A Modular Approach to Polymer Architecture Control via Catenation of Prefabricated Biomolecular Segments: Polymers Containing Parallel β-Sheets Templated by a Phenoxathiin-Based Reverse Turn Mimic

A biomolecular Lego set modular method whereby prefabricated building blocks are linked block by block has been developed and applied to the synthesis of peptide-based polymers containing parallel β-sheets induced by phenoxathiin derivatives acting as reverse turn mimics. Spectroscopic studies show that phenoxathiin is an effective template for β-sheet formation allowing even weak hydrogen acceptors such as ester amides to exist almost exclusively in intramolecularly hydrogen-bonded conformations. Replacing the phenoxathiin derivative with flexible hydrocarbon chains results in substantial loss of intramolecular hydrogen bonding. Solid state FTIR of the polymers revealed that the expected parallel β-sheets were retained in the polymer solely due to the presence of the rigid phenoxathiin template. Conformationally unrestricted units incapable of inducing sheet formation provide mostly random coils and contribute to interchain and intersheet antiparallel hydrogen bonding. The nature of the β-sheet domains has been confirmed through study of model octapeptides. DSC and TGA studies reveal that as the flexibility of the linkers decreases Tg and onset decomposition temperature also decrease. Powder X-ray diffraction of the unoriented polymers shows that they are semicrystalline