Control and Function of Hydrogen Bond of Bioactive Quinones on the Charge Separation

众所周知，自组装单分子层(SAM)电极被应用于电有机合成、生物传感器和电极界面电子传递的研究。本研究制备了SAM-AQ和SAM-NQ金电极，并结合分子轨道计算，通过SAM电极的电化学测量，研究了氢键对供氢客体电子转移的作用。SAM-AQ和SAM-NQ电极在循环伏安图上出现可逆波，分别对应于AQ和NQ自由基的产生。客体分子4-取代苯酚的存在使得波是不可逆的，显示了中介电流。调解取决于嘉宾的供氢能力。光谱电化学分析表明，它们的自由基阴离子在苯二酮的还原电势下产生在电极上。在分子轨道计算的基础上，讨论了氢键对电子转移的作用。研究发现，由于苯醌自由基与中性酚之间的氢键作用，使得电极上的苯醌对客体的电荷分离出现明显的上坡电荷分离，从而实现了能量平衡。在邻苯二酮氢键与二甲基脲的氢键体系中，证明了氢键络合物结构对氧化还原电位的控制机理。电化学测量和分子轨道计算表明，体系中的氧化还原电势受熵驱动的氢键形成稳定作用的控制。这些结果对进一步讨论生物对苯二酚作为电荷分离器的功能具有重要意义。

It is well known that self-assembled monolayer (SAM) electrodes are applied to electroorganic syntheses, biosensors, and investigation on the electron transfer at an electrode interface. In this study the SAM-anthraquinone (SAM-AQ) and SAM-naphthoquinone (SAM-NQ) gold electrodes were prepared, and the function of hydrogen bonds for electron transfer to the hydrogen-donating guests was investigated with the aid of electrochemical measurements with the SAM electrodes, combined with molecular orbital calculations.1. The SAM-AQ and SAM-NQ electrodes gave reversible waves on the cyclic voltammograms, corresponding to the generation of the AQ and NQ radical anions, respectively. The presence of the guest molecule of 4-substituted phenols allowed the waves to be irreversible, showing the mediation current. The mediation depended upon the hydrogen donating ability of the guests. Spectroelectrochemistry of the phenols showed that their radical anions generated on the electrodes at the reduction potential of quinones.2. The function of hydrogen bonds for the electron transfer has been discussed on the basis of MO calculations. It has been found that the apparently up-hill charge separation form the quinone on the electrode to the guests gets energy balance caused by large stabilization and potential shift arising from hydrogen bonding between the quinone radical and the neutral phenol.3. Mechanism on the control of the redox potentials with restrict of the structure of the hydrogen-bonding complexes has been demonstrated in the o-quinone hydrogen-bonding system with dimethylurea. Electrochemical measurements and molecular orbital calculations have revealed that the redox potentials in the system are controlled by the entropy driven stabilization for the hydrogen-bond formation.The results obtained here are important for the extended discussion on the function of biological quinones as a charge separator.