STABILIZED ALPHA-HELIX STRUCTURE OF POLY(L-LYSINE)-BLOCK-POLY(ETHYLENE GLYCOL) IN AQUEOUS-MEDIUM THROUGH SUPRAMOLECULAR ASSEMBLY

Stabilization of the alpha-helix structure of the poly(L-lysine) segme nt in a poly(ethylene glycol)-poly(L-lysine) block copolymer was evide nced in aqueous medium through circular dichroism (CD), H-1-NMR, and s tatic light scattering (SLS) measurements. It was revealed that poly(L -lysine) oligomers (M(w) = 2450), which themselves cannot form an alph a-helix structure due to substantially lower molecular weight, form an alpha-helix structure under high-pH condition when they are conjugate d with poly(ethylene glycol). Moreover, it has been suggested by stati c light scattering that the block copolymer under high-pH condition ex ists as a dimer with alpha-helical poly(L-lysine) segments. The alpha- helix content in the block copolymer gradually decreased in a linear m anner with the addition of urea, which is in sharp contrast to the hig h molecular weight poly(L-lysine) (M(w) = 170 000), showing a steep tr ansition from an alpha-helix structure to a random coil at a critical urea concentration (1.0 M). To completely shift the conformation of th e block copolymer to a random coil, 3.0 M urea is needed, suggesting a substantial significance of the stabilization effect. From BD-NOESY H -1-NMR and static light scattering measurement, it was suggested that the block copolymers selectively form dimers with a micelle-like struc ture: a poly(ethylene glycol) shell segregates the poly(L-lysine) segm ent from the aqueous medium to stabilize the alpha-helix structure.