Question

11 III COOH H3NCH Foo foo HINCH H2N-CH cod H2N-CH CH2 CH2 CH2 CH2 CH2 NH3 D pH 6 VA 0 0.5 3 1 1.5 2 2.5 Equivalents of OH

I have attached a titration of a mock amino acid with OH-. I have labeled the different charge states of the amino acid as I, II, III, IV. I have labeled different points on the titration curve as A, B, ....G. I want you to tell me which form(s) of the species I, II, III or IV predominates (is present at greatest concentration) at each of the lettered points on the curve. I would also like you to calculate the PI for the molecule. Relevant pKa's are below:

pKa alpha NH3 group 9.4    pka alpha COOH    1.95    pka side chain NH3   approximately 10

Answer 1

At point A and B, form I will predominate as pH<pKa of acid the acid group won't be deprotonated.

at point C and D, form II will predominate as 2<pH<9, alpha COOH group will be deprotonated but alpha NH2 won't.

At point E and F, form III will predominate as 9.5<pH<10, alpha NH3+ will be deprotonated.

At point G, form IV wiil predominate as 10<pH, side chain NH3+ will be deprotonated.

pI of the molecule would be

1/2(pKα(2) + pKα(3)) = 1/2/9.4 + 10) = 9.7