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

Question

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

BlackTom AI · Accepted Answer

To determine the region corresponding to the isoelectric point (pI) of glycine on a titration curve, we analyze the structure of the curve and the pH values at key regions.
Option I: At very low pH, the molecule is fully protonated with net positive charge; this region corresponds to the starting point of the titration where pH is near 0–1, not the pI. Therefore, identifying I as the pI would be incorrect because pI occurs where net charge is zero, not where the species is fully protonated.
Option II: The point around pH ~2.34 marks the first buffer region where deprotonation begins and the amino group loses a proton; this region is associated with a transition between fully protonated and zwitterionic forms, but the pI for glycine occurs at the neutral form where net charge is zero. Hence, II is not the pI.
Option III: The point at pH ~5.97 corresponds to the region where the molecule has balanced positive and negative charges, i.e., the zwitterion form of glycine with net zero charge. This is exactly the isoelectric point, where the molecule carries no net charge and the curve shows a plateau in charge balance. Thus, this is the correct location for pI.
Option IV: The region at pH ~9.60 aligns with the deprotonation of the carboxyl group and the predominance of the negatively charged species, giving a net negative charge; this is beyond the pI, representing the basic form rather than the point of zero charge. Therefore, IV is not the pI.
Option V: The high-pH region around pH ~11.30 marks the final deprotonation step where the molecule is strongly basic with a negative charge; this is far from the pI and does not reflect zero net charge. Hence, V is incorrect as the pI.
In summary, among the labeled regions, the one that corresponds to the isoelectric point of glycine is the point labeled III, at pH ~5.97, where the molecule exists predominantly as the zwitterion with no net charge.

AGRI20043_2025_SM1 Workshop 2 Quiz

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

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