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螺旋高分子是一类具有光学活性的手性材料,它们在分子识别、对映体拆分、不对称性催化、传感检测以及医药等领域展现了重要的应用前景[1-5]。螺旋高分子通常由不对称聚合反应制得,当前已有的螺旋高分子种类较多,包括聚苯乙烯[6]、聚异腈[7]、聚醛[8]、聚硅烷[9]、聚异氰酸酯[10-12]、聚胍[13]、聚甲基丙烯酸酯、聚炔等。这些高分子的空间螺旋构象的稳定性主要依赖其侧链基团的空间位阻效应,这使对螺旋结构的后控制与再设计较为困难。众所周知,天然蛋白质结构中的α-螺旋依靠分子内远程氢键作用形成螺旋结构。这一结构特征启发科研工作者侧重认识螺旋结构形成过程中的分子内弱相互作用力[14, 15]。将各种分子内超分子相互作用应用到螺旋结构的设计中,通过氢键、静电、π-π、金属配位等相互作用,可以使高分子链发生螺旋折叠,从而形成自折叠自稳定的螺旋高分子结构。聚合物链在发生螺旋折叠时,高分子能够形成明确的中空结构[16-19]。因此,这类高分子具有空间结构产生的螺旋孔道,是近些年发展的新颖的螺旋高分子孔道材料。螺旋高分子孔道材料除了孔结构材料常有的基本特征外,还具有独特的本征螺旋手性等特征,因而有望拓展全新的性质与功能。本文简要介绍聚合物链自发折叠形成螺旋高分子孔道的近期研究进展,阐述螺旋高分子孔道的设计方法与形成原理,展望它们在孔结构材料领域的应用潜力。
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螺旋高分子孔道结构作为一类非常独特的孔结构,在结构和功能方面具有很大的发展空间[20],其设计合成通常要满足下列条件:(1)高分子结构中分子内弱相互作用全程可控。将氢键、静电、π-π、金属配位等非共价相互作用引入到高分子结构中,能使高分子链发生折叠,在单分子水平上控制高分子的空间构象。只有高分子结构中分子内弱相互作用全程可控,才有望实现整个高分子构象的精确控制。当高分子结构中分子内弱相互作用不能连续控制高分子链折叠时,高分子的折叠将变得难以预测。(2)高分子链自发螺旋折叠。高分子链在弱相互作用下以螺旋方式进行折叠,其折叠的驱动力来自于占主导优势的空间构象。由于螺旋结构的周期性,重复单元的折叠方向确定后,整个高分子链的折叠方向亦可明确。(3)螺旋构象的自稳定性。在高分子链进行螺旋折叠后,螺旋构象的稳定性至关重要。需要注意的是,固态包括晶态的螺旋结构在溶液中并不一定能稳定存在。要获得在固态和溶液中都稳定的螺旋结构,高分子骨架需要特征结构。通常,刚性高分子是比较理想的结构体系,它们可以通过分子内π-π相互作用来稳定螺旋构象(其螺距约为0.35 nm),或者利用结构刚性的螺旋骨架产生更大的螺距。此外,其他超分子作用力,如氢键、疏溶剂效应等,也能有效稳定螺旋构象。(4)螺旋骨架内含有空腔。高分子链螺旋折叠的过程中可以产生空腔,形成具有本征手性的孔道结构。同时,根据高分子序列重复基元的化学多样性,能够获得不同的孔道内化学微环境以及孔道尺寸、形状、手性等,从而衍生出丰富的螺旋高分子孔道结构。螺旋高分子孔道的设计原理来源于对前期研究工作的总结和理解,以期促进螺旋高分子孔道材料的合成与发展。
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含孔道的螺旋高分子是一类独特的高分子结构材料。螺旋高分子孔道的合成困难制约了这类材料的快速发展。迄今为止,已建立的具有孔道的螺旋高分子种类依然很少,主要以氢键作用、静电排斥作用等来诱导刚性高分子骨架螺旋折叠而得到[21, 22]。以氢键主导的螺旋高分子孔道结构包括氨酰类[23-31]、酰肼类[32-34]、脲类[35, 36]等,以静电排斥作用驱动的螺旋高分子孔道结构有芳香三唑类[37, 38] 和芳香恶二唑类[39]等。
芳香酰胺类结构凭借其结构特征,已广泛应用于含孔道螺旋高分子的设计。近来,大量的芳香酰胺类螺旋结构已经被开发出来,黎占亭等[40]对此领域进行了详细的评述。尽管结构丰富的构筑基元可用于芳香酰胺螺旋高分子的合成,但是要制备高分子量的芳香酰胺螺旋高分子却遇到了巨大的挑战。黎占亭等[41]报道了分子内氢键驱动的芳香聚酰胺螺旋高分子(图1)。这类高分子能自发折叠形成螺旋构象,与参照的线性高分子空间结构不同,而且其螺旋手性可以通过手性基团诱导调控。螺旋高分子通过单体缩聚反应合成,产物数均分子量接近7.0×103,分子量分布较宽。在高分子链增长过程中,由于螺旋折叠引起的空间位阻效应通常导致反应活性迅速降低,难以获得高分子量的芳香酰胺螺旋高分子。为了排除螺旋折叠引起的空间位阻效应,龚兵等[42]利用保护基团打破折叠模式,在获得解折叠聚合物之后,通过去保护作用实现高分子链自发折叠成螺旋结构(图2)。研究表明,这个策略是可行的,可以用于高分子量螺旋高分子的合成。
