How nature reduces nitrogen - unravelling design features for a nitrogenase mimic

氮气循环将氮气转化为可用的形式，如铵、硝酸盐和亚硝酸盐。氮和氮化合物很重要，因为它们构成了我们的大气、我们的身体以及我们的地球环境。氮气可以通过固氮菌自然循环转化为氨/铵，也可以通过哈伯-博世工艺综合利用。细菌中的固氮遵循反应n2+8H++8e-2NH3+H2，并由16个三磷酸腺苷当量物的水解提供动力。Haber-Bosch工艺遵循N_2+3 H_2 NH_3的反应方案，并使用高压、温度和催化剂提供动力。采用Haber-Bosch工艺可获得97%的最佳氨收率。这些反应都使用双原子氮和氢，但最终产物不同，因为细菌固氮会释放氢气作为副产品。土壤中的细菌利用氮素创造能量来生长和繁殖，并引入氮素供其他物种使用。哈伯-博世工艺生产的铵可用于一系列活动，如生产化肥，甚至爆炸哈伯发明了一种维持地球三分之一人口的工艺：从氮气中生产氨肥。氮是生命所必需的，是DNA和蛋白质的重要组成部分，但即使氮是我们大气中最丰富的气体，我们的细胞也不能以大气的形式利用它，而是依赖其他过程将氮固定成生物可用的形式。一些微生物拥有可以执行这种化学反应的固氮酶，你体内大约一半的氮来自这些微生物。另一半来自哈伯-博世工艺(卡尔·博世将哈伯的工艺扩大到大规模工业水平)。哈伯-博世工艺消耗大量能源，一些研究人员认为，即使催化剂效率的微小改进也能带来巨大的节省。该项目旨在实现一种更根本的转变--寻找一种能够模仿大自然温和地从空气中固定氮气的催化剂。这一工业过程依靠化学锤击，一次快速切断原料气体的键，在催化剂表面散布着氮和氢原子，这些原子迅速结合在一起生成氨。相比之下，大自然使用质子和电子流一次一个键地分解氮素--这是一种由固氮酶策划的外科手术程序，固氮酶是一种在土壤根瘤菌等微生物中发现的酶。固氮酶有两个主要部分：充当电子输送工具的铁蛋白，以及用它们来分解氮素的钼铁蛋白。电子最终来自还原的铁氧还蛋白，这是光合作用过程中产生的一种强大的还原剂，而三磷酸腺苷(ATP)提供能量。生物固氮远不是效率的典范。将每个氮气分子转化为氨需要至少16个三磷酸腺苷分子，参与反应的电子有四分之一被浪费在制造氢气作为副产品上。这些细菌为此投入了大量能量，这告诉你固氮对生命是多么重要。该酶的催化核心被称为铁钼辅因子(FeMo-co)，由7个铁离子、1个钼离子和9个硫化物组成。科学家们仍然不确定它是如何工作的--事实上，直到2015年，X射线结晶学才明确地显示出该星团中间有一个单一的碳离子。基于硬数据和直觉的混合，目前的看法是，FEMO-CO中的四个铁原子形成了催化活性中心。一些人仍然怀疑固氮酶的铁原子结合和还原氮素。由模型化合物支持的另一种观点认为，钼是氮还原的关键。

The Nitrogen Cycle converts nitrogen gas into usable forms, such as ammonium, nitrates, and nitrites. Nitrogen and nitrogen compounds are important because they constitute our atmosphere, our bodies as well as our earthly environments. Nitrogen can be recycled into ammonia/ammonium naturally via nitrogen fixing bacteria, or synthetically using the Haber-Bosch process. Nitrogen fixation in bacteria follows the reaction N2 + 8 H+ + 8 e- 2 NH3 + H2 and is powered by the hydrolysis of 16 ATP equivalents. The Haber-Bosch process follows the reaction scheme of N2 + 3 H2 2 NH3 and is powered by using high pressures, temperatures, and catalysts. Using the Haber-Bosch process an optimum yield of 97% ammonium can be obtained. These reactions both use diatomic nitrogen as well as hydrogen, but differ in their final products as bacterial nitrogen fixation releases hydrogen gas as a byproduct. Bacteria in the soil use nitrogen to create energy to grow and reproduce as well as to introduce nitrogen for use by other species. The Haber-Bosch process produces ammonium which can be used for a range of activities such as the production of fertilizers, or even explosiveHaber invented a process that sustains one third of the population on earth: the production of ammonia fertilizer from nitrogen gas. Nitrogen is required for life, a crucial component of both DNA and proteins, but even though nitrogen is the most abundant gas in our atmosphere, our cells can't use it in its atmospheric form, relying on other processes to "fix" that nitrogen into a biologically available form. A few microorganisms possess nitrogenase enzymes that can perform this chemical reaction, and about half of the nitrogen in your body comes from these microorganisms. The other half comes from the Haber-Bosch process (Carl Bosch scaled up Haber's process to large-scale industrial levels).The Haber-Bosch process consumes vast amounts of energy, and some researchers argue that even tiny improvements in catalyst efficiency could yield big savings. This project aims for a more fundamental shift - finding a catalyst that emulates nature's ability to fix nitrogen gently from the air. The industrial process relies on a chemical hammer-blow to cleave the bonds of the feedstock gases in one swift stroke, littering the catalyst's surface with nitrogen and hydrogen atoms that rapidly combine to make ammonia. In contrast, nature uses a stream of protons and electrons to unpick dinitrogen one bond at a time - a surgical procedure orchestrated by nitrogenase, an enzyme found in microbes such as soil-dwelling Rhizobia.Nitrogenase has two major parts: an iron protein that acts as a delivery vehicle for electrons, and a molybdenum-iron protein that uses them to break dinitrogen apart. The electrons ultimately come from reduced ferredoxin, a powerful reducing agent generated during photosynthesis, while adenosine triphosphate (ATP) provides the energy. Biological nitrogen fixation is far from being a paragon of efficiency. Turning each molecule of nitrogen into ammonia takes at least 16 molecules of ATP, and a quarter of the electrons involved in the reaction are 'wasted' to make hydrogen gas as a by-product. 'These bacteria devote a lot of energy to it, which tells you how important nitrogen fixation is for life. The catalytic heart of the enzyme is called the iron-molybdenum cofactor (FeMo-co), a cluster of seven iron ions, one molybdenum ion, and nine sulfides. Scientists are still unsure how it works - indeed, it was only in 2015 that x-ray crystallography definitively revealed a single carbon ion right in the middle of the cluster.Based on a mixture of hard data and intuition, the current belief is that a quartet of iron atoms in the FeMo-co forms the catalytically active site. Some are still doubtful that nitrogenase's iron atoms bind and reduce dinitrogen. An alternative view supported by model compounds is that Mo is the key for nitrogen reduction.