Mechanistic investigations of lipase-catalyzed degradation of polycarbonate in organic solvents

The biodegradation of an engineering thermoplastic, poly (bisphenol-A carbonate) (BPAPC), was carried out using three different lipases from Candida antarctica (CAL), Candida rugosa (CRL) and porcine pancreas (PPL) in water-miscible (tetrahydrofuran) and water-immiscible (chloroform) solvents for 10 days. The degradation was monitored by gel permeation chromatography and Fourier transform infrared spectroscopy. Maximum degradation (ca. 60% reduction in Mn) of BPAPC was observed in THF with PPL when compared to the control without the enzyme. The degradation products in all the experiments were bisphenol-A and 4-α-cumyl phenol suggesting that the lipases act through an end-chain scission on the polymer. The degradation of BPAPC in THF was in the order of PPL > CAL > CRL, while in CHCl3 it was CRL > CAL > PPL. To understand this disparity, and to probe the mechanistic aspects of degradation, molecular dynamics investigations were performed on the lipases with model BPAPC in both the solvents. The results also suggested that catalytic triad (Ser, His, Asp/Glu) was involved in the hydrolysis of carbonate bond leading to release of bisphenol-A. These data provide us the basic understanding of the degradation mechanism and a novel methodology for degrading polycarbonate.