Biocatalytic Approaches to the Synthetic Manipulation of Silicones

Living organisms construct a tremendous variety of structures across a wide range of sizes, from bones to cells. Yet, the assembly of such structures ultimately rely on the organisation and production of building blocks that are essentially on the molecular to nanoscopic scale (Angstroms to nanometres). These structures, which are composed from a variety of compounds (proteins, fats, DNA) are synthesised by enzymes. These enzymes, which are the molecular machinery of all living organisms, are particularly interesting since they are able to perform a variety of reactions with high efficiency, giving mainly the desired compound with few unwanted by-products. Furthermore, they function under ambient conditions and do not rely on rare or toxic materials. As a result, many types of enzymes are now utilised in the production of medicines and other high-value chemicals.

One area in which enzymes have not been widely studied so far is in the chemistry of organic compounds containing silicon. Such "organosilicon" compounds are mainly used in the form of "silicone", a plastic-like material. These silicones are extremely widely used in all sectors of human activity, from industrial machine parts, lubricants and sealants; to consumer goods such as homeware, cosmetics and paints; as well as in electronic and surgical devices. Indeed, these materials are economically very important, with the global production and use of silicones giving rise to a multi-£billion turnover annually.

Unfortunately, current methods of producing silicones rely on chlorine-containing raw materials that are ecologically unfriendly and energy demanding to produce. In contrast, some species of marine sponges use silicon (in the form of glass-like silica) as part of their skeleton. To form this skeleton, the sponges employ a family of enzymes called "silicateins", which are able to react with silica.

Recent research by the lead investigator has shown that, remarkably, these enzymes are able to catalyse the formation, as well as degradation, of a range of organosilicon compounds under relatively mild conditions (less than 100 degrees C, using non-toxic starting materials). Thus, these enzymes could potentially offer a sustainable means of producing silicone compounds that would find use in many areas of the chemical industry. Furthermore, the silicateins could also be applied to decompose unwanted silicone waste into compounds that could be recycled, which cannot currently be achieved using conventional chemical methods.

Accordingly, the goals of this research are to investigate the feasibility of using silicateins for the efficient and precise synthesis of silicone materials, and develop modified versions of the enzymes that will be able to perform the production of silicones with a variety of chemical structures. The types of silicones that will be targeted include both silicones that are applicable to industrial applications, but also novel types that are otherwise difficult to synthesise by other means. In parallel, the feasibility of using them in the reprocessing and recycling of silicones will also be researched. In all cases, a major part of this research will be to study the chemical mechanisms by which the silicateins are able to perform these reactions. Such an understanding of how these enzymes function will therefore allow us to make modifications to improve their capabilities.

This research will study the silicateins, a group of enzymes originating from marine sponges that are able to form and cleave silicon-oxygen bonds; and investigate their use for the synthesis and depolymerisation of polysiloxane "silicone" polymers. Using insights gained from studies into its molecular mechanism of action, modified versions of the silicateins will be developed (by directed evolution) and applied to catalysis in organosilicon chemistry. Such engineered enzymes will give rise to new capabilities that are difficult or impossible to achieve through current techniques, as well as processes that are more sustainable. This research will therefore benefit a wide range of parties not only in academia (see Academic Beneficiaries), but also through external parties through the development of new biocatalytic tools.

Specifically, new methods for the synthesis of silicones that are both more sustainable and more precise would be of great benefit to the entire silicone manufacturing sector, across its very broad range of end applications. These include the production of bulk materials such as silicones for coatings, sealants and lubricants; to more demanding "high-tech" applications such as surgical implants and electronic device components. It is envisaged that they will benefit from the increased efficiency in terms of raw material and energy consumption together with the reduction of waste, leading to cost savings.

Furthermore, these enzymes may enable the synthesis of new and more complex types of silicon-based polymers that are of great interest in the materials sciences sector for development of new industrial products. For example, gas and ion permeable membranes for applications in gas separation and fuel cells; and fire resistant coatings that are relevant to applications in the aerospace and defence sectors.

Demonstrations of how enzymes can be used for selective silicon-oxygen bond synthetic manipulations are readily applicable to the synthesis of complex small molecules. Therefore this research will be relevant more generally to the fine chemical and pharmaceutical industries.

Additionally, the prospect of using these enzymes for the degradation of silicon-based waste materials would be of interest to organisations involved in efforts to recycle silicone materials. Currently, there are no economically or environmentally viable methods for the recovery or recycling of used silicones. As a result, they are primarily disposed by landfilling and consequently these molecules are now persistent environmental contaminants. Thus, any methods that enable the depolymerisation of silicones to monomers that can then be reused would be of high industrial and societal significance.

By the end of this project period, we expect to have tested the feasibility of using modified silicateins for silicone synthetic manipulations, allowing external parties to take our findings towards commercially viable tools. We will engage the potential end users through publishing our findings in scientific papers, engagement with academic-industry network meetings and, where appropriate, through the exploitation of our intellectual property (e.g. patents, consultancy). As noted in the Pathways to Impact, we will form a partnership with companies that deal with silicone materials to facilitate the dissemination of our findings.

More generally the outcome of this research will contribute to the UK's position as a major participant in the field of industrial ("white") biotechnology and frontier research in synthetic biology. This project will train new researchers that are engaged at these strategically important areas and enable the UK to better harness the potential economic and social benefits arising from this cutting-edge research. The more sustainable processes developed here will also reduce the environmental impact of human activity, enhancing the UK's "green" credentials and benefiting the wider global community.