Question

Explain the difference in the \({\bf{p}}{{\bf{K}}_{\bf{a}}}\) values of the carboxyl groups of alanine, serine, and cysteine.

Short answer

The increasing order of the of the \(p{K_a}\) amino acids is asfollows:

serine<cysteine<alanine.

The difference in the acidity of the amino acids is due to the electron-withdrawing groups present in them.

Step-by-step solution

pKa value

The value of pK_a is based on theacidityof a particular compound or the easiness of protonation. The compound having high acidity can protonate easily and has a low pK_a value.

Explaining the difference in the pKa values of the carboxyl groups of alanine, serine, and cysteine

It is expected that serine and cysteine have lower pK_a values than alanine since the hydroxyl-methyl and thiomethyl groups are more electron-withdrawing than a methyl group. As oxygen is more electronegative than sulfur, serine is expected to have a lower \(p{K_a}\) value than cysteine. The lower \(p{K_a}\) value of cysteine than serine can be explained by the stabilization of serine’s carboxyl proton by hydrogen bonding to the \(\beta \)-OH group of serine, which causes it to have less of a tendency to be removed by a base.

If the acidity or the easiness of protonation is high for a compound, the value of \(p{K_a}\) is less.

The increasing order of the \(p{K_a}\) of the amino acids is as follows:

serine<cysteine<alanine.

The difference in the acidity of the amino acids is due to the electron-withdrawing groups present in them.