Research Projects

Lytic polysaccharide monooxygenases (LPMOs) are a remarkably important family of enzymes indispensable for the degradation of polysaccharides such as cellulose and chitin, and hence capture enormous research interests relevant to processing of biomass to biofuels. While such enzymes were known to degrade cellulose since 1974, the structure of LPMOs and their functions in oxidizing C–H bond for cleaving the glycosidic bonds of polysaccharides caught enormous attention not more than 15 years ago, The active sites of LPMOs feature a mononuclear T-shaped copper site coordinated to a histidine residue in addition to a bidentate histidine-brace defined by the imidazole-N and the N-terminal of the second histidine residue. Notably, a weakly interacting tyrosine residue is situated within the non-covalent coordination sphere of copper for most of the LPMO variants (except in the chitin-active LPMO form AA10 containing phenylalanine). Previous studies on AA9 variant of LPMO illustrate that the removal of the Tyr164 residue from the coordination sphere of AA9 LPMO results in a significant loss of the monooxygenase activity presumably due to self-oxidation of the active site in the absence of tyrosine. Prior QM/MM-based computational investigations suggest that the tyrosine residue serves as a source of H+/e– (net H-atom), thereby providing protection against oxidative damage through a hole-hopping mechanism. Moreover, several questions remain open regarding the catalytic mechanism whether LPMO catalyzes C–H bond oxidation through O₂-dependent oxidase type activity (R–H + O₂ + 2H+ + 2e– ⟶ R–OH + H₂O) or employing H₂O₂-dependent peroxygenase reactivity (R–H + H₂O₂ ⟶ R–OH + H₂O). The questions regarding the identity of the principal cosubstrate (O₂ and/or H₂O₂), molecular mechanism for the cosubstrate activation, and role of tyrosine residue on the cosubstrate activation remain open. We hypothesize that the tyrosine residue in the secondary coordination sphere of the copper-site in LPMO is activated towards heterolytic as well as homolytic cleavage, and thus deemed to be suitable for modulating O₂/H₂O₂ activation, stabilization of intermediates, and substrate oxidation. Aiming to gain insights into the chemistry associated with the metal⋯phenol interactions and their subsequent role, this proposal aims the syntheses, structural characterization, and reactivity analyses of a series of phenol(ate) bound copper complexes relevant to the LPMO active sites. Moreover, a series of structurally characterized phenol(ate) complexes of first-row mid-to-late transition metals (Mn/Fe/Co/Ni/Cu/Zn) is proposed to be investigated for gaining valuable thermochemical insights such as pKa and O–H bond dissociation energy of the phenolic moieties. The incorporation of a phenolic moiety in the second coordination sphere can facilitate both proton/hydrogen-atom transfer, while simultaneously acting as a hydrogen-bond donor/acceptor. These functionalities are anticipated to stabilise (hydro)peroxo intermediates, assist in heterolytic/homolytic O–O bond cleavage depending on the nature of the metal centre, and thereby ultimately modulating the overall reactivity profile. Taken together, these understanding on the role of phenol (as a tyrosine model) in the second coordination sphere of the metal site would allow to mechanism driven small molecule activation for the development of new artificial catalysts for industrially relevant oxidative processes including biomass depolymerisation.