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Syntheses and conformational studies of poly(Nε-methyl-L-lysine), poly(Nδ-methyl-L-ornithine), poly(Nδ-ethyl-L-ornithine), and their carbobenzoxy derivatives
Authors:Hiroyuki Yamamoto  Jen Tsi Yang
Abstract:The helix–coil transitions of poly(Nε-methyl, Nε-carbobenzoxy-L -lysine), poly(Nδ-methyl, Nδ-carbobenzoxy-L -ornithine), and poly(Nδ-ethyl, Nδ-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, ΔH = ?370 cal/mol and ΔS = ?1.1 eu/mol for poly(Nε-methyl-L -lysine), and ΔH = ?540 cal/mol and ΔS = ?1.6 eu/mol for poly(Nδ-ethyl-L -ornithine) (all on molar residue basis). The absolute values of ΔH and ΔS 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(Nδ-methyl-L -ornithine) the coil-to-helix transition was not complete even when the polymer was uncharged at high pH.
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