Thermodynamic and Kinetic Stabilities of Complementary Double Helices Utilizing Amidinium-Carboxylate Salt Bridges
Thermodynamic and Kinetic Stabilities of Complementary Double Helices Utilizing Amidinium-Carboxylate Salt Bridges
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DOI：
10.1021/ja303701d
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发表时间：
2012-06-06
影响因子：
15
通讯作者：
Yashima, Eiji
中科院分区：
文献类型：
--
作者：
Yamada, Hidekazu;Wu, Zong-Quan;Yashima, Eiji
采用模块化方法合成了一系列由间三联苯骨架和非手性脒及非手性羧酸残基组成的二聚体，通过联乙炔、对二乙炔基苯和Pt（II）-乙炔基苯等连接基团连接，并通过吸收光谱、圆二色性（CD）、紫外吸收光谱和紫外吸收光谱研究了它们对互补双螺旋形成的手性性质。和1H-1 NMR谱。二聚体的热力学和动力学稳定性与它们的连接方式密切相关，热力学分析表明，二聚体的缔合常数按Pt（II）-乙炔基<对二乙炔基苯<联乙炔基的顺序增加，这与它们的体积大小顺序相反。脒基上的取代基对双链体的稳定性也有影响，其缔合常数按异丙基<（R）-1-苯乙基<环己基的顺序增大。此外，在分别连接脒和羧酸单元的对二乙炔基苯接头的苯基处引入给电子和/或吸电子取代基倾向于稳定互补双螺旋，特别是在极性溶剂如DMSO中，这是由于芳族接头之间的吸引性电荷转移相互作用，尽管盐桥形成在DMSO中受到阻碍。此外，双链体轴承二乙炔和对二乙炔基苯键的链交换反应的动力学分析表明，这些是缓慢的过程与负Δ S-双匕首值，这意味着链交换反应进行通过直接交换途径。与此相反，那些双链轴承Pt（II）-乙炔键是快速的过程支持的积极Δ S-双匕首值，这表明链交换反应进行通过解离交换的。变温H-1 NMR研究了由非手性脒组成的外消旋二聚体的螺旋反转动力学，结果表明螺旋反转的能垒依次为：Pt（II）-乙炔基连接体，对二乙炔基苯连接体<联乙炔连接体.
A series of dimer strands consisting of m-terphenyl backbones bearing complementary chiral or achiral amidines and achiral carboxylic acid residues connected by various types of linkers, such as diacetylene, Pt(II)-acetylide, and p-diethynylbenzene linkages, were synthesized by a modular strategy, and their chiroptical properties on the complementary double helix formations were investigated by absorption, circular dichroism (CD), and H-1 NMR spectroscopies. The thermodynamic and kinetic stabilities of the complementary double helices assisted by amidinium-carboxylate salt bridges are highly dependent on their linkages, and the thermodynamic analyses of the dimer duplexes revealed that the association constants increased in the order: Pt(II)-acetylide linker < p-diethynylbenzene linker < diacetylene linker, which is in agreement with the reverse order of their bulkiness. The substituents on the amidine groups were also found to affect the stabilities on the duplexes and the association constants increased in the order: isopropyl < (R)-1-phenylethyl < cyclohexyl. In addition, the introduction of electron-donating and/or electron-withdrawing substituents at the phenyl groups of the p-diethynylbenzene linkers connecting the amidine and carboxylic acid units, respectively, tends to stabilize the complementary double helices, especially in polar solvents, such as DMSO, due to the attractive charge-transfer interactions between the aromatic linkers, although the salt bridge formation is hampered in DMSO. Furthermore, the kinetic analyses of the chain exchange reactions for the duplexes bearing diacetylene and p-diethynylbenzene linkages showed that these were slow processes with negative Delta S-double dagger values, meaning that the chain exchange reactions proceed via direct exchange pathways. In contrast, those for the duplexes bearing Pt(II)-acetylide linkages were fast processes supported by positive Delta S-double dagger values, suggesting that the chain exchange reactions proceed via dissociation-exchange ones. The helix-inversion kinetics investigated for the racernic dimer duplexes composed of achiral amidines based on variable-temperature H-1 NMR measurements indicated that the barriers for the helix-inversion increased in the order: Pt(II)-acetylide linker, p-diethynylbenzene linker < diacetylene linker.