Self-Assembling Peptide−Polymer Conjugates Comprising (d-alt-l)-Cyclopeptides as Aggregator Domains

The synthesis of peptide−polymer conjugates comprising (d-alt-l)-cyclopeptides as aggregator domains and their self-assembly into tubelike structures is described. By coupling two well-defined poly(n-butyl acrylate) blocks to opposite sides of a preformed cyclic (d-alt-l)-α-octapeptide, a coil−ring−coil bioconjugate was accessed. The applied solution-phase coupling route allowed a multigram scale synthesis of the conjugate and assured both a controlled synthesis and ease of analysis. The controlled self-assembly of the conjugate leads to uniform tube structures. Atomic force microscopy (AFM) of these aggregates deposited on mica revealed a height of 1.4 ± 0.2 nm, a width of 5 nm, and roughly estimated lengths of up to 200−300 nm. A model is proposed, explaining the structure dimensions. This includes the formation of a tubular peptide core build via stacking of the cyclopeptides and a poly(n-butyl acrylate) shell wrapping around the peptide tube. The model is consistent with infrared spectroscopy and electron diffraction measurements, verifying that the peptide segment of the conjugate adopts a β-sheet structure, similar to unsubstituted (d-alt-l)-cyclopeptides. Hence, the stacks of peptide rings are stabilized along the fiber axis via inter-ring β-sheet H-bonding. The tube structures are capable to interact laterally, organizing further into weak networks as was evidenced by AFM and transmission electron microscopy.