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 (XC) of polyethylene terephthalate (PET) and the PET glass transition temperature (Tg) 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 XC was systematically varied by thermal annealing of PET disks (Ø 6 mm, thickness 1 mm). The XC affected the enzymatic product release rate that essentially ceased at XC 22-27% for the LCCICCG and at XC ∼17% for the DuraPETase. Scanning Electron Microscopy revealed that enzymatic treatment produced cavities on the PET surface when the XC was > 10% but resulted in a smooth surface on amorphous PET (XC ∼10%). The Tg of amorphous PET disks decreased from 75 °C to 60 °C during 24 h pre-soaking in water at 65 °C, while the XC remained unchanged. Enzymatic reaction on pre-soaked disks at 68 °C, i.e. above the Tg, 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.