The Enzymatic Synthesis of Perdeuterated D- and L-Lactic Acid-d(4) and Polymerization of Their Lactides to Polylactic Acid.

We report the synthesis of highly enantiopure perdeuterated poly-L-lactic acid and poly-D-lactic acid polymers with well-defined molecular weight by polymerization of perdeuterated lactides. Enantiopure D- and L-lactic acid-d(4) monomers were synthesized from sodium pyruvate-d(3) using D- and L-lactate dehydrogenase enzymes (D-LDH and L-LDH) as biocatalysts. The reduced form of the co-enzyme nicotinamide adenine dinucleotide-d(1) (NADH-d(1)) was generated in situ from the oxidized form nicotinamide adenine dinucleotide (NAD(+)) by formate dehydrogenase (FDH)-catalyzed oxidation of sodium formate-d(1) to carbon dioxide with concerted reduction of NAD(+) to NADH-d(1). For the conversion of the perdeuterated lactic acid monomers to the corresponding lactide dimers, we developed a process for generating these compounds in the high purity needed for the final anionic ring-opening polymerization step. This method enabled the generation of a range of perdeuterated polylactic acid polymers that are highly suitable for the characterization of polymer structure and dynamics using neutron scattering, infrared and nuclear magnetic resonance spectroscopy methods that are sensitive to deuterium. Furthermore, these deuterium-labeled polymers are well-suited to the study of the biodegradation of PLA-based plastics.