Carboxylate form the pkas of amino and carboxyl groups

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Unformatted text preview: r=c Acid Asparagine Glutamic Acid Glutamine Lysine 3 leDer Cys His Ile Met Ser Val Ala Gly Leu Pro Thr Arg Phe Tyr Trp Asp Asn Glu Gln Lys 1 leDer C H I M S V A G L P T R F Y W D N E Q K explana+on First le]er of the name First le]er of the name First le]er of the name First le]er of the name First le]er of the name First le]er of the name First le]er of the name First le]er of the name First le]er of the name First le]er of the name First le]er of the name aRginine Fenylalanine tYrosine tWiptophan (or, contains Double ring) asparDic Contains N (or asparagiN) gluE (or glutamEke) Q- tamine K is near L in the alphabet pH is controlled pH = − log[H + ] € •  Biological process is pH- dependent •  Arrangement of protons is essen=al for enzyme ac=vity Ioniza=on property of amino acids/Zwi]erions Carboxyl group pKa ≈ 2- 3 Amino group pKa ≈ 8- 9 (dipolar ions) At physiological pH (≈7), the amino groups are protonated and the carboxylic acid groups are in their conjugate base (carboxylate) form The pKas of α- amino and α- carboxyl groups are slightly different depending on the side- chain R- group pI •  At pI (isoelectric pH), a molecule carries no net electric charge •  For amino acids without ionizable side chains, pK1 + pK 2 pI = 2 Amino acids with Ionizable side chains Net charge •  His amino acid has 3 ionizable groups – 3 buffer regions. •  His side chain is protonated and charged +1 at lower pH •  pKR is near pH 7 - biological importance pI Amino acids with Ionizable side chains •  Each amino acid has different side chain pKa (pKR) -   many are not ionizable •  Glu side chain is protonated and charged 0 at low pH pI Q: indicate whether the following statements concerning histidine are true or false. Refer to the following titration __ (a) At pH 1.82 there are equal amounts of form 1 and form 2 __ (b) The α-carboxyl group is half dissociated at pH 9.17 __ (c) Histidine would be a good buffer at pH values near 1.8 and 9.2 __ (d) Histidine has biological importance because the pKa of its side chain is close to physiological pH __ (e) Histidine’s pI is between pH 1.82 an...
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