a Helix Poly(Glu) a Helix - Random conformation Poly(Lys) Random conformation O 2 4 6 8 10 12 14 pH Specific rotation Nelson & Cox, Lehninger Principles of Biochemistry, 8e, © 2021 W. H. Freeman and Company
Specific rotation is a measure of a solution's capacity to rotate circularly polarized light. 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. Polyglutamate, a polypeptide made up of only l-Glu residues, is an α helix at pH 3. When researchers raise the pH 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 researchers lower the pH to 7, the specific rotation also decreases, as shown in the graph.
The graph plots pH on the horizontal axis ranging from 0 to 14, labeled in increments of 2, against specific rotation on the vertical axis. The curve for poly (Lys) begins as a horizontal line just above the horizontal axis. This is labeled, random conformation. At 8.5, it bends sharply upward and becomes diagonal at 8.8 until 9.9, when it reaches a height of two-thirds of the height of the vertical axis and levels off to run horizontally until it ends at the same height at a p H of 14. The end is labeled alpha helix. The curve for poly (Glu) begins near the top of the vertical axis, where it is labeled alpha helix, and runs horizontally until it reaches 6.1, when it curves sharply down to a diagonal that continues to a height of about half the height of the vertical axis at p H 7.5, when it levels off to run horizontally to end at about half the height of the vertical axis at 12.2. This is labeled, random conformation. All data are approximate.
Question: Explain 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?
Trending now
This is a popular solution!
Step by step
Solved in 3 steps
