Separated Concerted Proton-Electron Transfer with Biomimetic Models of Peroxidase

描述（由申请人提供）：非常广泛的生物化学过程涉及质子和电子的运动，称为质子耦合电子转移（PCET）。这在包括细胞色素P-450、过氧化物酶、过氧化氢酶和许多其他酶在内的血红素酶的作用中尤为明显。这些酶对底物的氧化和对O2或H2O2的活化都需要精确控制质子和电子的运动。一个完整的生物系统的复杂性，即使在体外，也使得分析PCET的重要特征变得困难。本提案提出了一个实验方案，该方案将开发与这些酶的高价铁基氧中间体化合物I和化合物II相关的仿生系统中PCET的详细机制理解。特别重要的是质子和电子的转移发生在单个动力学步骤的机制，如协调质子-电子转移（CPET）。CPET机制可以避免在单独的质子转移（PT）或电子转移（ET）时产生的高能中间体。在这个建议中，一个重要的区别是分离协调质子电子转移（sCPET）的概念，其中质子和电子起源于或终止于不同的位置。这与氢原子转移（HAT）反应形成对比，其中PT和ET都是从一个单一物种到另一个单一物种进行的。在此，我们提出了电化学、光谱和化学动力学研究，使用定义明确的高价铁血红素（卟啉）-氧模型配合物和选择非血红素铁氧配合物。得到的关键热力学信息包括使用不同卟啉和轴向配体的铁基氧和相关中间体的酸度（pKa）和还原电位(E)。利用质子和电子供体，将开发一个全面的铁酰氧官能团质子化和还原的动力学曲线，这些质子和电子供体跨越了一系列的强度，而Eyring分析将为这些转化提供自由能垒。这将有助于在仿生系统中区分sCPET和分离的ET-PT或PT-ET机制，并将为将sCPET概念扩展到包含铁血红素和非血红素铁氧中间体的复杂生物系统提供实验基础。

DESCRIPTION (provided by applicant): A very wide variety of biochemical processes involve movement of protons and electrons, called proton-coupled electron transfer (PCET). This is particularly evident in the actions of heme enzymes, including cytochromes P-450, peroxidases, catalases, and many other enzymes. Both substrate oxidation and O2 or H2O2 activation by these enzymes require precise control of the movement of protons and electrons. The complexity of a full biological system, even in vitro, makes it difficult to analyze the features important to PCET. This proposal presents an experimental program that will develop a detailed mechanistic understanding of PCET in biomimetic systems relevant to the high-valent ferryl oxo intermediates compound I and compound II of these enzymes. Of particular importance are mechanisms where the transfer of protons and electrons occurs in a single kinetic step, as concerted proton-electron transfer (CPET). The CPET mechanism can avoid high-energy intermediates that would be otherwise be generated upon separate proton transfer (PT) or electron transfer (ET). An important distinction being made in this proposal is the concept of separated concerted proton-electron transfer (sCPET), in which the proton and electron to originate from or terminate at separate sites. This contrasts with hydrogen atom transfer (HAT) reactions where both PT and ET proceed from one single species to another single species. Herein, we propose electrochemical, spectroscopic, and chemical kinetic investigations using well-defined high-valent iron-heme (porphyrin)-oxo model complexes and select non-heme iron oxo complexes. Key thermodynamic information to be derived includes the acidity (pKa) and the reduction potential (E) of ferryl oxo and related intermediates using different porphyrin and axial ligands. A comprehensive kinetic profile for protonation and reduction of the ferryl oxo functional group will be developed using proton and electron donors that span a range of strengths, and Eyring analyses will provide the free energy barriers for these transformations. This will enable discrimination between sCPET and isolated ET-PT or PT-ET mechanisms in the biomimetic systems studied, and will provide an experimental foundation for extending the concept of sCPET to complex biological systems that incorporate ferryl heme and non-heme iron oxo intermediates. PUBLIC HEALTH RELEVANCE: Iron-containing enzymes execute remarkable chemical transformations related to cellular energy storage and conversion, photosynthesis, and neutralizing harmful materials. The work proposed here will provide greater understanding of how these enzymes achieve these tasks by developing simplified experimental models of these chemical transformations. Studies of these chemical model systems will provide important new insights into the functions and functioning of the full biological systems.