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- Thread starter fa21212
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Hi @fa21212 -
Excellent question! This actually is not an error; the titration curve you see in the passage is a real experimental finding (note also that it's a potentiometric titration curve, instead of having pH on the y-axis), and the best explanation for it is that the thiol group is too weakly acidic to produce a second distinct equivalence point. At the same time, it is true that classic amino acid titration curves have two distinct equivalence points, with the second one (for protons with a similar pKa, in the 9-10 range) occurring at a very high pH, but it may be the case that capturing that second equivalence point for the much more weakly acidic proton is beyond the ability of the method that was used. In any case, the task that you're faced with when answering this question is to determine which proton this equivalence point corresponds to, and the solidly acidic -COOH is the best bet.
Hope this helps, and keep up the careful & thoughtful review!
Excellent question! This actually is not an error; the titration curve you see in the passage is a real experimental finding (note also that it's a potentiometric titration curve, instead of having pH on the y-axis), and the best explanation for it is that the thiol group is too weakly acidic to produce a second distinct equivalence point. At the same time, it is true that classic amino acid titration curves have two distinct equivalence points, with the second one (for protons with a similar pKa, in the 9-10 range) occurring at a very high pH, but it may be the case that capturing that second equivalence point for the much more weakly acidic proton is beyond the ability of the method that was used. In any case, the task that you're faced with when answering this question is to determine which proton this equivalence point corresponds to, and the solidly acidic -COOH is the best bet.
Hope this helps, and keep up the careful & thoughtful review!