The unfolding of a polypeptide ?-helix to a randomly coiled conformation is acco

Question

The unfolding of a polypeptide ?-helix to a randomly coiled conformation is accompanied by a large decrease in a property called specific rotation, a measure of a protein’s capacity to rotate plane-polarized light. Polyglutamate, a polypeptide made up of only L-glutamate amino acid residues, has ?-helical conformation at pH 3. When the pH is raised to 5, there is a large decrease in the specific rotation of the solution. Similarly, polylysine (L-Lysine amino acid residues) is an ?-helix at pH 11.5, but when the pH is lowered to 9.5 the specific rotation also decreases.

a) What is the explanation for the change in conformation of poly(Glu) and poly(Lys) when the pH is altered?

b) Why does the transition occur over such a narrow range of pH?

Explanation / Answer

Ans:-(a) :- At pH values above 6 their occurs the deprotonation of the carboxylate side chains of poly(Glu) which leads to repulsion between adjacent negatively charged groups, and stablizes the alpha helix and results in unfolding. similarly at pH 7 protonation of amino group side chains of poly(Lys) causes repulsion between positively charged groups which leads to unfolding.

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