Amino acid with dissociation constant closest to physiological pH is -

Each amino acid have two ionizable group: (r) c-carboxyl group (-COOH), and (ii) c-amino group (-NHr). Beside these two groups, some amino acids have special ionizable groups. e.g. -SH of cysteine, imidazole of histidine. Dissociation constant (pka) is the value of pH at which any ionizable group of amino acid is in 50% ionized form and 50% non-ionized form. Thus, each amino acid has two pka values :- i) pk1 - pH at which 50% of amino acid molecules have a-carboryl group in ionized form (COO-) and 50% molecules have it in unionized form (COOH). ii) pk2 - pH at which 50% of amino acid molecules have a-amino group in ionized form (NH3+) and 50% molecules have it in unionized form (NH2). iii) The value of pk-l and pk-2 is different for different amino acids. But it ranges between 3.5-4.0 for pk-l (i.e. pka of a-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). iv) 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. v) Thus, imidazole group of histidine has pka value closest to physiological pH.