Premium
Conformation of poly( L ‐lysine) homologs in solution
Author(s) -
Tseng YuenWen,
Yang Jen Tsi
Publication year - 1977
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1977.360160414
Subject(s) - chemistry , circular dichroism , helix (gastropod) , crystallography , potentiometric titration , residue (chemistry) , lysine , sodium dodecyl sulfate , stereochemistry , methylene , amino acid , medicinal chemistry , chromatography , organic chemistry , biochemistry , ion , ecology , snail , biology
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.