Question

(2%) Indicate which secondary structure or structures (α -helix, β -pleated, random coil) will the following peptide adopt in an aqueous solution at pH 7

(2%) The unfolding of the alpha helix of a polypeptide to a randomly coiled conformation is accompanied by a large decrease in a property called specific rotation, a measure of
a solution’s capacity to rotate circularly polarized light.

Polyglutamate, a polypeptide made up of only L-Glu residues, has the alpha helical
conformation at pH 3. When the pH is raised to 7, there is a large decrease in the
specific rotation of the solution.

Similarly, polylysine (L-Lys residues) is an alpha helix at pH 10, but when the pH is
lowered to 7 the specific rotation also decreases, as shown by the following graph:

-α Helix Poly(Glu) - Helix Random conformation Poly (Lys) Random conformation 0 2 4 6 8 10 12 14 pH

  

What is the explanation for the effect of the pH changes on the conformations of
poly(Glu) and poly(Lys)? Why does the transition occur over such a narrow range of pH?

(2%) Draw the hydrogen bonding typically found between in an a-helix. Indicate which atoms and from which amino acids are involved in this hydrogen bonding.

(2%) Explain how circular dichroism UV spectroscopy could be used to measure the denaturation of a protein.

(2%) What is the main reason that both Gly and Pro are not usually found in a-helices?

Answer 1

ans-The folding and unfolding of the Poly(Glu) and Poly(Lys) a helices results from changes in the charge of the side-chains of these amino acids as a function of pH.As the pH of the solution containing Poly(Glu) is raised, the carboxyl group in the Glu side-chain leads to repulsion between adjacent negatively charged groups due to the deprotonation and becomes negatively charged cause destabilization of alpha helix and it becomes unfold.

For Poly(Lys), elevation of pH at 8 causes the proton on the a-amino side-chain to dissociate due to the repulsion between positively charged groups this narrow transition occurs because deprotonation of only a few of each of the amino acid donate charge to the helix and this leads to the repulsion and allows the unfolding of the alpha helix.