Question

Estimate the isoelectric point of a Ser–His dipeptide. Explain why this value is only an estimate.

Short answer

The isoelectric point of Ser-His is 6.69.

Step-by-step solution

Isoelectric point definition:

The isoelectric point is a point at which there is no net electric charge on the molecule, otherwise it will be a zwitterion.

Dipeptide Definition (Ser-His):

A bond formed by the peptide bond formation between the Serine and Histidine peptide is called dipeptide bond.

Isoelectric point calculation:

The pK1, pK₂and pK_Rvalues for both Serine and Histidine amino acids are written below:

pI- isoelectric point for individual amino acids .

Serine:

pk₁=2.19; pK₂=9.21

pI= 1/2(pK₁+pK₂) =5.7

Histidine:

pk₁=1.80, pK₂=9.33, pK_R= 6.04

pI=1/2(pK₁+ pK_R) =7.685

The isoelectric point in a dipeptide can be estimated by average pI of both serine and histidine.

Average

pI= (pI Ser + pI His)/2

pI = (5.7+7.685)/2

pI = 6.69

This a rough calculation because the actual value can only be determined by experimentation which can be more or less than 6.69.