Department of Chemistry and Chemical Biology , Northeastern University , 360 Huntington Avenue , Boston , Massachusetts 02115 , United States.
Barnett Institute of Chemical and Biological Analysis , Northeastern University , 360 Huntington Avenue , Boston , Massachusetts 02115 , United States.
J Am Chem Soc. 2018 Jun 20;140(24):7377-7380. doi: 10.1021/jacs.8b01136. Epub 2018 Jun 11.
This work addresses the need for chemical tools that can selectively form cross-links. Contemporary thiol-selective cross-linkers, for example, modify all accessible thiols, but only form cross-links between a subset. The resulting terminal "dead-end" modifications of lone thiols are toxic, confound cross-linking-based studies of macromolecular structure, and are an undesired, and currently unavoidable, byproduct in polymer synthesis. Using the thiol pair of Cu/Zn-superoxide dismutase (SOD1), we demonstrated that cyclic disulfides, including the drug/nutritional supplement lipoic acid, efficiently cross-linked thiol pairs but avoided dead-end modifications. Thiolate-directed nucleophilic attack upon the cyclic disulfide resulted in thiol-disulfide exchange and ring cleavage. The resulting disulfide-tethered terminal thiolate moiety either directed the reverse reaction, releasing the cyclic disulfide, or participated in oxidative disulfide (cross-link) formation. We hypothesized, and confirmed with density functional theory (DFT) calculations, that mono- S-oxo derivatives of cyclic disulfides formed a terminal sulfenic acid upon ring cleavage that obviated the previously rate-limiting step, thiol oxidation, and accelerated the new rate-determining step, ring cleavage. Our calculations suggest that the origin of accelerated ring cleavage is improved frontier molecular orbital overlap in the thiolate-disulfide interchange transition. Five- to seven-membered cyclic thiosulfinates were synthesized and efficiently cross-linked up to 10-fold faster than their cyclic disulfide precursors; functioned in the presence of biological concentrations of glutathione; and acted as cell-permeable, potent, tolerable, intracellular cross-linkers. This new class of thiol cross-linkers exhibited click-like attributes including, high yields driven by the enthalpies of disulfide and water formation, orthogonality with common functional groups, water-compatibility, and ring strain-dependence.
这项工作满足了对能够选择性形成交联的化学工具的需求。例如，当代的硫醇选择性交联剂修饰所有可及的硫醇，但仅在亚基之间形成交联。由此产生的孤立硫醇的末端“死端”修饰物是有毒的，会混淆大分子结构的交联研究，并且是聚合物合成中不可避免的、目前不可避免的副产物。我们使用铜/锌超氧化物歧化酶（SOD1）的硫醇对，证明了包括药物/营养补充剂硫辛酸在内的环状二硫化合物能够有效地交联硫醇对，但避免了死端修饰。硫醇定向亲核攻击环状二硫化合物导致硫醇-二硫键交换和环裂解。所得的二硫键连接的末端硫醇部分要么引导相反的反应，释放环状二硫化合物，要么参与氧化二硫（交联）形成。我们假设，并通过密度泛函理论（DFT）计算证实，环状二硫化合物的单 S-氧代衍生物在环裂解时形成末端亚磺酸，从而避免了以前的限速步骤，即硫醇氧化，并加速了新的速率决定步骤，即环裂解。我们的计算表明，加速环裂解的起源是在硫醇-二硫键交换过渡中改善了前线分子轨道重叠。合成了五到七个原子的环状硫代亚磺酸酯，并有效地将其交联速度提高了五到七倍，比其环状二硫代前身物快；在生物浓度的谷胱甘肽存在下发挥作用；并作为细胞渗透性、有效、耐受和细胞内交联剂发挥作用。这种新的硫醇交联剂类表现出类似点击的属性，包括由二硫键和水形成的焓驱动的高收率、与常见官能团的正交性、水相容性和环应变依赖性。
