Design and Application of Nanostructured Flavin Photocatalysts

Flavin compounds are found in Nature within a range of flavin-containing enzymes (flavoenzymes) that are responsible for metabolic, antioxidant and photoreception processes across animal, plant and bacteria kingdoms. Their broad redox and photochemistry has seen the field of flavin-based catalysis emerge as a powerful addition to sustainable catalysis thanks to being cheap, non-toxic and highly active. Flavin photocatalysis in particular has shown great promise in a range of synthetic procedures such as benzylic oxidations, sulfoxidations, decarboxylative transformations, isomerisations and [2+2] cycloadditions to name a few. It has been shown that flavin photocatalysts can be tuned to a specific application either through chemical structure modification or by the design of advanced systems through heterogeneous or polymer carrier attachment. The latter has shown encouraging results to widen flavin photocatalyst applicability within organic synthesis but has lacked in displaying characteristics like flavoenzymes which enable highly efficient and selective catalysis of industrially relevant products with precise stereochemical control. This thesis details the design and application of novel flavin photocatalysts through the combination of polydopamine (PDA). It is shown that PDA not only acts as a carrier of the flavin, but actively engages in the catalytic mechanism and improves flavin photostability. In the first instance, copolymer flavin-polydopamine (FLPDA) nanoparticles were synthesised and their photocatalytic activity was characterised through model oxidation and reduction reactions. This study revealed enzyme-like kinetics of the catalysed reactions and improved photostability of the conjugated flavin moieties. Additionally, the biocompatibility of the nanoparticles was assessed through in vitro cell studies to ensure applicability to other fields such biomedicine or water remediation. Subsequently, the flavoenzyme specific oxidation of indole to indigo and indirubin dyes was explored using the FLPDA photocatalyst. The results showed that the nanoparticle system exhibited higher production of the valuable dyes over a homogeneous flavin photocatalyst. This increase in activity was investigated through reactive oxygen species (ROS) scavenging experiments which revealed that FLPDA’s mechanism of action in this reaction partially resembled natural flavoenzymes and therefore enhanced key product formation. Finally, reduction reactions catalysed by flavoenzymes were investigated using FLPDA and a chiral flavin-polydopamine system (RCPDA). The light-driven reduction of azobenzene dyes was first explored using FLPDA which provided evidence that hydride could be transferred from PDA-conjugated flavin moieties to a substrate upon irradiation in the presence of an electron donor reagent. Next, the reduction of C=C bonds within α,β-unsaturated ketones and aldehydes was explored by initially screening buffered electron donor reagents and homogeneous flavin photocatalysts with key substrates. The optimal conditions and compatible substrates were then applied to the nanoparticle RCPDA system which produced the saturated products in comparable yields to the homogeneous photocatalysts with very low catalyst loading (<1 mol% vs. 10 mol%). The inclusion of a chiral linkage between PDA and flavin and its effect on the stereochemical outcome of the reaction was also explored, with preliminary data showing that adopting such a strategy could enable some enantioselectivity over the product. In summary, this thesis provides new methodologies to design advanced flavin photocatalysts with enzyme-like characteristics that will help to further develop the field of sustainable catalysis.UKRI DTP 2016-2017 EP/N509620/

Flavin Conjugated Polydopamine Nanoparticles Displaying Light-Driven Monooxygenase Activity.

A hybrid of flavin and polydopamine (PDA) has been explored as a photocatalyst, drawing inspiration from natural flavoenzymes. Light-driven monoxygenase activity has been demonstrated through the oxidation of indole under blue light irradiation in ambient conditions, to afford indigo and indirubin dyes. Compared to riboflavin, a flavin-polydopamine hybrid is shown to be more resistant to photobleaching and more selective toward dye production. In addition, it has been demonstrated that it can be recycled from the solution and used for up to four cycles without a marked loss of activity, which is a significant improvement compared to other heterogenous flavin catalysts. The mechanism of action has been explored, indicating that the PDA shell plays an important role in the stabilization of the intermediate flavin-peroxy species, an active component of the catalytic system rather than acting only as a passive nanocarrier of active centers

Flavin Conjugated Polydopamine Nanoparticles Displaying Light-Driven Monooxygenase Activity

A hybrid of flavin and polydopamine (PDA) has been explored as a photocatalyst, drawing inspiration from natural flavoenzymes. Light-driven monoxygenase activity has been demonstrated through the oxidation of indole under blue light irradiation in ambient conditions, to afford indigo and indirubin dyes. Compared to riboflavin, a flavin-polydopamine hybrid is shown to be more resistant to photobleaching and more selective toward dye production. In addition, it has been demonstrated that it can be recycled from the solution and used for up to four cycles without a marked loss of activity, which is a significant improvement compared to other heterogenous flavin catalysts. The mechanism of action has been explored, indicating that the PDA shell plays an important role in the stabilization of the intermediate flavin-peroxy species, an active component of the catalytic system rather than acting only as a passive nanocarrier of active centers

