Allosteric Regulation of the Ligand-Binding Ability of Zinc Porphyrins with Sterically Bulky Shielding Units by Metal Complexation
作者:Yoshikazu Ninomiya、Masatoshi Kozaki、Shuichi Suzuki、Keiji Okada
DOI:10.1246/bcsj.20140197
日期:2014.11.15
An artificial allosteric system composed of a zinc porphyrin moiety as a ligand-binding site, diphenylanthracene moieties as sterically bulky shielding units, and bipyridine terminals as Fe(II) ion-binding sites, was designed, synthesized, and characterized. The allosteric system forms an [Fe(bpy)3]-type complex in the presence of Fe(II) ions and induces a large conformational change in the alkyl side chains and shielding units, in which the anthracene groups would cover both the top and bottom of the porphyrin Zn(II) center. Consequently, the ligand-binding ability of the zinc porphyrin unit was allosterically suppressed. We found that the formation of the Fe(II) complex resulted in a 9.5-fold reduction in the binding constant between the zinc porphyrin and 3,5-bis(3,5-di-tert-butylphenyl)pyridine at 20 °C. Comparisons of entropy and enthalpy changes in the absence and presence of Fe(II) ions led to the conclusion that the allosteric suppression was mainly due to the destabilization of the Zn(II)-coordinated complex owing to the steric repulsion between the ligand and the shielding unit.
设计、合成并表征了一种人工变构系统,该系统由锌卟啉部分作为配体结合位点、二苯基蒽部分作为体积庞大的屏蔽单元、以及双吡啶末端作为Fe(II)离子结合位点组成。在存在Fe(II)离子的条件下,该变构系统形成了[Fe(bpy)3]类型的复合物,并引发了烷基侧链和屏蔽单元的巨大构象变化,其中特别是蒽基团会覆盖卟啉Zn(II)中心的上下部分。因此,锌卟啉单元的配体结合能力受到变构抑制。我们发现,Fe(II)复合物的形成导致锌卟啉与3,5-双(3,5-二叔丁基苯基)吡啶之间的结合常数在20°C下减少了9.5倍。在缺乏和存在Fe(II)离子的情况下熵和焓变化的比较得出结论,变构抑制主要是由于配体与屏蔽单元之间的立体排斥导致Zn(II)配位复合物的不稳定性。