Imidazolyl cyclodextrins: Artificial serine proteases enabling regiospecific reactions
Imidazolyl cyclodextrins: Artificial serine proteases enabling regiospecific reactions
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DOI：
10.1002/anie.200701156
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Fujita, Kahee
中科院分区：
文献类型：
--
作者：
Yuan, De-Qi;Kitagawa, Yumika;Fujita, Kahee
受到酶的高选择性和高效率的启发，科学家们在创造模仿酶的基本结构和催化特征的合成分子方面付出了巨大的努力。[1, 2] Breslow 及其同事证明，即使存在其他没有保护基团的更具反应性的功能，人工酶也可以靶向单一功能。[3]武尔夫等人。制备了一些基于印迹聚合物的人工酶，其具有很强的催化活性，明显超过相应的催化抗体。 [4]许多其他人工酶也表现出有趣的选择性和显着的速率加速。[1,2,5]然而，很少有工作证明只有在许多其他功能存在的情况下特定定位的功能才能积极参与催化，这是天然酶的一般特征。胰凝乳蛋白酶使用 Asp102-COOÀ/His57-Im/Ser195-OH 三联体催化肽底物的水解，并且 Ser195-OH 基团在第一步中被酰化。 [6]这种酶促功能长期以来引起了人们的兴趣，并被广泛模仿。[1, 2] 环糊精 (CD) 通过与蛋白酶密切相关的机制显着加速（高达 600 万倍）硝基苯酯的水解。[7]酰基转移到CD的次级侧，但尚不清楚是2-OH还是3-OH基团首先攻击酯。 [8]咪唑基 CD 表现出改进的催化能力，而在这种情况下，对于酰基转移到哪里一无所知。 [9]另一方面，通过引入两个或多个不同的高选择性官能团来对CD进行功能化仍然是一个巨大的挑战。 [10]这些缺点阻碍了更复杂的人工酶的合理设计。在此，我们证明，在咪唑基环糊精的众多羟基中，只有特定位置的羟基参与催化，这使得建立一种新型有效的环糊精异双功能化和三功能化策略成为可能。 6-咪唑基-b-CD (1b) 催化乙酸对硝基苯酯的水解。反应混合物的 HPLC 显示形成了一种新的 CD 物质，其保留时间比 1b 更长。结构分析表明产物是异构纯的单乙酸酯2b（方案1），这意味着20个羟基中的一个可以接收底物的酰基。详细的 NMR 分析（支持信息中的图 1S）揭示了一个伯羟基 (6-OH) 的乙酰化，但无法指定该位点。考虑到环糊精磺酸盐在环糊精功能化中的重要性，我们研究了1b与均三甲苯磺酰氯的反应，以阐明咪唑基和环糊精羟基之间的相互作用，并为环糊精的异双功能化或多功能化提出了一个新概念。反应在室温下在含有33％二甲基甲酰胺(DMF)的pH 8的磷酸盐缓冲液中进行。在估计的最多 20 种异构单磺酸盐中，磺酸盐 3b 与未反应的 1b（63%，方案 2）一起以 13% 的产率被识别和分离。在这些条件下，b-CD 没有观察到明显的磺酰化。 3b 的结构测定表明 6E-OH 基团发生磺酰化，这意味着尽管 6D-OH 和 6E-OH 基团相对于咪唑基部分的位置相似，但只有 6E-OH 基团积极参与反应。 [11]这些结果强调，需要精确的几何匹配……
Inspired by the high selectivity and high efficiency of enzymes, scientists have made tremendous efforts in creating synthetic molecules that mimic the basic structural and catalytic features of enzymes.[1, 2] Breslow and co-workers demonstrated that artificial enzymes can target a single functionality even in the presence of other more reactive functionalities without protective groups.[3] Wulff et al. prepared some imprinted polymer-based artificial enzymes that have strong catalytic activity that clearly surpasses the corresponding catalytic antibodies.[4] Many other artificial enzymes have also displayed interesting selectivity and remarkable rate acceleration.[1, 2, 5] However, little work has been done to demonstrate that only the specifically located functionalities in the presence of many others could actively take part in the catalysis, which is the general feature of natural enzymes. Chymotrypsin uses the Asp102-COOÀ/His57-Im/Ser195-OH triad in catalyzing the hydrolysis of peptide substrates and the Ser195-OH group is acylated during the first step.[6] This enzymatic function has attracted a long-standing interest and has been extensively mimicked.[1, 2] Cyclodextrins (CDs) remarkably accelerate (by a factor of up to six million) the hydrolysis of nitrophenyl esters by a mechanism closely related to that of proteases.[7] The acyl group is transferred to the secondary side of the CD, but it is not known whether the 2-OH or 3-OH group attacks the ester first.[8] Imidazolyl CDs demonstrate improved catalytic ability, whereas in this case, nothing is known about where the acyl group is transferred to.[9] On the other hand, it still represents a great challenge to functionalize CDs by introducing two or more different functional groups with high selectivity.[10] These drawbacks have hampered the rational design of more sophisticated artificial enzymes. Herein we demonstrate that among the many hydroxy groups of imidazolyl CDs, only the one at a specific position is engaged in the catalysis, which enables the establishment of a novel efficient strategy for the hetero-bifunctionalization and-trifunctionalization of CDs. 6-Imidazolyl-b-CD (1b) catalyzes the hydrolysis of pnitrophenyl acetate. HPLC of the reaction mixture revealed the formation of a new CD species with a longer retention time than that of 1b. Structural analysis indicated that the product is the isomerically pure monoacetate 2b (Scheme 1), thus implying that one of the 20 hydroxy groups can receive the acyl group of the substrate. Detailed NMR analysis (Figure1S in the Supporting Information) revealed the acetylation of one primary hydroxy group (6-OH), but it did not allow the specification of that site. Considering the significance of CD sulfonates in the functionalization of CDs, we examined the reaction of 1b with mesitylenesulfonyl chloride to elucidate the cooperation between the imidazolyl and the hydroxy groups of CDs as well as to develop a new concept for the hetero-bifunctionalization or-multifunctionalization of CDs. The reaction was carried out at room temperature in a phosphate buffer at pH 8 containing 33% dimethylformamide (DMF). The sulfonate 3b, among the estimated maximum 20 isomeric monosulfonates, was recognized and isolated in 13% yield together with unreacted 1b (63%, Scheme 2). No obvious sulfonylation was observed with b-CD under these conditions. Structure determination of 3b indicated the sulfonylaton of the 6E-OH group, implying that only the 6E-OH group is actively engaged in the reaction although the 6D-OH and 6E-OH groups are similarly located with respect to the imidazolyl moiety.[11] These results emphasize that precise geometrical matching is required for the …