Site-Specific Mineralization of a Polyester Hydrolysis Product in Natural Soil

Poly(4-methylcaprolactone) (P4MCL) has been successfully incorporated into mechanically competitive materials with potential for biodegradability in engineered and natural systems. The mineralization of the hydrolysis product of P4MCL, 6-hydroxy-4-methylhexanoic acid (4MHA), was herein investigated by synthesizing tailor-made molecules with 13C labels in the carboxylic acid group (4MHA-13COOH) or the methyl group (4MHA-13CH3) and incubating each separately in a soil. Isotope-sensitive cavity ringdown spectroscopy on the efflux gas was then used to quantitatively monitor the mineralization of each isotopomer. These experiments clearly demonstrated that 4MHA was assimilated and utilized by the soil microorganisms and provided insight into position-specific mineralization. The 13CO2 evolution rate profiles and overall extents of mineralization to 13CO2 (∼85% and ∼46% for carboxyl- and methyl-labeled carbons, respectively) are consistent with the methyl carbon being preferentially incorporated into biomass rather than respired, whereas the carboxyl carbon is preferentially used for energy production and thus mineralized more rapidly (presumably by decarboxylation). These findings agree with previous reports regarding variations in the extents of mineralization of carbon atoms in different oxidation states. Moreover, this work demonstrates the value of systematically probing biodegradation of polymer hydrolysis products by the precise design of 13C-labeled molecules.