Influence of substrate crystallinity and glass transition temperature on enzymatic degradation of polyethylene terephthalate (PET)

This work examines the significance of the degree of crystallinity (X-C) of polyethylene terephthalate (PET) and the PET glass transition temperature (T-g) on enzymatic degradation of PET at elevated temperatures using two engineered, thermostable PET degrading enzymes: LCCICCG, a variant of the leaf-branch compost cutinase, and DuraPETase, evolved from the Ideonella sakaiensis PETase. The X-C was systematically varied by thermal annealing of PET disks (empty set 6 mm, thickness 1 mm). The X-C affected the enzymatic product release rate that essentially ceased at X-C 22-27% for the LCCICCG and at X-C similar to 17% for the DuraPETase. Scanning Electron Microscopy revealed that enzymatic treatment produced cavities on the PET surface when the X-C was > 10% but resulted in a smooth surface on amorphous PET (X-C similar to 10%). The T-g of amorphous PET disks decreased from 75 degrees C to 60 degrees C during 24 h pre-soaking in water at 65 degrees C, while the X-C remained unchanged. Enzymatic reaction on pre-soaked disks at 68 degrees C, i.e. above the T-g, did not affect the enzymatic product release rate catalyzed by LCCICCG. These findings improve the understanding of enzymatic PET degradation and have implications for development of efficient enzymatic PET upcycling processes.