-α Helix Poly(Glu) -α Helix Random conformation Poly(Lys) Random conformation 0 2 4 8 10 12 14 pH Specific rotation 6.
The unfolding of the α 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 α-helix 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 α helix at pH 10, but when the pH is lowered to 7 the specific rotation also decreases, as shown by the following graph.
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?
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