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.

View Explanation

Log in for full answers

Similar Questions

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 one decimal point.

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

Which one of the following amino acids has the highest isoelectric point? (all amino acids are shown at the same pH)

The pI is the pH where a molecules charge is overall neutral . This is sometimes at near pH 7.

What is the pI of the given amino acid?

The titration curve for glycine is shown below. The key points are designated I to V. Select the the point on the titration curve where the average net charge of glycine is zero

The titration curve for glycine is shown below. The key points are designated I to V. Click on the region of the titration curve that represents the isoelectric point (pI) of glycine

In a consumer society, many adults channel creativity into buying things

More Practical Tools for Students Powered by AI Study Helper

Homework AI Solver

Stylized AI Paper Writer

Plagiarism Checker Assistant

Citation AI Academic Writing Tool

In-Class Translation Assistant

AI Note Generator

AI Quiz Answers

Past Exam Questions from University Test Bank

Smart Practice Assistant

Adaptive Practice

Making Your Study Simpler