Question

You are observing, by NMR, the Hα resonance of the N-terminal methionine residue in a small peptide. You find that the chemical shift of the resonance is pH dependent, as tabulated below. Provide the simplest explanation for this pH dependence.

pH δ (pH)

3.5 4.50 ppm

4.5 4.50

5.5 4.49

6.5 4.40

7.0 4.26

7.5 4.00

8.0 3.74

8.5 3.60

9.5 3.51

10.5 3.50

11.5 3.50

Plot δ vs pH for the Hα resonance. What is the pKa of the ionizable group?

Answer 1

As pH value increases H⁺ ion concentration decreases. At lower pH as H⁺ ion concentration is more hence the possibility of protonation of amino group –NH2 is more and forms quaternary ammonium species as we can see in diagram. Hence the electron withdrawing capacity of -N increases which pulls electron density from alpha C and ultimately make alpha H dieshielded and hence at lower pH side we can see higher chemical shift value.

The graph of chemical shift verses pH value shown in graph.

Pka of ionizable group is the ratio of concentration of ionized species to the concentration of non-ionized group. For –COOH and –NH3⁺ is shown in diagram.