 = 2.83pKa2 = 9.39pI = 5.89 Tryptophan

  24. H2NCCH2 O Table 27.2Amino Acids with Neutral Side Chains O H pKa1 = 2.02pKa2 = 8.80pI = 5.41 + – H3N O C C Asparagine

  25. O H + – H3N O C C H2NCCH2CH2 O Table 27.2Amino Acids with Neutral Side Chains pKa1 = 2.17pKa2 = 9.13pI = 5.65 Glutamine

  26. O H + – H3N O C C CH2OH Table 27.2Amino Acids with Neutral Side Chains pKa1 = 2.21pKa2 = 9.15pI = 5.68 Serine

  27. Table 27.2Amino Acids with Neutral Side Chains O H pKa1 = 2.09pKa2 = 9.10pI = 5.60 + – H3N O C C Threonine CH3CHOH

  28. OCCH2 O Table 27.3Amino Acids with Ionizable Side Chains • For amino acids with acidic side chains, pI is the average of pKa1 and pKa2. O H pKa1 = 1.88pKa2 = 3.65pKa3 = 9.60 pI = 2.77 + – H3N O C C Aspartic acid

  29. OCCH2CH2 O Table 27.3Amino Acids with Ionizable Side Chains O H pKa1 = 2.19pKa2 = 4.25pKa3 = 9.67 pI = 3.22 + – H3N O C C Glutamic acid

  30. O H + – H3N O C C CH2 OH Table 27.3Amino Acids with Ionizable Side Chains pKa1 = 2.20pKa2 = 9.11pKa3 = 10.07 pI = 5.66 Tyrosine

  31. O H + – H3N O C C CH2SH Table 27.3Amino Acids with Ionizable Side Chains pKa1 = 1.96pKa2 = 8.18pKa3 = 10.28 pI = 5.07 Cysteine

  32. O H + – H3N O C C + CH2CH2CH2CH2NH3 Table 27.3Amino Acids with Ionizable Side Chains • For amino acids with basic side chains, pI is the average of pKa2 and pKa3. pKa1 = 2.18pKa2 = 8.95pKa3 = 10.53pI = 9.74 Lysine

  33. O H + – H3N O C C CH2CH2CH2NHCNH2 + NH2 Table 27.3Amino Acids with Ionizable Side Chains pKa1 = 2.17pKa2 = 9.04pKa3 = 12.48pI = 10.76 Arginine

  34. O H + – H3N O C C CH2 N NH Table 27.3Amino Acids with Ionizable Side Chains pKa1 = 1.82pKa2 = 6.00pKa3 = 9.17 pI = 7.59 Histidine