Periodic Reporting for period 1 - MOMIMIC (Multi-layered biomimetics of lytic polysaccharide monooxygenases)
Período documentado: 2018-01-08 hasta 2020-01-07
Lytic polysaccharide monooxygenases (LPMO) are a family of enzymes that use oxygen or hydrogen peroxide to oxidatively cleave the glycosidic bonds in recalcitrant polysaccharides. This has been shown to greatly enhance the enzymatic degradation of cellulose, making these enzymes of significant interest for the conversion of lignocellulosic biomass into biofuels or commodity chemicals. The cleavage reaction occurs via hydroxylation of the 1- or 4-position C-H bonds flanking the glycosidic linkage. As the dissociation energy for these bonds is quite high (near 100 kcal/mol), a better understanding of all of the factors that allow LPMO to perform this reaction is important not just from the standpoint of biomass conversion, but also for developing synthetic catalysts able to perform similar transformations. The active sites of all members of the LPMO family contain a mononuclear copper site with the metal bound in a T-shaped N3 coordination environment, described as the histidine brace, comprised of the amine and imidazole of an N-terminal histidine and another histidine imidazole. Though the specific features of the histidine brace that contribute to LPMO reactivity are not fully understood, the similar binding site in particulate methane monooxygenase indicates it may be essential for the demonstrated oxidative power of the enzymes. Additionally, beyond the active site, multiple amino acids in the second coordination sphere are also thought to contribute to the reactivity. Studying the role these structural features with synthetic models of the active site could lead to a better understanding of the enzyme and help address multiple questions related to the mechanism of the oxidation. Still, despite the apparent simplicity of the coordination environment, no model complexes to date have been reported that exactly replicate the histidine brace.
The objectives of MOMIMIC are to address the challenges of developing structural models through the use of molecular scaffolds based on aromatic oligoamide foldamers to engineer ligands with fully-defined multi-layered coordination environments and to study the properties and reactivity of the complexes with copper. New synthetic approaches for obtaining imidazole/histamine functionalized monomers and for their incorporation into oligomers are explored leading to a series of ligands. Through metalation, the spectroscopic properties of the resulting complexes can be compared to both the enzyme and smaller model systems to see the effects of the scaffold. By studying the reactivity of these systems with oxygen, these complexes can eventually provide important spectroscopic models for better understanding of LPMO.
The objectives of MOMIMIC are to address the challenges of developing structural models through the use of molecular scaffolds based on aromatic oligoamide foldamers to engineer ligands with fully-defined multi-layered coordination environments and to study the properties and reactivity of the complexes with copper. New synthetic approaches for obtaining imidazole/histamine functionalized monomers and for their incorporation into oligomers are explored leading to a series of ligands. Through metalation, the spectroscopic properties of the resulting complexes can be compared to both the enzyme and smaller model systems to see the effects of the scaffold. By studying the reactivity of these systems with oxygen, these complexes can eventually provide important spectroscopic models for better understanding of LPMO.
The work towards the scaffold complexes was divided into three parts: Monomer synthesis, Oligomer synthesis, and metalation. As the oligomer scaffold designs required the development of several new monomers an important initial focus was on obtaining these units. Significant synthetic work went into the optimization and scaling up of the reactions for these monomers. New synthetic protocols for incorporating the imidazole and histamine units into quinoline based monomers were developed that allowed these monomers to be obtained on the gram quantities needed for the oligomer synthesis. Additionally, a short and efficient route to a 2,7-diamino-1,8-diazaanthracene monomer, used as a linker between the quinolines, was developed. Once the monomers were obtained, their incorporation into the oligomer scaffolds was performed. Selectively monoprotecting the 2,7-diamino-1,8-diazaanthracene unit to avoid side reactions proved challenging, giving only low yields for the desired product. Thus, procedures were optimized for coupling of the quinoline acids directly to the unprotected diamino unit. These proved efficient and gave a more direct route to the coupled products. Despite the simplicity of the amide coupling steps, several challenges were encountered. Notably, deprotection of the trityl and boc groups in the final scaffold required multiple steps in order to ensure sufficient purity of the final scaffold. Finally, metalation of the obtained scaffolds was performed with several copper salts and studied by UV-vis spectroscopy. Depending on the copper source, the resulting spectra show an absorbance band between 600-700 nm, similar to what is reported for LPMO. Research is still ongoing and focuses on the full structural characterization of the ligands and complexes, as well as, testing of their reactivity for the oxidative degradation of model saccharide substrates.
The synthetic work performed during MOMIMIC has generated significant progress beyond the state of the art. The novel structures (both monomers and oligomers) that were developed and the synthetic procedures for obtaining them not only provides new units and designs for aromatic oligoamide foldamers, but also to the broader synthetic chemistry community. Notably, imidazole functionalized quinolines like those developed in MOMIMIC are synthetically challenging targets that can be of potential interest for medicinal chemistry applications. We have developed a new and reliable method for obtaining them.
Additionally, the strategy developed for generating model complexes of enzyme active sites represents a good progress beyond the state of the art. The use of stable and predictable aromatic oligoamides for the scaffolding approach used to obtain the specific ligand environment offers a valuable new tool for the design of biomimetic complexes and can have important impacts for coordination chemistry, bioinorganic chemistry and potentially catalysis. Once monomers are synthesized, the modular nature of these structures can allow ready exchange of the building blocks for incorporating multiple interactions or for ligand modification. It is expected that the development of such next-level model structures will have significant implications for the understanding of enzyme mechanisms and their reactivity. In the case of LPMO, future work with the model complexes developed in MOMIMIC will be invaluable both for interpretation of spectroscopic studies for better understanding the enzyme and also for understanding features that must be built into synthetic catalysts for increasing their oxidative capabilities. This latter point could lead to advances in the catalytic conversion of biomass into value added chemicals.
Additionally, the strategy developed for generating model complexes of enzyme active sites represents a good progress beyond the state of the art. The use of stable and predictable aromatic oligoamides for the scaffolding approach used to obtain the specific ligand environment offers a valuable new tool for the design of biomimetic complexes and can have important impacts for coordination chemistry, bioinorganic chemistry and potentially catalysis. Once monomers are synthesized, the modular nature of these structures can allow ready exchange of the building blocks for incorporating multiple interactions or for ligand modification. It is expected that the development of such next-level model structures will have significant implications for the understanding of enzyme mechanisms and their reactivity. In the case of LPMO, future work with the model complexes developed in MOMIMIC will be invaluable both for interpretation of spectroscopic studies for better understanding the enzyme and also for understanding features that must be built into synthetic catalysts for increasing their oxidative capabilities. This latter point could lead to advances in the catalytic conversion of biomass into value added chemicals.