The isoelectric point of a molecule can be calculated from the pKa values of its ionisable groups.Calculate the isoelectric point of the molecule below. Give your answer to two decimal points.

Question

BlackTom AI · Accepted Answer

Begin by identifying the ionisable groups shown in the molecule and their pKa values. The diagram indicates two key ionisable sites: a carboxylic acid with pKa = 2.9 and an amine group with pKa = 10.6.

Consider how the molecule is charged at very low pH: the amine is protonated (NH3+) giving a positive charge, while the carboxyl group is in the COOH form (uncharged). Overall, the molecule carries a +1 charge at low pH.

As pH increases and passes each pKa, deprotonation occurs at the respective group. First, around pH 2.9, the carboxyl group loses a proton to become COO−, while the amine remains protonated. This yields a net charge of 0 because +1 from the ammonium is balanced by −1 from the carboxylate.

As the pH continues to rise, the amine group (NH3+) deprotonates around pH 10.6 to form NH2, removing that positive charge. At this stage, with the carboxylate still present as COO− and the amine neutral, the net charge would be −1. However, the isoelectric point is defined as the pH at which the molecule has zero net charge, which occurs when the carboxylate is already deprotonated (−1) but before the amine deprotonates to remove the +1, i.e., in the interval between the two pKa values.

Thus, the isoelectric point is given by the average of the two pKa values that surround the neutral species: pI = (pKa(carboxyl) + pKa(amine)) / 2 = (2.9 + 10.6) / 2.

Compute the average: 2.9 + 10.6 = 13.5; 13.5 / 2 = 6.75. Rounding to two decimal places gives 6.75.

If you want to cross-check, recall that for amino-like molecules with a basic amine and acidic carboxyl group, the neutral form occurs between deprotonation steps, and the pI is simply the mean of those two pKa values. The numerical result 6.75 is consistent with that rule of thumb.

My LMS Subjects Quiz 3

The isoelectric point of a molecule can be calculated from the pKa values of its ionisable groups.Calculate the isoelectric point of the molecule below. Give your answer to two decimal points.

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