An optically active porphyrinâpentapeptide conjugate 1, actually a porphyrinato zinc complex covalently linked with a glycinylâalanylâglycinylâalanylâglycine (GAGAG) peptide chain, was designed and synthesized. The self-assembly properties of this novel porphyrinâpentapeptide 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 (1â:â3) and polar THF/water (1â:â3) system depending on the cooperation between intramolecular or intermolecular hydrogen bonding interaction with chiral discrimination between pentapeptide chains and porphyrinâporphyrin 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 (1â:â3) and THF/water (1â:â3). 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 porphyrinâpentapeptide 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 porphyrinâpeptide conjugates. In addition, both nanofibers and nanotubes were revealed to show good semiconducting properties.
本研究设计并合成了一种具有光学活性的
卟啉五肽共轭物 1,它实际上是一种与甘
氨酰甘
氨酰甘
氨酰甘
氨酸(GAGAG)肽链共价连接的
卟啉锌络合物。通过电子吸收、圆二色(CD)、红外光谱、透射电子显微镜(
TEM)、扫描电子显微镜(
SEM)和 X 射线衍射(XRD)技术,比较研究了这种新型
卟啉五肽共轭物在 THF/n-hexane 和 THF/water 中的自组装特性。共轭物 1 在不同溶剂体系中与
卟啉环共价连接的五肽链具有不同的次级构象,根据 CD 光谱结果,在无极性 THF/n-hexane (1â. â3) 和极性 THF/n-hexane (1â. â3) 溶液中,共轭物 1 的自组装导致在
卟啉发色团堆叠中形成右手螺旋排列的纳米纤维和左手螺旋排列的纳米管:3)和极性
四氢呋喃/
水(1â:â3)体系中,分子内或分子间的
氢键相互作用与五肽链之间的手性辨别以及
卟啉与
卟啉在平行于相邻共轭分子的四
吡咯环方向上的相互作用之间的合作关系。红外光谱结果清楚地显示了在 THF/n-hexane (1â:â3) 和 THF/water (1â:â3) 中形成的纳米结构中,
卟啉核心上附着的五肽链分别采用了δ-螺旋和δ-片二级构象。X 射线衍射(XRD)结果证实,在纳米管中,通过两个共轭分子之间的分子间
氢键作用,形成了二聚超分子双层结构。这些结果清楚地揭示了共轭分子中五肽链的次级构象对
卟啉发色团的堆积模式、超分子手性和自组装纳米结构的形态的影响。本成果不仅是首个由共价连接的
卟啉五肽共轭物自组装成有机纳米结构的实例,更重要的是首次尝试通过调节肽在不同溶剂体系中的次级构象来控制和调节
卟啉纳米结构的形态,特别是超分子手性,这有助于理解、设计、制备和模拟
天然存在的
卟啉肽共轭物的结构和作用。此外,纳米纤维和纳米管都显示出良好的半导体特性。