[Aymeric]

Hello,

The chapter in my course on aminoacids explains that they are amphoteric because the amine functions behave as a base, with the ability to gain a proton (NH2 becoming NH3+) while the carboxylic function behaves as an acid with the ability to lose a proton (COOH becoming COO-).
I would have two questions about this:

1/Since at isoelectric pH both functions are ionized, this means the COOH has already lost its proton when the NH3+ hasen’t lost its proton yet. Does this mean that N has a tendency to hold on to its protons more than O, in a protonic version of electronegativity as it were?

2/Am I right in understanding that "behaving as a base" doesn’t mean that NH2 is going to alkalinize the solution and "behaving as an acid" does not mean that COOH is going to acidify it, and that their charges simply depend on ambient pH but in themselves they have no effect on it? And that, for example, adding a lot of dixarboxylic aminoacids to a solution is not going to acidify it, because all they do is fixate the extra protons in an acidic solution and let them float around in an alkaline one?

Thanks!

[Babcock_Hall]

I infer that your book is using the standard Bronsted-Lowry definition of acid and base in this passage.

1) Consider there values of pK_a1 (R-COOH) and pK_a2 (R'-NH₃⁺) of Alanine, which are 2.34 and 9.69, respectively.  Which group is the stronger acid?  What does that indicate about the relative strengths of the two conjugate bases?

2) I don't fully understand your question, but I am very hesitant to say "no effect."  It is sometimes helpful to think about the forms of the ionizable groups on an amino acid in a well-buffered solution at a given pH.  If the buffering capacity is sufficient then the addition of an amino acid will have at best a slight effect on the pH.  Alternatively, one can imagine titrating the most protonated form of an amino acid using hydroxide ion, and examine what the dominant form(s) is(are) as a function of pH.

[Aymeric]

I infer that your book is using the standard Bronsted-Lowry definition of acid and base in this passage.

1) Consider there values of pK_a1 (R-COOH) and pK_a2 (R'-NH₃⁺) of Alanine, which are 2.34 and 9.69, respectively.  Which group is the stronger acid?  What does that indicate about the relative strengths of the two conjugate bases?

Thank you! This comes up much later in the course, I had completely forgotten about pK’s... I’ll try to be more patient

2) I don't fully understand your question, but I am very hesitant to say "no effect."  It is sometimes helpful to think about the forms of the ionizable groups on an amino acid in a well-buffered solution at a given pH.  If the buffering capacity is sufficient then the addition of an amino acid will have at best a slight effect on the pH.  Alternatively, one can imagine titrating the most protonated form of an amino acid using hydroxide ion, and examine what the dominant form(s) is(are) as a function of pH.

Sorry if my question wasn’t very clear, I must say the whole notion of "behaving as acids or bases" is confusing to me. I guess my problem is that if you take COOH, yes it can release a proton and thus "behave as an acid". But where did this specific proton come from in the first place? if it came from the solution itself because pH had been previously lowered, and then released again because pH has since then gone back up, then the net contribution of COOH to the acidity of the solution is zero, right?

More concretely my course really discusses biochemistry applied to nutrition so pH in general is assumed to be physiological except when talking about urine.
But when we are taught that "at physiological pH, the carboxylic and amine groups of aminoacids are ionized", should we assume that they were "manufactured" like that originally, and thus if we take dicarboxylic aminoacids as an example, their dual carboxylic groups will never actually behave as acids because they were already ionized to begin with (and the opposite for the NH3+ group)? or should we assume that dicarboxylic aminoacids are manufactured with COOH and NH2 and only later on do they really "behave" like an acid and a base by releasing two protons from their COOH, and picking up a proton on their NH2, thus contributing to a net acidification of the solution (in which case I wonder at what point bodily pH would allow the carboxylic group to *not* have been deprotonated)?

I hope my question is a bit clearer, sorry if it isn’t but it’s hard to explain a question about something one doesn’t understand, especially in English while my course is all in French.

Thanks!