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Syntheses and conformational studies of poly( N ε ‐methyl‐ L ‐lysine), poly( N δ ‐methyl‐ L ‐ornithine), poly( N δ ‐ethyl‐ L ‐ornithine), and their carbobenzoxy derivatives
Author(s) -
Yamamoto Hiroyuki,
Yang Jen Tsi
Publication year - 1974
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.1974.360130603
Subject(s) - chemistry , lysine , chloroform , ornithine , dichloroacetic acid , stereochemistry , side chain , helix (gastropod) , titration , crystallography , polymer , polymer chemistry , amino acid , organic chemistry , arginine , biochemistry , ecology , snail , biology
The helix–coil transitions of poly( N ε ‐methyl, N ε ‐carbobenzoxy‐ L ‐lysine), poly( N δ ‐methyl, N δ ‐carbobenzoxy‐ L ‐ornithine), and poly( N δ ‐ethyl, N δ ‐carbobenzoxy‐ L ‐ornithine) in chloroform–dichloroacetic acid and their corresponding decarbobenzoxylated polypeptides in alkaline solutions were followed by optical rotation measurements. The introduction of a methyl or an ethyl group to the side chains of the carbobenzoxy derivatives of poly( L ‐lysine) and poly( L ‐ornithine) appeared to weaken the helical conformation at 25°C. The thermodynamic quantities of the three water‐soluble polypeptides were calculated from the data on potentiometric titrations at several temperatures. For uncharged coil‐to‐helix transition, Δ H = −370 cal/mol and Δ S = −1.1 eu/mol for poly( N ε ‐methyl‐ L ‐lysine), and Δ H = −540 cal/mol and Δ S = −1.6 eu/mol for poly( N δ ‐ethyl‐ L ‐ornithine) (all on molar residue basis). The absolute values of Δ H and Δ S dropped in the region of pH‐induced transition and eventually both quantities became positive. The initiation factor σ was about 2 × 10 −3 , which was essentially independent of temperature. For poly( N δ ‐methyl‐ L ‐ornithine) the coil‐to‐helix transition was not complete even when the polymer was uncharged at high pH.