Hydrogen bonding interactions affect the hierarchical self-assembly and secondary structures of comb-like polypeptide supramolecular complexes displaying photoresponsive behavior

Abstract

In this study we blended 4-(4-hexadecylphenylazo)pyridine (AzoPy-C16), synthesized through facile diazonium and monoetherification reactions, with polytyrosine (PTyr) to form comb-like polypeptide supramolecular complexes stabilized through hydrogen bonding between the pyridyl ring of each AzoPy-C16 unit and the OH groups of PTyr. The secondary structure of PTyr transformed from an α-helical to a β-sheet conformation upon the addition of AzoPy-C16, because the long alkyl chains of the AzoPy-C16 units disrupted the weak intramolecular hydrogen bonds between the CO and NH groups in the α-helical conformation, as determined using Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopy. Small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) suggested a hierarchical lamellae-within-lamellae structure for the PTyr/AzoPy-C16 supramolecular complex, featuring long-range-ordered lamellae arising from the antiparallel β-pleated sheet conformation of PTyr (d = 1.15 nm) within long-range-ordered lamellae arising from the alkyl groups of AzoPy-C16 units (d = 5.94 nm), oriented in a perpendicular manner. The d-spacing and long-range order of the lamellar structure formed from the alkyl groups decreased upon UV irradiation as the rod-like trans isomers of the AzoPy-C16 units transformed into V-shaped cis counterparts. This phenomenon also led to a change in the water contact angle of the supramolecular material, with the hydrophobic surface of PTyr/trans-AzoPy-C16 (94.2°) transforming to a hydrophilic surface for PTyr/cis-AzoPy-C16 (61.3°).