Modulating artificial membrane morphology: pH-induced chromatic transitionand nanostructural transformation of a bolaamphiphilic conjugated polymer from blue helical ribbons to red nanofibers

Design and characterization of helical ribbon assemblies of a bolaamphiphil ic conjugated polymer and their color-coded transformation into nanofibers are described. An L-glutamic acid modified bolaamphiphilic diacetylene lipi d was synthesized and self-assembled into right-handed helical ribbons with micron scale length and nano scale thickness under mild conditions. The ri bbon structures were further stabilized by polymerizing well-aligned diacet ylene units to form bisfunctional polydiacetylenes (PDAs). Transitions from flat sheets to helical ribbons and tubes were observed by transmission ele ctron microscopy. The helical ribbons appear to originate from the rupture of flat sheets along domain edges and the peeling off between stacked lipid layers. These results point to the applicability of chiral packing theory in bolaamphiphilic supramolecular assemblies. Contact mode atomic force mic roscopy observations revealed that high order existed in the surface packin g arrangement. Hexagonal and pseudorectangular packings were observed in fl at and twisted regions of the ribbons, respectively, suggesting a correlati on between microscopic morphologies and nanoscopic packing arrangements. Th e tricarboxylate functionalities of the bolaamphiphilic lipid provide a han dle for the manipulation of the bisfunctional PDAs' morphology. Increasing solution pH caused the fraying of helical ribbons into nanofibers accompani ed by a sharp blue-to-red chromatic transition. A dramatic change in circul ar dichroism spectra was observed during this process, suggesting the loss of chirality in packing. A model is proposed to account for the pH-induced morphological change and chromatic transition. The color-coded transition b etween two distinct microstructures would be useful in the design of sensor s and other "smart" nanomaterials requiring defined molecular templates.