芳香酰肼类螺旋高分子与酰胺类螺旋高分子的设计原理相似,但在其结构刚性稍微降低的同时,反应活性却大大增加。黎占亭等[33, 34, 43, 44]近来报道了一类芳香酰肼螺旋聚合物(结构式示于图3),其数均分子量达到1.5×104,聚合度约为10。芳香酰肼结构容易制备高分子量聚合物,主要是由于酰肼的反应活性较高。另一类反应活性较高的异氰酸酯基团也可用于芳香脲类螺旋高分子的合成。近年来,Meijer和龚兵等[35, 45]分别报道了芳香脲类螺旋聚合物(结构式示于图4)的合成,其数均分子量可以超过2.0×104。芳香脲类聚合物通过分子内氢键驱动高分子链自发折叠,其最稳态为螺旋构象。将芳香酰肼和脲类螺旋高分子的分子折叠原理应用到结构设计中,可以通过不同的单体来构筑结构多样的芳香酰肼和脲类螺旋高分子。考虑到增加反应活性的问题,将烷基伯胺引入到螺旋骨架中能够使高分子量螺旋高分子的合成变得相对容易[46],但需要注意螺旋结构的稳定性问题。
除了氢键驱动高分子链自发螺旋折叠外,静电排斥作用也被用来设计螺旋结构[47]。静电排斥作用能够使高分子链朝着单一方向折叠形成螺旋结构,高分子骨架的刚性是螺旋形成的关键。黎占亭和赵新等[48]报道了一类芳香三唑螺旋寡聚物,其在静电排斥和C―H…O氢键作用下,形成了自折叠的螺旋构象(图5)。这类螺旋寡聚物以最稳定的螺旋构象存在,其内部空腔直径将近1.8 nm。这项研究表明芳香杂环结构在螺旋高分子的设计与制备方面还有较大的发展空间。最近,本课题组构筑了一类芳香恶二唑螺旋高分子(图6)[39]。我们先用芳香单体的缩聚反应制备线性高分子,再用脱水反应成环,环化后的聚合物链在静电排斥作用下可自发折叠形成螺旋结构。这种合成方法制备高分子量的螺旋聚合物简单易行。研究发现,芳香恶二唑螺旋高分子的数均分子量可达到1.8×104以上,其聚合度约为30。芳香恶二唑螺旋高分子通过分子内π-π相互作用进一步稳定螺旋构象,其螺距为0.36 nm,每圈螺旋大约有3.2个重复单元,其内部空腔直径约为0.55 nm[49]。芳香恶二唑螺旋高分子是独特的共轭高分子,在其分子量增加的过程中,分子内π-π相互作用增强,螺旋构象更稳定。利用扫描透射显微镜技术可以清楚地观察到单根螺旋高分子结构。这类芳香恶二唑螺旋高分子具有非常精准的孔道尺寸和空间结构[50]。值得注意的是,利用不同的芳香单体,可以制备不同孔道尺寸和空间结构的刚性共轭螺旋高分子。
含孔道螺旋高分子结构的设计与制备得益于化学结构的多样性。除了上述代表性例子外,其他含孔道螺旋高分子体系也在发展中。例如,通过动态共价键形成可以自发折叠的螺旋高分子已有报道[51, 52]。动态共价螺旋高分子通过聚合物链内弱相互作用实现最稳定空间结构朝着单一方向折叠,从而得到螺旋结构[53]。有趣的是,动态共价螺旋高分子的合成相对简单,其利用两种单体的自发反应就可以形成自稳定的螺旋折叠的聚合物(图7)[52],其数均分子量达到3.1×104,内部空腔的直径约为1 nm。对比共价和动态共价螺旋高分子,含孔道的非共价螺旋高分子未来有望通过超分子合成方法来构筑。
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含孔道螺旋高分子具有潜在的光学活性,除了可用于研究手性的转移、放大、记忆等基本规律外,它们还能应用到三维显示、信息存储等领域。从螺旋高分子结构来看,含孔道螺旋高分子的外表面可以空间排列功能基团实现仿生识别[54-56],在分子识别、对映体拆分、不对称性催化、传感检测以及医药等领域具有重要的应用潜力。值得注意的是,含孔道螺旋高分子的内腔结构是被隔离的独特化学微环境,具有明显的受限空间效应。因而,螺旋高分子孔道能展现独特的分子相互作用模式,有望发展全新的性质和功能,并在分子识别、仿生催化、传感检测等领域获得新的认识和理解。
近年来,利用螺旋的内腔进行分子识别的研究已经获得了广泛关注,特别是可以在螺旋孔道内识别的物质已经从离子、小分子扩展到更大的生物分子[57-59]。对生物分子的选择性识别不仅可以应用到检测、分离与催化等领域,而且有助于理解螺旋孔道结构与选择性识别的分子基础,促进新的性质与功能开发。例如,含孔道的螺旋高分子材料可以作为新型手性固定相,实现从手性识别到对映体拆分的功能升级,从而发展出高效的手性拆分材料。
含孔道螺旋高分子在膜分离材料中将有重要应用前景。螺旋高分子具有预组装的孔道结构,其形成的膜分离材料有望保留其孔道特征,从而推进膜材料的设计策略。当然,要理解基于含孔道螺旋高分子的膜分离材料的性能,单个孔道的性质研究是基础。因此,实现膜分离材料的性能设计主要依赖螺旋高分子孔道结构的性质。目前,研究螺旋高分子孔道性质的方法并不多,理想的研究体系主要是仿生跨细胞膜传输模型。近来,本课题组[49]利用仿生跨细胞膜传输模型研究了一类芳香恶二唑螺旋高分子的孔道性质,并发展了超稳定的仿生跨膜通道(图8)。为了实现这一仿生功能,我们通过螺旋高分子的长度与侧链设计,使其与磷脂膜组装形成稳定的超分子体系。研究发现,此螺旋高分子孔道(直径0.55 nm)几乎无选择性地传输阴离子,而对阳离子具有适当的选择性(选择性顺序为:Rb+ > K+ > Cs+ > Na+ > Li+)(图8)。
