Nanoparticle Formation from Hybrid, Multiblock Copolymers of Poly(Acrylic Acid) and VPGVG Peptide

Elastin-mimetic hybrid copolymers with an alternating molecular architecture were synthesized via the step growth polymerization of azide-functionalized, telechelic poly(tert-butyl acrylate) (PtBA) and an alkyne-terminated, valine and glycine-rich peptide with a sequence of (VPGVG)₂ (VG2). The resultant hybrid copolymer, [PtBA-VG2]_n, contains up to six constituent building blocks and has a polydispersity index (PDI) of ~1.9. Trifluoroacetic acid (TFA) treatment of [PtBA-VG2]_n gave rise to an alternating copolymer of poly(acrylic acid) (PAA) and VG2 ([PAA-VG2]_n). The modular design permits facile adjustment of the copolymer composition by varying the molecular weight of PAA (22 and 63 repeat units). Characterization by dynamic light scattering indicated that the multiblock copolymers formed discrete nanoparticles at room temperature in aqueous solution at pH 3.8, with an average diameter of 250-270 nm and a particle size distribution of 0.34 for multiblock copolymers containing PAA₂₂ and 0.17 for those containing PAA₆₃. Upon increasing the pH to 7.4, both types of particles were able to swell without being disintegrated, reaching an average diameter of 285-300 nm for [PAA₂₂-VG2]_n and 330-350 nm for [PAA₆₃-VG2]_n, respectively. The nanoparticles were not dissociated upon the addition of urea, further confirming their unusual stability. The nanoparticles were capable of sequestering a hydrophobic fluorescent dye (pyrene), and the critical aggregation concentration (CAC) was determined to be 1.09 × 10^-2 or 1.05 × 10^-2 mg/mL for [PAA₂₂-VG2]_n and [PAA₆₃-VG2]_n, respectively. We suggest that the multiblock copolymers form through collective H-bonding and hydrophobic interactions between the PAA and VG2 peptide units, and that the unusual stability of the multiblock nanoparticles is conferred by the multiblock architecture. These hybrid multiblock copolymers are potentially useful as pH-responsive drug delivery vehicles, with the possibility of drug loading through concerted H-bonds and hydrophobic interactions.