The Rational Design of Artificial Nanozymes for Urease Mimicking: the Stoichiometric Release of NH3 from Urea Hydrolysis at Ambient and Elevated Temperature

Description

Since the first synthesis of urea by Wöhler in 1828, more than 200 million tonnes of urea was used as a nitrogen-release fertilizer in 2018 to feed the growing population worldwide. However, the increasing discharge of urea from mammal and factories waste has made urea pollution a major environmental problem. Urea is a very stable molecule in solution and enters the ecosystem mainly through wastewater; in ocean, it triggers algae blooms and excretion of domoic acid which threatens aquatic life. Currently, the industrial removal of urea from wastewater relies on chemical hydrolysis at high temperature/pressure, which is cost intensive on a large scale. Although industrial synthesis of urea from ammonia (NH3) is well-established, the development of catalysts that hydrolyze urea back to “two NH3molecules” has been rarely explored. In nature, this reaction is catalyzed by ureases in many soil bacteria. However, the remedy of this environmental problem with natural enzymes is not feasible as they are easily denatured and requires high costs in preparation/storage. To tackle these disadvantages, the development of artificial alternatives with advantages such as low production cost, simple storage and pH/thermal stability has attracted much attention over the past decade.This proposal aims to design and synthesize urease mimetics with activity comparable to their native counterpart. As revealed in literature, urease-catalyzed urea hydrolysis involves (1) the effective adsorption of urea at Ni-O(H)-Ni center and (2) subsequent formation of leaving group (i.e. -NH3+) on urea. These two steps facilitate the nucleophilic attack of water on urea carbon to produce NH3 and carbamic acid. The second NH3is readily released from carbamic acid in water even without ureases. Preliminary screening of catalysts suggests that Lewis acid (LA) and Brönsted acid (BA) sites are both required for this reaction, however, the LA strength is the key factor affecting not only urea adsorption (step 1) but also the activation of proton transfer from the BA sites (step 2). The adsorption/activation structure of urea on the promising HNb3O8catalysts (surprisingly not NiO) found in our preliminary study will be firstly unraveled by advanced characterization tools to establish a structure-acidity-activity relationship. We will then carefully compared their activity with native ureases in a wide range of pH/temperature. The activities of other Nb-based layered catalysts with various LA/BA strength will also be tested. The success of this proposal is expected to provide guidelines for the rational design of urease mimetics with high activity.