Why does alanine salt have a pH of 6.113?

[fangrz]

I understand that alanine salt ooks like this:

However, does it look like that in solution? Does the salt dissociate? I know that alanine salt has a pH of about 6.113 because it is the average of the 2 pKas. However, if this salt dissociates, it seems that the carboxyl group is a weak base. Does this mean that the amine group is a weak acid? I cannot find online that the amine group goes from NH2 to NH (unless in a protein backbone), but I do see that it can go from NH2 to NH3+?

 

[epenguin]

In the last words you seem to have the essential of the answer to your question.

 

[fangrz]

In the last words you seem to have the essential of the answer to your question.

Thank you! I gather that the salt is soluble, so there is Na+ in solution, and its effect on pH is negligible. So, I am now looking at the amino acid portion of the salt. I understand how normal dehydrolysis works to make the protein backbone. However, there can't seem to be a dehydrolysis reaction occurring here in order to make NH2 go to NH since the carboxyl group is deprotonated?

 

[epenguin]

You really need to revise on almost any book ionisations of amines, carboxylic acids, and amino acids, then you could ask better questions. You will see no reaction "NH2 go to NH" just -NH₂ to -NH₃⁺.

Actually the sodium alanine salt will not have a pH near 6.1. Neutral alanine, e.g. what you get out of the bottle labeled alanine will give you that pH and essentially all the molecules are zwitterions (look it up). If instead you get out of the bottle labeled "alanine Na Salt" (it could be called sodium alaninate but it never is) then you have essentially what you have pictured but if you dissolve it it will be alkaline. Yes it is fully dissociated.

 

[fangrz]

You really need to revise on almost any book ionisations of amines, carboxylic acids, and amino acids, then you could ask better questions. You will see no reaction "NH2 go to NH" just -NH₂ to -NH₃⁺.

Actually the sodium alanine salt will not have a pH near 6.1. Neutral alanine, e.g. what you get out of the bottle labeled alanine will give you that pH and essentially all the molecules are zwitterions (look it up). If instead you get out of the bottle labeled "alanine Na Salt" (it could be called sodium alaninate but it never is) then you have essentially what you have pictured but if you dissolve it it will be alkaline. Yes it is fully dissociated.

Thank you! I apologize if my terminology is bad; I have not taken organic chemistry yet. I looked up zwitterions and pIs; is it true that the pI is essentially the pH of the neutral pure amino acid? Also, the alaninate salt is basic because of the -COO- group accepting protons, right?

 

[epenguin]

Thank you! I apologize if my terminology is bad; I have not taken organic chemistry yet. I looked up zwitterions and pIs; is it true that the pI is essentially the pH of the neutral pure amino acid?

That's right.

Also, the alaninate salt is basic because of the -COO- group accepting protons, right?

I wouldn't say that's a good way of putting it, in fact not right. If you dissolve some alanine it will be pH about 6, and essentially all zwitterIonic ⁺NH₃CH₃CHCOO^- . The salt is what you get if to that per mole of alanine you add one mole of NaOH. You would expect that to also make it alkaline. Doing that you won't have done anything to the -COO^- group which will remain negatively charged but rather you will have deprotonated the amino -NH₃⁺ group to make it -NH₂.

You don't need to know everything about the chemistry to get the basic ideas of amino acids (we are talking only of their physical aspect) but you need to at least some basics of them, and physical chemistry of acids and bases in general, at least enough to be comfortable with these formulae.

 

[fangrz]

That's right.

I wouldn't say that's a good way of putting it, in fact not right. If you dissolve some alanine it will be pH about 6, and essentially all zwitterIonic ⁺NH₃CH₃CHCOO^- . The salt is what you get if to that per mole of alanine you add one mole of NaOH. You would expect that to also make it alkaline. Doing that you won't have done anything to the -COO^- group which will remain negatively charged but rather you will have deprotonated the amino -NH₃⁺ group to make it -NH₂.

You don't need to know everything about the chemistry to get the basic ideas of amino acids (we are talking only of their physical aspect) but you need to at least some basics of them, and physical chemistry of acids and bases in general, at least enough to be comfortable with these formulae.

Thank you again! When you dissolve alanine in water, doesn't it form a polymer? Is the reason why it is acidic is that at the end of the chain, the carboxyl group acts as a weak acid, overriding the weak base effect of the amine group? In this regard, are there multiple polymer chains in solution? I am not quite sure what alanine dissolved in water actually looks like. I really appreciate your help.

 

[epenguin]

Thank you again! When you dissolve alanine in water, doesn't it form a polymer? Is the reason why it is acidic is that at the end of the chain, the carboxyl group acts as a weak acid, overriding the weak base effect of the amine group? In this regard, are there multiple polymer chains in solution? I am not quite sure what alanine dissolved in water actually looks like. I really appreciate your help.

No not in water. You are right that there is a sort of chemical possibility there in principle of polymerization, but the equilibrium is all against it. Look at the supposed polymerization reaction - it's actually getting rid of water. In fact you can get a sort of messy polymerization by heating nearly dry amino acids, I forget exact conditions, which has interested some Origin of life people. Of course proteins in life are synthesized from amino acids but the latter need to be activated by adenylyation if that means anything to you, plus a whole lot of other stuff which is a large fraction of molecular biology, not suitable for one paragraph summaries! .

 

[fangrz]

No not in water. You are right that there is a sort of chemical possibility there in principle of polymerization, but the equilibrium is all against it. Look at the supposed polymerization reaction - it's actually getting rid of water. In fact you can get a sort of messy polymerization by heating nearly dry amino acids, I forget exact conditions, which has interested some Origin of life people. Of course proteins in life are synthesized from amino acids but the latter need to be activated by adenylyation if that means anything to you, plus a whole lot of other stuff which is a large fraction of molecular biology, not suitable for one paragraph summaries! .

Thank you! Am I right, though, that for zwitterion amino acids, the NH2 acts as a weak base and the COOH acts as a weak acid, and the weak acid overrides the effect of the weak base to make the pH of the solution acidic? Thanks again (I keep saying this but I really appreciate your explanations!)

 

[Borek]

Am I right, though, that for zwitterion amino acids, the NH2 acts as a weak base and the COOH acts as a weak acid, and the weak acid overrides the effect of the weak base to make the pH of the solution acidic?

It depends on their relative strengths, so in general, depending on the amino acid, the solution can be either slightly acidic or slightly basic.

 

[fangrz]

No not in water. You are right that there is a sort of chemical possibility there in principle of polymerization, but the equilibrium is all against it. Look at the supposed polymerization reaction - it's actually getting rid of water. In fact you can get a sort of messy polymerization by heating nearly dry amino acids, I forget exact conditions, which has interested some Origin of life people. Of course proteins in life are synthesized from amino acids but the latter need to be activated by adenylyation if that means anything to you, plus a whole lot of other stuff which is a large fraction of molecular biology, not suitable for one paragraph summaries! .

It depends on their relative strengths, so in general, depending on the amino acid, the solution can be either slightly acidic or slightly basic.

Thank you everyone! I realized I had it mixed up, since amino acids exist in +NH3CH3CHCOO- form.