Transport Mechanism and Improvement
Ideally, chemicals could attack compound middle lamella with the highest lignin concentration to release fibers during chemical treatment of biomass. Unfortunately, it is opposite. Starting from the pores, the chemical generally diffue gradually from lumen to cell layers and to middle lamella at the end. The pores are the key for chemical transpotation.
Similar mechanism seems to happen during enzymatic hydrolysis of biomass, which is called tunneling mechanism: the enzyme complex attacks cellulose by penetrating into the interior of particle rather than eroding the outer surface (Ladisch et al 1992). The following pictures demonstarte the progress of enzymatic hydrolysis of corn stover.
After pretreatment
After 3 hours enzymatic hydrolysis
After 168 hours enzymatic hydrolysis
(Ref. Zeng and Ladisch, 2007)
Obviously, transport of chemicals or enzymes into the fiber walls is regulated by the sizable pores. It has been found that there is a linear relationship between the initial hydrolyzability of a ligonocellulosic substrate and its accessibility to a molecule of nominal diameter 51 Å (Grethlein et al.,1984 ). However, CBH I (cellobiohydrolase I) from Trichoderma reesei is tadpole shaped proteins with total length of 180 Å and a dimension of the catalytic core: 50x60x40 Å (Divine et al., 1993, 1994; Esterbauer et al., 1991). To initiate and promote its penetration, it requires sort of opened channels at the very beginning. That is why we need pretreatments of biomass before any enzymatic hydrolysis.
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