(i, i + 4) Ion pairs stabilize helical peptides derived from smooth muscle caldesmon

The central region of smooth muscle caldesmon contains 10 repeats of a 13-amino-acid residue motif that is rich in charged side chains. This region has been predicted to have a strong alpha-helical propensity, and the helical structure was thought to be stabilized by the interactions between oppositely charged side chains of residues at positions i and i + 4 (Wang, C.-L. A., Chalovich, J.M., Graceffa, P., Lu, R.C., Mabuchi, K, and Stafford, W.F., J.Biol. Chem. 266, 13958-13963, 1991). Two synthetic peptides corresponding to these repeats, each containing 25 and 60 amino acid residues, indeed assume an alpha-helical conformation that is stable over a wide range of salt concentration and pH, and exhibit a typical helix-coil transition upon heating. Most significantly, when the amino acid sequence of the 25-mer is randomized without losing the i-to-i + 4 ion pairs, the peptide maintains a helical content comparable to that of the wild-type peptide, whereas another peptide variant with a sequence rearranged to eliminate all i-to-(i + 4) ion pairs has much less helicity, suggesting that specific interactions between residues with (i,i + 4) spacings are important determinants for the maintenance of secondary structure in this region of caldesmon.