Syntheses and conformational studies of poly(Nε‐methyl‐L‐lysine), poly(Nδ‐methyl‐L‐ornithine), poly(Nδ‐ethyl‐L‐ornithine), and their carbobenzoxy derivatives

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.