Effect of the secondary structure of poly(L-lysine) segments on the micellization in aqueous milieu of poly(ethylene glycol) poly(L-lysine) block copolymer partially substituted with a hydrocinnamoyl group at the Nε-position

A series of poly(ethylene glycol)-poly(L-lysine) block copolymers partially substituted in varying degrees with a hydrocinnamoyl group at the N-epsilon-position was prepared by a coupling reaction of hydrocinnamic acid to the epsilon-amino group of the poly(L-lysine) segment of the block copolymer using (benzotriazol-1-yloxy)tris(dimethylamino)hexanuorophosphate (BOP Reagent) as a condensation reagent. The NE-substituted block copolymers with less than 90% substitution formed clear solutions in 0.1 M phosphate buffer (pH 7.4) by dialyzing from DMSO. Light scattering measurements revealed multimolecular micelle formation for the block copolymers with 65-70% of the substitution degree. The cumulant diameter of the micelles was approximately 40 nm with a moderate polydispersity (mu(2)/Gamma(2) similar to 0.15). A fairly low critical association concentration (similar to 40 mg/L) was estimated using pyrene as a fluorescence probe molecule. At a pH as high as 11, the block copolymer with a substitution degree even as low as 50%, which gave only a subtle indication of micellization at pH 7.4, clearly formed multimolecular micelles, suggesting that the balance between the side-chain (hydrocinnamoyl group) interaction and electrostatic repulsion of the protonated E-amino groups may play a substantial role in the micelle stabilization Furthermore, in the circular dichroism spectra of the solutions, a remarkable change in the secondary structure of the copolymers was observed, that is, a change from random coil to beta-sheet structure, either with an increased substitution degree or with a pH increment. Multimolecular micellization concomitantly occurred with beta-sheet formation. Presumably, the layered packing of beta-sheets through the side-chain interaction of aromatic groups and intermolecular hydrogen bonding may stabilize the micelle structure. It is of interest that beta-sheet structures were preserved even below the critical association concentration, suggesting the existence of intra- and/or interstrand interaction of the block copolymers under highly diluted conditions.