Development of Modularly Assembled Synthetic Esterases

Abstract

This thesis describes the development of two conceptually distinct, modular artificial esterases based on cooperative catalysis. The first system exploits the self-assembly of amphiphiles bearing catalytically active head groups to form vesicular catalysts. The modular nature of this self-assembled system enables rapid catalyst discovery and optimisation by varying the identity and ratio of amphiphiles in solution, without the need for synthetic modification. Optimal activity was achieved using a 1:1:1 mixture of imidazole-, guanidine-, and di-(2-picolyl)amine-functionalised surfactants in the presence of Zn²⁺. Mechanistic studies suggest nucleophilic catalysis by imidazole, transition-state stabilisation by guanidine via hydrogen bonding, and Zn²⁺-mediated activation of a secondary nucleophile bound by di-(2-picolyl)amine, enabling catalyst turnover.

Comparison with reported artificial esterases revealed that more rigid and structurally defined systems typically display higher catalytic efficiencies. To address this limitation, a second catalytic platform was developed based on a silver-thiol coordination polymer, introducing structural rigidity while retaining modularity. These polymers were formed in situ from catalytically functionalised thiols, allowing systematic variation of active components. A 1:1:1 combination of imidazole-, guanidine-, and di-(2-picolyl)amine functionalised ligands produced the highest activity, notably in the absence of Zn²⁺. Mechanistic analysis indicates that while the initial nucleophilic attack mirrors that of the vesicular system, the second catalytic step proceeds via the pyridine units of di-(2picolyl)amine acting as either nucleophiles or general bases.

The silver-thiol polymer catalyst exhibited a 2.8-fold increase in catalytic efficiency relative to the vesicular system, underscoring the importance of rigidity and organisation in artificial enzyme design. Catalytic activity could be reversibly switched off and on by disrupting and reforming the silver-thiol coordination bonds using iodide and Ag⁺, respectively, confirming their essential structural role. Overall, this work demonstrates how modular, self-assembled catalytic systems enable rapid optimisation of artificial esterases with minimal synthetic effort, while highlighting the critical role of cooperative interactions and structural organisation in achieving enzyme-like performance. These principles provide a foundation for the development of future artificial esterases for applications such as the hydrolytic degradation of ester-based plastics, including polyethylene terephthalate (PET).