Enzymatic degradation of polyethylene terephthalate nanoplastics analyzed in real time by isothermal titration calorimetry

Plastics are globally used for a variety of benefits. As a consequence of poor recycling or reuse, improperly disposed plastic waste accumulates in terrestrial and aquatic ecosystems to a considerable extent. Large plastic waste items become fragmented to small particles throughmechanical and (photo)chemical processes. Particles with sizes ranging from millimeter (microplastics, <5 mm) to nanometer (nanoplastics, NP, <100 nm) are apparently persistent and have adverse effects on ecosystems and human health. Current research therefore focuses on whether and to what extent microorganisms or enzymes can degrade these NP. In this study, we addressed the question of what information isothermal titration calorimetry, which tracks the heat of reaction of the chain scission of a polyester, can provide about the kinetics and completeness of the degradation process. The majority of the heat represents the cleavage energy of the ester bonds in polymer backbones providing real-time kinetic information. Calorimetry operates even in complex matrices. Using the example of the cutinase-catalyzed degradation of polyethylene terephthalate (PET) nanoparticles, we found that calorimetry (isothermal titration calorimetry-ITC) in combination with thermokinetic models is excellently suited for an in-depth analysis of the degradation processes of NP. For instance, we can separately quantify i) the enthalpy of surface adsorption Delta H-Ads = 129 +/- 2 kJ mol(-1), ii) the enthalpy of the cleavage of the ester bonds Delta H-EB = -58 +/- 1.9 kJ mol(-1) and the apparent equilibrium constant of the enzyme substrate complex K = 0.046 +/- 0.015 g L-1. It could be determined that the heat production of PET NP degradation depends to 95% on the reaction heat and only to 5% on the adsorption heat. The fact that the percentage of cleaved ester bonds (eta = 12.9 +/- 2.4%) is quantifiablewith the newmethod is of particular practical importance. The newmethod promises a quantification of enzymatic and microbial adsorption to NP and their degradation in mimicked real-world aquatic conditions. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

Plastics waste poses adverse effects on ecosystems and human health when not properly managed. Current research focuses on microbial or enzymatic degradation of microplastics, while using isothermal titration calorimetry in combination with thermokinetic models for in-depth analysis of degradation processes. The new method allows quantification of enzymatic and microbial adsorption to nanoparticles in simulated aquatic conditions.