摘要/Abstract
环二肽由两个氨基酸通过肽键环合形成，在氢键相互作用驱动下具有较强的自组装倾向.本工作研究了c-SF，c-SY，c-SH及c-DF等四种环二肽的自组装行为和组装体的荧光特性.实验结果表明，c-SH为无规卷曲而其他三种环二肽均采取β-sheet二级结构，且除c-SH未形成明显组装体外，其他三种环二肽均形成不同尺寸的纳米纤维.荧光光谱检测发现环二肽在不同波长的激发下存在多个不同的荧光发射峰；对于c-SH，侧链咪唑基官能团与Zn（II）配位可以增大荧光发射的强度；对于c-SY，侧链酚羟基的氧化也可以增强荧光强度.推测在氢键作用的驱动下环二肽分子可以逐个堆叠形成纳米纤维，自组装导致的分子聚集和分子的侧链结构均可使环二肽具有可调变的荧光性能.
关键词: 环二肽, 2,5-二酮哌嗪, 自组装, 纳米纤维, 荧光性能
Cyclic dipeptide (CDP) is a kind of the smallest cyclic peptide with two amino acids cyclization through amide bonds. The two amide bonds with four hydrogen bonding sites give CDPs a high self-assembly propensity, mainly driven by the hydrogen bonding interactions. In this paper, we have designed four CDPs, c-SF, c-SY, c-SH and c-DF, and studied their self-assembly performance in aqueous solution with circular dichroism spectroscopy (CD) and atomic force microscopy (AFM), including the effects of pH and zinc ion coordination on self-assembly. The fluorescence properties of CDP self-assemblies have also been studied. CD results showed that c-SF, c-SY and c-DF adopted a β-sheet conformation, while c-SH was random coil secondary structure at the concentration of 2.0 mmol/L and pH 5.0. AFM results showed that c-SF, c-SY and c-DF could form nanofibers with different diameters ranged from 1.0 to 3.0 nm. In addition, c-SY self-assembled hierarchically over time. Not only the nanofiber diameter gradually increased, but also the nanofibers entangled into 3D networks. Although c-SH did not self-assemble at the concentration of 3.0 mmol/L and pH 7.0, it could form monolayers with the induction of zinc ion at pH 9.0. The self-assemblies of each CDP had different multiple fluorescent emission peaks with excitation of different wavelengths. Especially, c-SF emitted green fluorescent light under UV light of 365 nm. The fluorescent emission intensity of CDPs was much stronger than their corresponding linear dipeptides. It was assumed that the diketopiperazine structure contributed to the fluorescence enhancement. Moreover, the fluorescent emission intensity of CDP self-assemblies was much higher than that of their free molecules, which meant that the ordered aggregation made a significant contribution to the fluorescent properties. Both the coordination of zinc ions with the imidazole groups on histidine and the oxidation of phenolic hydroxyl groups in tyrosine could enhance the fluorescent emission intensity of CDPs. It was assumed that CDP molecules stacked one by one to form nanofibers during self-assembly. The diketopiperazine ring of CDPs and its self-assembly endowed CDPs with special fluorescent properties.
Key words: cyclic dipeptide, diketopiperazine, self-assembly, nanofiber, fluorescence
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