Salt bridges do not stabilize polyproline II helices.
Interactions between side chains, and in particular salt bridges, have been shown to be important in the stabilization of secondary structure. Here we investigate the contribution of a salt bridge formed between a lysine and a glutamate to the polyproline II (P(II)) helical content of proline-rich peptides. Since this structure has precisely three residues per turn, charged residues spaced three residues apart are on the same side of the helix and are best situated to interact. By contrast, computer simulations show that charged residues spaced four residues apart are both too far apart to interact strongly and are oriented such that interactions are unlikely. We have measured the P(II) content of peptides containing a lysine and glutamate pair spaced three or four residues apart using circular dichroism spectroscopy. Somewhat surprisingly we find that the P(II) content is insensitive to both the spacing and the pH. These findings indicate that i --> i + 3 salt bridges do not stabilize the P(II) helical conformation. The implications of these observations for both P(II) helix formation and denatured protein conformations are discussed.
Whittington
Shelly J
SJ
Center for Structural Biology, Department of Molecular and Cellular Biochemistry, University of Kentucky, 800 Rose Street, Lexington, Kentucky 40536-0298, USA.
Creamer
Trevor P
TP
D002942:Circular Dichroism; D003198:Computer Simulation; D018698:Glutamic Acid / Q000737:chemistry*; D006860:Hydrogen Bonding; D008239:Lysine / Q000737:chemistry*; D008958:Models, Molecular; D009010:Monte Carlo Method; D009842:Oligopeptides / Q000138:chemical synthesis; D010455:Peptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D012492:Salts / Q000737:chemistry
D009842:Oligopeptides; D010455:Peptides; D012492:Salts; C011083:polyproline; D018698:Glutamic Acid; D008239:Lysine