Module 2: Lesson 2 Amino Acid Ionization

Amino Acid: Ionization and Environment Contain at least two ionizable protons, each with its own pKa The carboxyl group has an acidic pKa and will be protonated at an acidic (low) pH:   COOH ↔ COO- + H+ The amino group has a basic pKa and will be protonated until basic pH (high) is achieved: -NH2 ↔ -NH3+ Environmental Effects * α-Carboxyl group is much more acidic than in carboxylic acids * α-Amino group is slightly less basic than in amines Due to intramolecular interactions pKa 2 4 6 8 10 12 |---|---|---|---|---|---| Methylammonium CH3-NH3+ Carboxyl and amino groups in glycine <--- α-Amino Acid (glycine) H-C-COOH NH3+ Acetic Acid CH3-COOH Methylamine CH3-NH2 <--- α-Amino Acid (glycine) H-C-COO- NH2 The protonated amino group withdraws electrons from the carboxyl group, lowering its pKa Electronegative oxygen atoms in the carboxyl group withdraw electrons from the amino group, lowering its pKa Ionization and Titration Curves Low pH → amino acid exists in a positively charged form, cation. High pH → amino acid exists in a negatively charged form, anion. Between the pKa for each group, the amino acid exists in a zwitterion form, in which a single molecule has both a positive   and negative charge.   Cation ↔ Zwitterion ↔ Anion Zwitterion Predominate at pH values between the pKa values of the amino and carboxyl groups Amino acids without ionizable side chains, the isoelectric point (equivalence point, pI) is: pI = (pKa1 + pKa2) / 2 Example: Glycine pKa1 = 2.34 pKa2 = 9.60 pI = (2.34 + 9.60) / 2 = 5.97 Net charge = 0 -AA is least soluble in water -AA does not migrate in electric field [Image of titration curve for glycine] NH3+ NH3+ | | R-C-COOH R-C-COO- | | H H Amino Acid Zwitterion NH3+ NH2 | | R-C-COO- R-C-COO- | | H H low pH Amino Acids Can Act as Buffers Amino acids with uncharged side chains, such as glycine, have two pKa values: * The pKa of the   α-carboxyl group is 2.34 * The pKa of the α-amino group is 9.6 As buffers   can prevent a change in pH close to the pKa, glycine can act as a buffer in two pH ranges. Amino Acids with Ionizable R-groups Ionizable side chains also have pKa and act as buffers. Ionizable side chains can also influence the pI of the amino acid. Titration curves of 2+ pKa units are now more complex, as each pKa has a buffering zone of 2 pH units. Identifying species that carries a net zero charge pKa1 = carboxyl pKa2 = amino pKa3 = R-group Amino Acid | pKa1 | pKa2 | pKa3 | pI ------------|------|------|------|----- Glycine | 2.34 | 9.60 | - | 5.97 Alanine | 2.34 | 9.69 | - | 6.02 Valine | 2.32 | 9.62 | - | 5.97 Leucine | 2.36 | 9.60 | - | 6.02 Isoleucine | 2.36 | 9.60 | - | 6.02 Methionine | 2.28 | 9.21 | - | 5.74 Phenylalanine| 1.83 | 9.13 | - | 5.48 Tyrosine | 2.20 | 9.11 | 10.07 | 5.66 Tryptophan | 2.46 | 9.39 | - | 5.89 Asparagine | 2.02 | 8.80 | - | 5.41 Glutamine | 2.17 | 9.13 | - | 5.65 Cysteine | 1.96 | 10.28 | 8.18 | 5.07 Aspartic Acid | 2.09 | 9.82 | 3.65 | 2.77 Glutamic Acid | 2.19 | 9.67 | 4.25 | 3.22 Lysine | 2.18 | 8.95 | 10.53 | 9.74 Arginine | 2.17 | 9.04 | 12.48 | 10.76 Histidine | 1.82 | 9.17 | 6.00 | 7.59   NH3+ ↔ NH2 CH3 | CH3 | CH3 COOH ↔ COO- ↔ COO-