title: pH-responsive self-assembly and conformational transition of partially propyl-esterified poly(alpha,beta-L-aspartic acid) as amphiphilic biodegradable polyanion.
abstract: Poly(alpha,beta-L-aspartate) (PAsp) was partially esterified to afford an amphiphilic biodegradable polyanion, poly(sodium aspartate-co-propyl aspartate) (PAsp-Na/PAsp-P). The synthesized polyanion could be assembled into the nano-scaled aggregates in aqueous medium. The aggregate morphologies were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as a function of pH. It was demonstrated that micellization of this random copolymer occurred with stimulus of pH changes to form various morphological micelles. The copolymer existed as precipitate at low pH (pH<2). When pH increased to 4, the polymers were associated into spherical micelles with the core of poly(propyl aspartate) (PAsp-P) hydrophobic units and shell of some negatively charged poly(sodium aspartate) (PAsp-Na) units. At higher pH (pH>5), toroidal nanostructures of the micelles were formed because rigid polyamide chains directly assemble into the large hollow spheres. The CD study showed that the conformation underwent a transition between alpha-helix and random coil at pH 3-7. The cooperative transitions were regulated by the degree of ionization of carboxylic side chains. When they were protonated (neutralized), the molecular backbone was in favor of the regular helical structure; when deprotonated (ionized), the electrostatic repulsions among side chains destabilized the intramolecular hydrogen bonds, thus randomizing the regular conformation.
authors.affiliation: The Key Laboratory of Functional Polymer Materials (Ministry of Education), Institute of Polymer Chemistry, Nankai University, Tianjin 300071, People's Republic of China.
authors.forename: Yong
authors.initials: Y
authors.lastname: Wang
authors.forename: Yinong
authors.initials: Y
authors.lastname: Wang
authors.forename: Guolin
authors.initials: G
authors.lastname: Wu
authors.forename: Yunge
authors.initials: Y
authors.lastname: Fan
authors.forename: Jianbiao
authors.initials: J
authors.lastname: Ma
mesh_terms: D001672:Biocompatible Materials / Q000138:chemical synthesis / Q000737:chemistry*; D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D008968:Molecular Conformation; D000071228:Polyelectrolytes; D011108:Polymers / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D001672:Biocompatible Materials; D000071228:Polyelectrolytes; D011108:Polymers