Question

Determine the isolelectric point (pI) for peptide CHEMTRY.

Draw a rectangle to identify the six atoms that are coplanar with the peptide bond between residue 2 and 3 in the peptide CHEMTRY.

Answer 1

Ans. In a peptide chain, the N-ter and C-ter group of all residues except the terminal residues participate peptide bond formation.

So, the following groups are included for pI calculation-

I. The –NH2 group of N-terminal residue

II. The –COOH group of C-terminal residues

III. The ionizable side chains of all residues

Now,

pI of a peptide = (Sum of pKa of all ionizable groups) / No. of groups pKa involved

Or, pI = (10.28 +6.04 + 4.15 + 12.10+ 10.10 + 2.24) /6 = 7.485

#. The atoms constituting the peptide bond (NH-CO) and the adjacent C-atoms on either side of the peptide bond skeleton are planer.