Titration Curve of Amino Acids

tion Experiment 1 Titration curves of amino group blisterings General structure of amino virulents (amphoteric type) Zwitter ion C * ? carbon ? amino acid NH2 ? amino group, basic (proton acceptor) COOH ? carboxyl group, acidic (proton donor) R side chain of amino acid Classification of amino acids depending on the nature of side chain * Neutral * canonical amino acid with extra amino group (NH2) in its side chain ( histidine, arginine, lysine ) * Acidic amino acid with extra carboxylic acid group (COOH) in its side chain (aspartic acid & glutamic acid) * With phenol group in its side chain (Tyrosine) * With sulfide group in its side chain (Cysteine) Isoelectric point (pI) It is the pH at which the net charge on the molecule in solution is zero (Zwitter ion). Zwitter ion will not migrate in electric field, it is electrically neutral. pI is the average of the closest values of pKas. The pI for the acidic amino acids is the average of pKa1 and pKa2, while the pI for basi c amino acids is the average of pKa2 and pKa3 pKa range for ? COOH ( 1. 71 2. 63 ) pKa range for ? NH3+ ( 8. 8 10. 78 ) Acidity of neutral amino acid ? COOH ? NH3+ For weak acids Henderson Hasslbalch equation pH = pKa + log A- HA When A- = HA pH = pKa Acid- base titration An experiment in which a measured amount of base ( or acid) are added to a measured amount of acid ( or base) to calculate something unknown such as molarity, pH.. etc.Equivalence point The point at which an acid is exactly neutralized with a base. Inflection point The point in an acid- base titration at which 0. 5 mole of base has been added to one mole of acid to achieve A- = HA and then pKa = pH Example Inflection points 0. 5 1 1. 5 2 * Alanine is an type on neutral amino acid which has 2 acidic protons (diprotic acids ) ? COOH and ? NH3+ , these neutral amino acids have two titration curves.Net charge +1 0 -1 Zwitter ion Titration curve of Alanine * aspartic acid is an example on acidic amin o acid which has 3 acidic protons (triprotic acids ) ? COOH, ? NH3+ and R- COOH. Acidic amino acids have three titration curves. Acidity of acidic amino acid ? COOH R- COOH ? NH3+ Net charge +1 0 -1 -2 Zwitter ionTitration curve of aspartic amino acid * Lysine is an example on basic amino acid which has 3 basic protons (triprotic acids ) ? COOH, ? NH3+ and R- NH3+. Basic amino acids have also three titration curves. Acidity of basic amino acid ? COOH ? NH3+ R- NH3+ Net charge +2 +1 0 -1 Zwitter ion Tiration curve of Lysine order at low pH 1 1. 3 amino acid is fully protonated.