Calculate pi of lysine code#

For acidic amino acids, the isoelectric point will be at lower pH as the acidic side chain will introduce an extra negative charge and for basic amino acids, the isoelectric point will be at higher pH as the basic side chain will introduce an extra positive charge. pHs are: aspartate (D), glutamate (E), lysine (K), arginine (R), and histidine (H).

For neutral amino acids, the side chains are neutral and the isoelectric point is given simply by the average of the pK a values of carboxylic acid and amine. The isoelectric point is the pH at which the net charge is zero. The protons on half the alpha-amino groups and R-Groups are removed so the sum of their charge is +1. As we titrate with more hydroxide ions, we reach the pI at pH 10.25, half way between pK a2 (10) and pK a3 (10.5). At this pH, the fraction of lysine in the form H3L2+ is negligible. So, the net charge on the lysine molecules is a positive 0.5. We know that the isoelectric point will be near (pff2 + pA3) 9.95.

Use this condition to calculate the isoelectric pH of lysine. Therefore, at the isoelectric point, 2H3L2+ + H2L+ L.

Calculate pi of lysine code#

Note: The isoelectric point is given by the average of the pK a values that involve the zwitterions, not just by the pK a values that describe the carboxylic acid group and the amine group. pKa and pI values of amino acids Amino acid 3-letter code 1-letter code pKa C-COOH pKa C-NH 3 + pKa side chain Isoelectric point (pI) Alanine Ala A 2.34 9.69 - 6.02 Arginine Arg R 2.17 9.04 12.48 10.76 Asparagine Asn N 2.02 8.80 - 5.41 Aspartic acid Asp D 2.09 9.82 3.86 2.98 Cysteine Cys C 1.71 10.78 8.33 5. The isoelectric point is the pH at which the average charge of lysine is 0. Then calculate the isoelectric point (pI): Strategy: First. Since the isoelectric point is given by the average of the pK a values that involve the zwitterion, so we can write the formula for lysine as: Identify the charge states for aspartic acid (Asp), leucine (Leu) and Lysine (Lys) at pH 1, 7 and 12.