Morphology and chirality controlled self-assembled nanostructures of porphyrin–pentapeptide conjugate: effect of the peptide secondary conformation

Abstract

An optically active porphyrin–pentapeptide conjugate 1, actually a porphyrinato zinc complex covalently linked with a glycinyl–alanyl–glycinyl–alanyl–glycine (GAGAG) peptide chain, was designed and synthesized. The self-assembly properties of this novel porphyrin–pentapeptide conjugate in THF/n-hexane and THF/water were comparatively investigated by electronic absorption, circular dichroism (CD), IR spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) technique. Associated with the different secondary conformation of the pentapeptide chain covalently linked to the porphyrin ring in different solvent systems, self-assembly of conjugate 1 leads to the formation of nanofibers with right-handed helical arrangement and nanotubes with left-handed helical arrangement in a stack of porphyrin chromophores according to the CD spectroscopic result in apolar THF/n-hexane (1 : 3) and polar THF/water (1 : 3) system depending on the cooperation between intramolecular or intermolecular hydrogen bonding interaction with chiral discrimination between pentapeptide chains and porphyrin–porphyrin interactions in the direction parallel to the tetrapyrrole ring of neighboring conjugate molecules. IR spectroscopic result clearly reveals the α-helix and β-sheet secondary conformation, respectively, employed by the pentapeptide chain attached at the porphyrin core in the nanostructures formed in THF/n-hexane (1 : 3) and THF/water (1 : 3). The X-ray diffraction (XRD) result confirms that in the nanotubes, a dimeric supramolecular bilayer structure was formed through an intermolecular hydrogen bonding interaction between two conjugate molecules which, as the building block, self-assembles into the target nanostructures. These results clearly reveal the effect of a secondary conformation of pentapeptide chain in the conjugate molecule on the packing mode of porphyrin chromophore, supramolecular chirality, and morphology of the self-assembled nanostructure. The present result represents not only the first example of organic nanostructures self-assembled from a covalently linked porphyrin–pentapeptide conjugate, but more importantly the first effort towards controlling and tuning the morphology and in particular the supramolecular chirality of porphyrin nanostructures via tuning the secondary conformation of peptides in different solvent systems, which is helpful towards understanding, designing, preparing, and mimicking the structure and role of naturally occurring porphyrin–peptide conjugates. In addition, both nanofibers and nanotubes were revealed to show good semiconducting properties.