(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 α-helical propensity, and the helical structure was thought to be stabilized by the interactions between oppositely charged side chains of residues at positionsiandi+ 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 α-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 thei-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 alli-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.