图 8 芳香恶二唑螺旋高分子仿生跨膜通道:(a)芳香恶二唑螺旋高分子的结构;(b)螺旋高分子仿生跨膜通道示意图;(c)螺旋高分子的平面膜片钳微电流测试信号图;(d)螺旋高分子仿生通道的离子选择性测试曲线[49]
Figure 8. Biomimetic transmembrane channels based on aromatic oxadiazole helical polymers:(a)Chemical structure of aromatic oxadiazole helical polymers;(b)Cartoon of biomimetic transmembrane channel by helical polymers;(c)Electrophysiology channel recordings of helical polymer channel;(d)Representative fluorescence traces of anion and cation selectivity tests of helical polymer channel[49]
芳香恶二唑螺旋高分子的孔道结构和性质可通过单体来调控。针对天然蛋白质通道的特异性功能,我们利用芳香恶二唑螺旋高分子孔道来构筑人造钾离子通道,实现了选择性传输性质(图9)[50]。该人造钾离子通道展示了高效的K+/Na+选择比,而其选择性传输的本质来源于选择性的分子识别能力。尽管螺旋高分子的传输速率还有待深入研究,但是理论模拟证实螺旋高分子通道与碳纳米管有相似的传输速率(数据未报道)[60]。因此,基于含孔道螺旋高分子的膜分离材料的孔道性质、传输效率等方面的潜在特征使其有希望成为一类新型先进功能材料。
针对螺旋高分子孔道的结构特点,结构导向的性质和功能将是螺旋高分子领域研究的重点。通过精确的孔道结构来实现螺旋高分子性质与功能的设计,将极大地促进螺旋高分子材料的发展与应用。含孔道的螺旋高分子材料具有非常广泛的应用前景,可以相信,它们在检测传感、手性拆分、物质运输以及孔测序等领域都将展现出空前的性质与功能。
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目前,螺旋高分子的螺旋手性已经能够完全控制,长度3 nm以上的孔道合成具有挑战性,需要发展新的含孔道螺旋高分子制备方法。根据高分子序列重复基元的分子结构多样性,能够实现不同的孔道内化学微环境以及孔道大小、形状和手性等,从而可以发展丰富的螺旋高分子孔道结构。含孔道螺旋高分子凭借其独特的结构,有望实现性质与功能的可设计性,将会在分子识别、催化、物质传输、检测与分离等领域产生新的技术与应用方法。
螺旋高分子孔道材料的研究进展
Recent Progress on Pore-Containing Helical Polymer Materials
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摘要: 含孔道的螺旋高分子是一类独特的高分子材料。目前已报道的含孔道的螺旋高分子种类较少,主要以氢键、静电相互作用等来诱导刚性高分子骨架以螺旋方式折叠而得到。螺旋高分子孔道的合成困难,制约了这类材料的快速发展。因此,亟待解决的是发展螺旋高分子孔道的合成方法,制备结构丰富的螺旋高分子孔道。鉴于螺旋高分子孔道的独特结构,它们在分子识别、对映体拆分、不对称催化、传感检测等领域具有重要的应用前景。本文简要概述了含孔道螺旋高分子的研究进展,重点阐述其结构特征、设计理念、结构类型以及在仿生输运功能与膜分离材料方面的应用潜力。Abstract: Porous materials display many important applications in our daily life. Usually, the applications of porous materials largely depend on the properties and structures of the pores. Although the study of porous materials has been of several decades, the features and functions of porous materials are still unpredictable. Indeed, the function-directed design of porous materials remains challenging. A " bottom-up” preparation method will be promising to make progress in the filed of porous materials, wherein porous materials are composed of preorganized pore structures. Therefore, it is crucial to create preorganized pores for the