Conformation of poly( L ‐lysine) homologs in solution

The effect of the number of methylene groups in the side chains on the conformation of polypeptides is assessed for three poly( L ‐lysine) homologs with R = –(CH 2 ) n NH 2 . Circular dichroism studies show a pH‐induced helix–coil transition in 0.05 M KCl with midpoints at 9.6, 9.0, and 8.7 for n = 5, 6, and 7, respectively, as compared with 10.1 for (Lys) x ( n = 4). Homologs with n = 6 and 7 could be partially helical even when the side groups are fully charged (with n = 7, the compound is highly aggregated above pH 9.1). Thus, the longer the number of methylene groups the more stable is the helical conformation of these homologs. Potentiometric titration of the n = 5 homolog gives a ΔG° of −310 cal/mol (residue) for the uncharged coil‐to‐helix transition at 25°C. The corresponding Δ H ° and Δ S ° are −1740 cal/mol (residue) and −4.8 e.u./mol (residue). Unlike (Lys) x , the uncharged helix‐to‐β transition is slow and incomplete even after heating at 80°C for 1 hr. Addition of methanol enhances the helical formation in neutral solution with midpoints at 72, 52, and 27% methanol ( v / v ) for n = 5, 6, and 7, respectively [cf. 88% for (Lys) x ]. Addition of sodium dodecyl sulfate induces a coil‐to‐helix transition for all three homologs in contrast with the β form of (Lys) x under similar conditions.