Conformation of poly(L-lysine) homologs in solution

✍ Scribed by Yuen-Wen Tseng; Jen Tsi Yang

• Publisher: Wiley (John Wiley & Sons)
• Year: 1977
• Tongue: English
• Weight: 704 KB
• Volume: 16
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1977.360160414

✦ Synopsis

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 = -(CHz),,NHz. Circular dichroism studies show a pH-induced helix-coil transition in 0.05 M KCl with midpoints a t 9.6, 9.0, and 8.7 for n = 5, 6, and 7, respectively, as compared with 10.1 for (Lys), ( 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 AGO of -310 cal/mol (residue) for the uncharged coil-to-helix transition a t 25°C. The corresponding AHo and ASo are -1740 cal/mol (residue) and -4.8 e.u./mol (residue). Unlike (Lys),, the uncharged helix-to$ transition is slow and incomplete even after heating a t 80°C for 1 hr. Addition of methanol enhances the helical formation in neutral solution with midpoints at 72,52, and 27% methanol ( u / u ) for n = 5,6, and 7, respectively [cf. 88% for (Lys),]. Addition of sodium dodecyl sulfate induces a coil-to-helix transition for all three homologs in contrast with the p form of (Lys), under similar conditions.