Craig Whitney R, Baker Tessa W, Marts Amy R, DeGenova Daniel T, Martin David P, Reed Garrett C, McCarrick Robert M, Crowder Michael W, Cohen Seth M, Tierney David L
Department of Chemistry and Biochemistry, Miami University , Oxford, Ohio 45056, United States.
Department of Chemistry and Biochemistry, University of California, San Diego , La Jolla, California 92093, United States.
Inorg Chem. 2017 Oct 2;56(19):11721-11728. doi: 10.1021/acs.inorgchem.7b01661. Epub 2017 Sep 12.
A combination of XAS, UV-vis, NMR, and EPR was used to examine the binding of a series of α-hydroxythiones to CoCA. All three appear to bind preferentially in their neutral, protonated forms. Two of the three clearly bind in a monodentate fashion, through the thione sulfur alone. Thiomaltol (TM) appears to show some orientational preference, on the basis of the NMR, while it appears that thiopyromeconic acid (TPMA) retains rotational freedom. In contrast, allothiomaltol (ATM), after initially binding in its neutral form, presumably through the thione sulfur, forms a final complex that is five-coordinate via bidentate coordination of ATM. On the basis of optical titrations, we speculate that this may be due to the lower initial pK of ATM (8.3) relative to those of TM (9.0) and TPMA (9.5). Binding through the thione is shown to reduce the hydroxyl pK by ∼0.7 pH unit on metal binding, bringing only ATM's pK close to the pH of the experiment, facilitating deprotonation and subsequent coordination of the hydroxyl. The data predict the presence of a solvent-exchangeable proton on TM and TPMA, and Q-band 2-pulse ESEEM experiments on CoCA + TM suggest that the proton is present. ESE-detected EPR also showed a surprising frequency dependence, giving only a subset of the expected resonances at X-band.
采用X射线吸收光谱（XAS）、紫外可见光谱（UV-vis）、核磁共振（NMR）和电子顺磁共振（EPR）相结合的方法，研究了一系列α-羟基硫酮与钴咔咯（CoCA）的结合情况。这三种物质似乎都以其中性的质子化形式优先结合。其中两种物质明显以单齿方式结合，仅通过硫酮硫原子。基于核磁共振结果，硫代麦芽酚（TM）似乎表现出一定的取向偏好，而硫代焦袂康酸（TPMA）似乎保留了旋转自由度。相比之下，别硫代麦芽酚（ATM）最初以中性形式结合，大概是通过硫酮硫原子，最终形成一种五配位的配合物，通过ATM的双齿配位实现。基于光学滴定，我们推测这可能是由于ATM的初始pK值（8.3）相对于TM（9.0）和TPMA（9.5）较低。通过硫酮的结合显示，在金属结合时，羟基的pK值降低约0.7个pH单位，仅使ATM的pK值接近实验pH值，促进去质子化以及随后羟基的配位。数据预测TM和TPMA上存在可与溶剂交换的质子，并且对CoCA + TM进行的Q波段2脉冲电子自旋回波包络调制（ESEEM）实验表明该质子存在。电子自旋回波检测的电子顺磁共振还显示出令人惊讶的频率依赖性，在X波段仅给出预期共振的一个子集。