preparation of porous materials. Recently, pore-containing helical polymers have been developed as specific advanced functional polymer materials, and have attracted much attention owing to their potential applications in various fields such as molecular recognition, isomer separation, asymmetric catalysis, sensing, enantioseparation, delivery, and sequencing. Typically, these helical structures were spontaneously formed by folding of polymeric chains driven by intramolecular noncovalent interactions, such as hydrogen bonding, electrostatic interactions, and π-π interactions. In general, the pore structures strongly relied on the stability of helical conformation, thus the helical polymers with rigid backbones were required. The pore-containing helical polymers can be further used as preorganized pore structures for the preparation of porous materials through a " bottom-up” preparation approach. In this review, the recent progress on pore-containing helical polymer materials is outlined. In particular, the structural features and designing strategies of pore-containing helical polymers will be emphasized, and a few examples on pore-containing helical polymers have been highlighted. Additionally, these helical polymers show very important properties, such as biomimetic transmembrane transport, and can be applied in the development of advanced membrane materials.
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Key words:
- helical polymers /
- pore-containing helix /
- molecular folding /
- substance transport /
- membrane materials
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图 8 芳香恶二唑螺旋高分子仿生跨膜通道:(a)芳香恶二唑螺旋高分子的结构;(b)螺旋高分子仿生跨膜通道示意图;(c)螺旋高分子的平面膜片钳微电流测试信号图;(d)螺旋高分子仿生通道的离子选择性测试曲线[49]
Figure 8. Biomimetic transmembrane channels based on aromatic oxadiazole helical polymers:(a)Chemical structure of aromatic oxadiazole helical polymers;(b)Cartoon of biomimetic transmembrane channel by helical polymers;(c)Electrophysiology channel recordings of helical polymer channel;(d)Representative fluorescence traces of anion and cation selectivity tests of helical polymer channel[49]
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