Lignin-first pretreatment combined with photostimulated enzymatic hydrolysis enables yield-gaining conversion of wood biomass

Light-driven enzymatic degradation of lignocellulose could help boosting the economy of biorefineries by either shortening the bioprocessing time, or accelerating enzymatic kinetics and so possibly spare some precious enzymes. The redox Lytic Polysaccharide Monooxygenases - LPMOs, a key enzyme for cellulose degradation, had been shown to benefit from photobiocatalysis settings thanks to the in-situ photocatalytic reduction of O2 into H2O2, a co-substrate of the enzyme and perhaps through a faster reduction of its catalytically active metal Cu-II to Cu-I. Despite the successful photoactivation of LPMO, a clear gain in saccharification yield during enzymatic hydrolysis of industrial-relevant lignocellulosic substrate was never achieved. The previous attempts had seen the use of externally added photosensitizers i.e. chlorophyllin or melanin, or settings where the lignin already present in the biomass was exploited as photosensitizer, in both cases undergoing an uncontrolled photocatalytic production of the H2O2. Eventually, it appears that the choice of the lignocellulosic pretreatment technology - PT, the ratio between glycoside hydrolases - GHs and LPMO in the enzymatic cocktail, and the light irradiation could all be key to set a yield-gaining light-driven biorefinery. In this work, we successfully found that the reductive catalytic fractionation – RCF, a lignin-first PT, is suitable to fractionate birchwood lignin so to provide lignin-oil, and a remaining lignocellulose consisting of a photoactivable lignin and highly digestible cellulose. Up to 20% yield gain was obtained when conducting the enzymatic hydrolysis under intermitted light irradiation using a lab-made cocktail containing GHs and LPMOs. Moreover, the exchange of photo-excited electrons among the various components had been electrochemically characterized. Eventually, confocal laser scanning microscopy had been extensively used to characterize the oxidative status of the lignin during photo-driven catalysis in the presence of LPMO.