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

自组装单层（SAM）电极被广泛应用于有机电合成、生物传感器以及电极界面电子转移的研究等领域。本研究制备了SAM-蒽醌（SAM- aq）和SAM-萘醌（SAM- nq）金电极，并结合分子轨道计算，利用电化学测量方法研究了SAM-蒽醌（SAM- aq）和SAM-萘醌（SAM- nq）金电极上的氢键在给氢客体上的电子传递功能。SAM-AQ和SAM-NQ电极在循环伏安图上产生可逆波，分别对应于AQ和NQ自由基阴离子的产生。4-取代酚的客体分子的存在使得波是不可逆的，显示了中介电流。调解取决于客人的供氢能力。对酚类化合物的光谱电化学分析表明，在醌类化合物的还原电位下，它们的自由基阴离子在电极上产生。在MO计算的基础上，讨论了氢键对电子转移的作用。研究发现，醌自由基与中性苯酚之间的氢键产生的较大的稳定化和电位转移，使电极上醌向客体的明显的上坡电荷分离得到能量平衡。在邻醌与二甲基脲的氢键体系中，研究了限制氢键配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，系统中的氧化还原电位由熵驱动的氢键形成稳定化控制。所得结果对进一步探讨生物醌作为电荷分离器的作用具有重要意义。

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.