Isoelectric point and the influence of pH in an Amino Acid

[onedirection]

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Just a quick question because of a contradiction in something I've learned

if the pH were to increase what happens and if it were to decrease what happens

if pH increases: there are fewer protons around and the pH is greater than the isoelectronic point pH therefore the Amino acid is overall negative because it loses the available proton from the carboxylic end? Or does the carboxylic and Amino end both give up protons for the negative charge?


if pH decreases, then there are more protons around. Are both ends of the amino acid protonated to lead to a positive charge? Or the carboxylic side is protonated and the amino side left the same?

 

[Ansar]

I'm not entirely sure if this is the right way to think about it, but basically, equilibrium always wants to be reached, so in a more basic solution, there are less H+ floating around, so the amino acid (zwitterion) will give off it's H (from the -OH) in order to balance things out, hence leaving you with an amino acid that has the carboxylic acid group as COO- (the reverse for a low pH, where there is an overabundance of H+ and the amine group of the zwitterion 'sweeps' up the excess H+ that it can turning your -NH2 into -NH3+)

 

[onedirection]

Awesome. Thank you!!

 

[Endure]

Awesome. Thank you!!

It also helps to look at both the carboxylic acid terminal and the amino terminal of an amino acid during a pI experiment (basically, a fancy titration for amino acids).

In an acidic solution, the amino group and carboxylic acid are protonated with pKa values of 9 and 5, respectively. As the solution becomes more basic, the carboxylic acid tosses a proton and forms a carboxylate anion. At the first equivalence point, all of the original amino acid has its C-terminal deprotonated (a zwitterion). As the pH of the solution continues to increase, the ammonium cation at the N-terminal tosses a proton and forms an anime.

Take away: as the solution becomes increasingly basic, the carboxylic acid tosses its proton first, followed by the amino group.