Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.
Phys Chem Chem Phys. 2020 Nov 18;22(44):25467-25473. doi: 10.1039/d0cp04265j.
Low-barrier H-bonds form when the pKa values of the H-bond donor and acceptor moieties are nearly equal. Here, we report redox potential (Em) values along two redox-active low-barrier H-bonds in the water-oxidizing enzyme photosystem II (PSII), using a quantum mechanical/molecular mechanical approach. The low-barrier H-bond between D1-Tyr161 (TyrZ) and D1-His190 is located in the middle of the electron transfer pathway. When the proton is at D1-His190, Em(TyrZ) is the lowest and can serve as an electron donor to the oxidized chlorophyll PD1˙+. Em(TyrZ) and Em(D1-His190) are equal, and the TyrZD1-His190 pair serves as an electron acceptor to Mn4CaO5 when the proton is at TyrZ. In the low-barrier H-bond between D1-His215 and plastoquinone QB, located at the terminus of the electron transfer pathway, the driving force of electron transfer and electronic coupling between QA and QB are maximized when the proton arrives at QB. It seems likely that local proton transfer along redox-active low-barrier H-bonds can alter the driving force or electronic coupling for electron transfer.
低势垒氢键是在氢键供体和受体部分的 pKa 值几乎相等时形成的。在这里，我们使用量子力学/分子力学方法报告了在水氧化酶光系统 II（PSII）中两条氧化还原活性低势垒氢键的氧化还原电势（Em）值。位于电子传递途径中间的 D1-Tyr161（TyrZ）和 D1-His190 之间的低势垒氢键。当质子位于 D1-His190 时，Em(TyrZ) 最低，可以作为氧化叶绿素 PD1˙+的电子供体。Em(TyrZ) 和 Em(D1-His190) 相等，当质子位于 TyrZ 时，TyrZD1-His190 对作为 Mn4CaO5 的电子受体。在电子传递途径末端的 D1-His215 和质体醌 QB 之间的低势垒氢键中，当质子到达 QB 时，电子转移和 QA 和 QB 之间的电子耦合的驱动力最大化。似乎局部质子转移沿着氧化还原活性低势垒氢键可以改变电子转移的驱动力或电子耦合。
