Why does the pKA of -COOH change when -NH3+ is present in the same molecule?

[Bayoudh101]

Its a very simple question . So when the carboxylic acid radical -COOH is standalone in an aliphatic molecule without any other acid or basic radicals Pka would be around 4.5 . But when something like -NH3+ is present in the same molecule the Pka for -COOH changes , even though -NH3+ wouldn't start reacting until the value of ph is a lot higher ...

 

[Borek]

And the question is...?

 

[Bayoudh101]

And the question is...?

Why is the Pka relative to -COOH change when -NH3+ is in the same molecule ?

 

[epenguin]

Indeed relative to the typical monobasic carboxylic acids with pK_a's somewhat above 4, the amino acids are moderately strong acids with pK_a's a little above 2. I don't know if there is any official line on this but an obvious explanation would be that the positive charge on the amino group stabilises a negative one on the carboxyl.

Then this simpleminded explanation does not work at all well for what happens at the amino group. Amino acid pK_a's are typically somewhat below 10, whereas those of primary amines are well above 10. I - you - could give a handwavy explanation in terms of which way the water molecules of pointing. In fact I am sure that all the explanation fundamentally involves the surrounding water.

The two negative charges on cysteine manage to push up the pK_a of the amino group to 10.7. Maybe a better comparison is glutamine/glutamic acid - the extra negative charge on the latter is worth only 0.35 pH units for the pK of the amino group. Or between aspragine and aspartic acid there is a pK difference of 1. But then the pK of Asn is exceptionally low at 8.8 for some reason. And in glutamine or glutamic acid there is some story I can't recall offhand about the carbon chain bending back allowing internal hydrogen bonding. All very confusing really.

It is a good question, but if there is any answer I think it will be relatively advanced stuff.

I should say that most biochemists tend to be concerned only with what the charges are on side groups and protein end groups at around neutral pHs

 

[lavoisier]

I'm not sure the OP's question is completely clear.
First of all, amino acids are commonly reported with (at least) 2 distinct pK_A's, one for the fully protonated species (including the amino group) going to electrically neutral molecule, and one for the latter going to anion.
The OP was essentially asking why the pK_A changes when a H atom alpha to the carboxyl is replaced by an NH₂ group, BUT he/she also mentioned NH₃⁺, which leaves open to interpretation which pK_A he/she was referring to.

To be more concrete, here's some data from the CRC Handbook of Chemistry and Physics, 85^th edition, page 8-47.
- pK_A of acetic acid (CH₃COOH / CH₃COO^-) = 4.756
- pK_A of monoprotonated glycine (H₃N⁺CH₂COOH / H₃N⁺CH₂COO^-) = 2.35
- pK_A of electrically neutral glycine (H₃N⁺CH₂COO^- / H₂NCH₂COO^-) = 9.78

So, monoprotonated glycine is a stronger acid than acetic acid, not unexpectedly I'd say, given that we need to abstract a proton from a positively charged species.

On the other hand, electrically neutral glycine, which I suppose is in equilibrium between H₃N⁺CH₂COO^- and H₂NCH₂COOH, is a much weaker acid than acetic acid.
I don't have an explanation for this. It appears that the stability of the acetate anion compared to acetic acid is much higher than the one of the glycine anion (assuming that's what it's called) compared to electrically neutral glycine.
@epenguin 's theory may be correct: electrically neutral glycine may be comparatively more stable because of the two charges in the zwitterionic form. Or maybe because it has two possible forms? And could it be that the glycine anion is comparatively unstable because it has to bear a negative charge close to an electron-rich amino group? But this is all speculation from my part. I am sure of the first bit about the protonated amino acid, though: I distinctly remember they explained it to us when we were doing titrations of amino acids at university.