Flavin-conjugated iron oxide nanoparticles as enzyme-inspired photocatalysts for Azo Dye degradation

In this work, a new photocatalytic system consisting of iron oxide nanoparticles (IONPs), coated with a catechol-flavin conjugate (DAFL), is synthesized and explored for use in water remediation. In order to test the efficiency of the catalyst, the photodegradation of amaranth (AMT), an azo dye water pollutant, was performed under aerobic and anaerobic conditions, using either ethylenediaminetetraacetic acid (EDTA) or 2-(N-morpholino)ethanesulfonic acid (MES) as electron donors. Depending on the conditions, either dye photoreduction or photooxidation were observed, indicating that flavin-coated iron-oxide nanoparticles can be used as a versatile enzyme-inspired photocatalysts.Funded by EPSRC DTC Studentship, grant number EP/N509620/

Light-driven assembly of biocompatible fluorescent chitosan hydrogels with self-healing ability.

Nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) was successfully used to cross-link complementary tetrazole and maleimide chitosan derivatives into hydrogel networks using irradiation. The photo-click reaction resulted in the formation of robust fluorescent hydrogels with an emission signal at around 530 nm. The degree of cross-linking and the resulting hydrogel properties such as pH sensitivity and swelling were influenced by the tetrazole/maleimide ratio and the length of irradiation. Interestingly, rheological studies demonstrated self-healing character of the novel hydrogels as indicated by instantaneous recovery of the storage modulus to the initial values under different oscillatory strains without any additional external trigger. Finally, in addition to their photo-tuneable and self-healing properties, the novel chitosan hydrogels were also found to be biocompatible and susceptible to in vitro enzymatic degradation, making them suitable for design of traceable biomaterials for biomedical applications

Tuning riboflavin derivatives for photodynamic inactivation of pathogens.

The development of effective pathogen reduction strategies is required due to the rise in antibiotic-resistant bacteria and zoonotic viral pandemics. Photodynamic inactivation (PDI) of bacteria and viruses is a potent reduction strategy that bypasses typical resistance mechanisms. Naturally occurring riboflavin has been widely used in PDI applications due to efficient light-induced reactive oxygen species (ROS) release. By rational design of its core structure to alter (photo)physical properties, we obtained derivatives capable of outperforming riboflavin's visible light-induced PDI against E. coli and a SARS-CoV-2 surrogate, revealing functional group dependency for each pathogen. Bacterial PDI was influenced mainly by guanidino substitution, whereas viral PDI increased through bromination of the flavin. These observations were related to enhanced uptake and ROS-specific nucleic acid cleavage mechanisms. Trends in the derivatives' toxicity towards human fibroblast cells were also investigated to assess viable therapeutic derivatives and help guide further design of PDI agents to combat pathogenic organisms

Endosomolytic Bioreducible Poly(amido amine disulfide) Polymer Conjugates for the <i>in Vivo</i> Systemic Delivery of siRNA Therapeutics

Efficient siRNA delivery is dependent not only on the ability of the delivery vehicle to target a specific organ but also on its ability to enable siRNA entry into the cytoplasm of the target cells. Polymers with endosomolytic properties are increasingly being used as siRNA delivery vehicles due to their potential to facilitate endosomal escape and intracellular delivery. Addition of disulfide bonds in the backbone of these polymers was expected to provide degradability through reduction by glutathione in cytosol. This paper describes the synthesis of new endosomolytic bioreducible poly­(amido amine disulfide) polymers whose lytic potential can be masked at physiological pH, but can be restored at acidic endosomal pH. These polymer conjugates gave good <i>in vitro</i> knockdown (KD) and did not demonstrate cytotoxicity in a MTS assay. Efficient mRNA KD for apolipoprotein B in mouse liver was observed with these polyconjugates following intravenous dosing

Security governance : making the concept fit for the analysis of a multipolar, global and regionalized world

Recently introduced in the academic and political debate, the concept of “security governance” still needs to be clarified. In particular, to make the concept more useful for an assessment of current security dynamics, four main shortcomings need to be overcome: in the first place, attention has been devoted more to “governance” than to “security”, while greater attention should be paid to how security is understood and perceived by the actors involved in the governance system. Second, the literature is divided in two main branches, one looking at security governance predominantly by/through governmental organizations and one dealing with non-state actors: attempts should be made to give sense of coordination efforts (or lack thereof) among different actors and layers of governance, even when focussing attention predominantly on one type of actor (e.g. regional state powers). Third, the literature (with notable exceptions though) has predominantly focused on Europe and the transatlantic area: an effort should be made to look at extra-European dynamics, also with an aim to evaluate the relationship between political/security culture and security governance, as well as between political/economic development and security governance. Finally, the literature on security governance has been too often detached from reflections over regionalism, while it would be useful to explore further the relationship between cognitive definitions of regional and security dynamics. This is all the more important when considering the progressive emergence of non-European regional powers, possibly interpreting security challenges in different terms and displaying different likelihoods and modalities to arrange coordination patterns and solve security problems