Deuteration Network Members Involved: ESS, FZJ

Project Description: This aim of this collaborative project was to synthesise stereo-defined, deuterium-labelled samples of poly(lactic acid) (Figure 1: stereochemistry omitted).

Figure 1. The structure of poly(lactic acid) (undefined stereochemistry).

Poly(lactic acid) is an increasingly important polymer because it can be derived from renewable resources and is recyclable and biodegradable.

Figure 2. Cups made from poly(lactic acid).

Physical properties of poly(lactic acid) relating to biodegradability can be tuned by changing the stereochemistry of the monomer from which it is synthesised, lactide. Lactide exists as three enantiomers, (R,R), (S,S) and (R,S), and poly(lactic acid) can exist in several forms, including isotactic (from all (R,R)-lactide, or all (S,S)-lactide) and hetero- and syndiotactic (from a mixture of (R,R)- and (S,S)-lactide, or from all (R,S)-lactide).

Determining the stereochemistry of poly(lactic acid) can be challenging. The technique of labelling molecules with the stable isotope deuterium should allow us to analyse samples of poly(lactic acid) using neutron scattering techniques, as neutrons interact in a more useful way with deuterium atoms than the more abundant hydrogen atoms.

This project required the synthesis of enantiopure, deuterium-labelled D- (R) and L- (S)-lactic acid. The ESS DEULAB utilised lactate dehydrogenase enzymes to produce perdeuterated lactic acid from perdeuterated pyruvate (Scheme 1). Lactate dehydrogenase enzymes exist as L-lactate dehydrogenase and D-lactate dehydrogenase and these enzymes are highly enantiospecific; their ‘stereochemistry’ is transferred to the chiral products (ie D-lactate dehydrogenase produces D-lactic acid and L-lactate dehydrogenase produces D-lactic acid). These enzymes require a co-factor, NADH, which in this case was required to contain a single deuterium atom. In order to produce the deuterated cofactor, the ESS lab coupled a second enzymatic reaction: using formate dehydrogenase to produce deuterated NADH-d1 from commercially available sodium formate-d1. The substrate for the enzymes, pyruvate, was deuterated using standard chemical techniques.

The polymer chemists at FZJ then synthesised the stereo-defined, deuterium-labelled monomers of lactide, and completed the polymerisation reaction to produce stereo-defined, deuterium-labelled poly(lactic acid). The next step it to perform neutron experiments on these samples.