Stabilized α-Helix Structure of Poly(l-lysine)-block-poly(ethylene glycol) in Aqueous Medium through Supramolecular Assembly

Stabilization of the α-helix structure of the poly( l -lysine) segment in a poly(ethylene glycol)−poly( l -lysine) block copolymer was evidenced in aqueous medium through circular dichroism (CD), 1 H-NMR, and static light scattering (SLS) measurements. It was revealed that poly( l -lysine) oligomers ( M w = 2450), which themselves cannot form an α-helix structure due to substantially lower molecular weight, form an α-helix structure under high-pH condition when they are conjugated with poly(ethylene glycol). Moreover, it has been suggested by static light scattering that the block copolymer under high-pH condition exists as a dimer with α-helical poly( l -lysine) segments. The α-helix content in the block copolymer gradually decreased in a linear manner with the addition of urea, which is in sharp contrast to the high molecular weight poly( l -lysine) ( M w = 170 000), showing a steep transition from an α-helix structure to a random coil at a critical urea concentration (1.0 M). To completely shift the conformation of the block copolymer to a random coil, 3.0 M urea is needed, suggesting a substantial significance of the stabilization effect. From 2D-NOESY 1 H-NMR and static light scattering measurement, it was suggested that the block copolymers selectively form dimers with a micelle-like structure: a poly(ethylene glycol) shell segregates the poly( l -lysine) segment from the aqueous medium to stabilize the α-helix structure.