| The complex structural recalcitrance of lignocelluloses is the key factor to inhibit the efficiency of bioethanol production. White rot fungal pretreatment is an environmental friendly and low energy comsuption technology, which can reduce the resistance of lignocelluluose to enzymatic hydrolysis and enchance the efficiency of bioethanol conversion. Fungal pretreatment has a great potential in the production of biofuels from lignocelluloses. However, the mechanism of biological pretreatment to improve the hydrolysis of lignocelluloses is not clear and the efficiency of fungal pretreatment is still to be improved. In this study, a simple and efficient bio-pretreatment was established by investigating the key factor to biological pretreatment using the novel white rot fungus Irpex lacteus, and the mechanism of the key factor Mn2+to the biological pretreatment was clarified; on the basis, by analysising the differences of structures of lignin and lignocelluloses and the interactions between cellulase and substrates under different pretreatment conditions, it was clarified that the mechanism of the reduction of structural resistance to improve the efficiency of enzymatic hydrolysis by biological pretreatment.According to the study of effects of environmental factors and additives to biological pretreatment using/. lacteus, it showed that Mn2+was the key factor to promote the efficiency of fungal pretreatment. An novel and efficient technology to produce bioethanol from lignocelluloses was established:the efficiency of biological pretreatment has been achieved the highest level in the current studies, the yield of glucose and ethanol from the pretreated corn stover were308.98mg/g and144.03mg/g respectively with additive of0.01mM/g MnSO4in substrates. Moreover, the correlation of component composition of corn stover, reaction of cellulases and the production of ethanol was also investigated, results showed that the efficiency of enzymatic hydrolysis was closely related to the the content of lignin in pretreated corn stover.The determination of extracellular enzymes from I. lacteus during pretreatment and the degradation of lignin model compounds by I. lacteus showed that manganese peroxidase played an important part in the process of biodegradation and biomodification of lignin. The supplement of Mn2+improved the activity of MnP from I. lacteus. On the other hand, the ability of I. lacteus to generate free radicals was also enhanced during biological pretreatment. The production of superoxide anion radical was enhanced by3.5-fold compared with conventional biological pretreatment, thereby promoting the selective modification of lignin during fungal pretreatment.Structural characteristics of pretreated lignin with Mn2+present and absent during biological pretreatment were evaluated by the technologies of infrared spectroscopy, nuclear magnetic resonance spectroscopy and pyrolysis-gas chromatography/mass spectroscopy, etc. Results showed that the improvement of enzymatic hydrolysis after fungal pretreatment was closely related to deconstruction of key structures of/β-O-4ether bond, hydroxyl group, methoxy group and phenyl ring in lignin. Mn2+promoted the process of lignin bio-modification during pretreatment using I. lacteus. In comparsion with the structural alteration of pretreated lignin with Mn2+absent in biomass, the content of β-O-4ether bond decreased by50%, hydroxyl and methoxy group were further reduced and more than30%of guaiacyl lignin and syringyl lignin was deconstructed. Structural modification of lignin resulted in a significant depolymerization of macromolecule lignin and enhancement of modification to side chain of phenyl ring in lignin, which was contributed to the deconstruction of phenyl ring in lignin and decompostion of the network structure of lignin.The relationships of structural modification of lignin, physical and chemical characteristics of corn stover and the interaction between cellulase and substrates were further investigated. Results showed that the suface area of corn stover was increased by49.51%and hydrophilicity of corn stover was also increased after pretreated by I. lacteus, thereby reducing the steric hindrance of corn stover to enhance hydrolysis of cellulose. The network structure of macromolecular lignin was further destructed caused by the enhancement of modification to the key structure of lignin in the efficient biological pretreatment with Mn2+present in substrates. In comparasion to the pretreated corn stover with Mn2+absent in substrates, there was an increase of the pore size distribution within the range of15-30nm and60-100nm and the hydrophilicity of corn stover was further increased, causing the improvement of adsorption properties of corn stover to cellulase, the adsorption of cellulase increased by15.38%. Supplement of Mn2+in corn stover has greatly improved the efficiency of biological pretreatment. The recalcitrance of corn stover to hydrolysis was almost completely removed, which greatly promoted the efficiency of enzymatic hydrolysis and ethanol production from biopretreated corn stover.This research was established on the basis of improvement of enzymatic hydrolysis with the supplement of key additive during biological pretreatment, which clarifying the relationship between enhancement of enzymatic hydrolysis and structural modification of corn stover. A simple and efficient biological pretreatment was established, moreover, the key target of structural modification of corn stover to improve efficiency of hydrolysis was also investigated. In this work, the key factor of the resistance of lignocelluloses is clarified and the relationship between enzymatic hydrolysis and structural modification of lignin was elucidated, which provides a theoretical basis for the artificial modification of lignin to enhance the efficiency of bioethanol production from lignocelluloses. |
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