Question

How many (appoximatly) equivalents of NaOH would be needed to completely ionize the peptide HIKLMN of you started at pH = 0?

Answer 1

1 equivalent of OH from NaOH would convert one equivalent of COOH to COO^-.

Example : An equivalent is the amount of NaOH in moles needed to convert COOH to COO^- . Therefore if the amount of an amino acid is 0.1 moles then equal moles of NaOH i.e 0.1 moles would be needed to convert its COOH group to COO^- ion.

Coming to the question At low ph =0 all of amino acid side chains with ionizable groups (imidazole ring of histidine and lysine side chain), amino terminal and carboxyl terminal will be protonated, because H⁺ concentration in the solution is high. One equivalent of NaOH will ionize COOH to COO- group. There are 3 other ionizable groups (amino terminal group, side chain of histidine and lysine). Therefore three more equivalents of NaOH would be needed to ionize these groups. In total 4 equivalents of NaOH would be required to completely ionize the peptide.

First ionizable group is terminal COOH (pka 3.8 )

Second ionizable group is imidazole ring of histidine (pka 6)

third ionizable group is amino terminal of peptide (pka 7.8)

fourth ionizable group is lysine side chain amino group (pka 10.53)