<prompt> Amino acid with dissociation constant closest to physiological pH isSerineHistidineThreonineProline  <response>: To understand dissociation constant pKa of amino acids one must know the ionizable groups of amino acids.Each amino acid has two ionizable groups i α-carboxyl group -COOH, and ii α-amino group -NH2.Besides these two groups, some amino acids have special ionizable groups. e.g. -SH of cysteine, imidazole of histidine.The dissociation constant pKa is the value of pH at which any ionizable group of the amino acid is in 50 ionized form and 50 non-ionized form.Thus, each amino acid has two pKa values:-pk1:- pH at which 50 of amino acid molecules have a-carboxyl group in the ionized form COO- and 50 molecules have it in unionized form COOH.pk2:- pH at which 50 of amino acid molecules have an a-amino group in the ionized form NH3 and 50 molecules have it in unionized form NH2.The value of pk-1 and pk-2 is different for different amino acids. But it ranges between 3.5-4.0 for pk-1 i.e. pKa of α -carboxyl group is 3.5-4.0 and 8.0-9.0 for pk-2 i.e. pKa of a-amino group is 8.0-9.0.Beside a-carboxyl and a-amino group, some amino acids contain special ionizable group, which will provide one more pKa value pk3. For example pKa of imidazole group of histidine ranges between 6.5-7.4. That means somewhere between pH 6.5-7.4, 50 of histidine molecules in a solution will have imidazole group in ionized form and 50 molecules will have in unionized form 