Low-temperature NMR characterization of reaction of sodium pyruvate with hydrogen peroxide

Abstract

It was proposed that the reaction of sodium pyruvate and H2O2 generates the intermediate 2-hydroperoxy-2-hydroxypropanoate, which converts into acetate, CO2, and H2O ( Aleksankin et al. Kernenergie 1962 , 5 , 362 - 365 ). These conclusions were based on the products generated in (18)O-enriched water and H2O2 reacting with pyruvic acid at room temperature; however, the lifetime of 2-hydroperoxy-2-hydroxypropanoate at room temperature is too short for direct spectroscopic observation. Therefore, we applied the combination of low-temperature and (13)C NMR techniques to verify, for the first time, the formation of 2-deuteroperoxy-2-deuteroxypropanoate in mixtures of D2O and methanol-d4 and to monitor directly each species involved in the reaction between D2O2 and (13)C-enriched pyruvate. Our NMR results confirm the formation of 2-deuteroperoxy-2-deuteroxypropanoate, where the respective chemical shifts are supported by density functional theory (DFT) calculations. At near-neutral apparent pD (pD*) and -35 °C, the formation of 2-deuteroperoxy-2-deuteroxypropanoate occurred with k = 2.43 × 10(-3) dm(3)·mol(-1)·s(-1). The subsequent decomposition of 2-deuteroperoxy-2-deuteroxypropanoate into acetate, CO2, and D2O occurred with k = 2.58 × 10(-4) s(-1) at -35 °C. In order to provide a full kinetic analysis, we also monitored the equilibrium of pyruvate and methanol with the hemiacetal (2-deuteroxy-2-methoxypropanoate). The kinetics for the reaction of sodium pyruvate and D2O2 were fitted by taking into account all these equilibria and species.

Figure 1

HMBC data of a 1:1 mixture (0.015 dm³·mol⁻¹) of ¹³C1 and ¹³C2 enriched sodium pyruvate in 99.8% D₂O/0.2% H₂O at near-neutral pD*. The spectrum was recorded at room temperature; frequency 1 (f1), carbon chemical shift in ppm; frequency 2 (f2), hydrogen chemical shift.

Figure 2

HMBC spectrum of a 1:1 mixture (0.015 dm³·mol⁻¹) of ¹³C1- and ¹³C2-enriched sodium pyruvate in S₀ at near-neutral pD*. The spectrum was recorded at room temperature; frequency 1 (f1), carbon chemical shift in ppm; frequency 2 (f2), hydrogen chemical shift.

Figure 3

HMBC spectrum of a 1:1 mixture of ¹³C1 and ¹³C2-enriched sodium pyruvate (0.015 dm³·mol⁻¹) in S₀ ca. 30 min after the addition of 0.15 dm³·mol⁻¹ hydrogen peroxide at near-neutral pH and −35 °C. Frequency 1 (f1), carbon chemical shift in ppm; frequency 2 (f2), hydrogen chemical shift.

Figure 4

HMBC spectrum of a 1:1 mixture of ¹³C1- and ¹³C2-enriched sodium pyruvate (0.015 dm³·mol⁻¹) in S₀ at pD* <2. The spectrum was recorded at room temperature; frequency 1 (f1), carbon chemical shift in ppm; frequency 2 (f2), hydrogen chemical shift.

Figure 5

Representative 1-D ¹³C NMR spectrum of ¹³C2-enriched pyruvate in S₀ after 34 min of reaction with hydrogen peroxide at −33.6 °C (pD* = 6.8).

Figure 6

Signal intensities vs time during reaction of ¹³C1-enriched pyruvate with D₂O₂ at −34.85 °C. Pyruvate (●, blue); CO₂ (■, green); hemiacetal (▲, orange); intermediate ( ×, red).

Figure 7

Signal intensities vs time during the reaction of ¹³C1-enriched pyruvate with D₂O₂ at −13.94 °C. Pyruvate (●, blue); CO₂ (■, green); hemiacetal (▲, orange); intermediate ( ×, red).

Figure 8

Eyring plot of dependence of k₁ (▲, blue) and k₃ (●, red) on temperature. Linear regression was performed and k values were rejected that led to a p-value above 0.05. For k₁, values at temperatures −19.18, −25.52, and −33.64 °C were rejected, because the p-value exceeded 0.05. For k₃, no values were rejected.

Scheme 1

Reaction of Pyruvate and D₂O₂ Forming an Intermediate and Breakdown to Products

Scheme 2

Reaction Pathway on Which Calculation of Kinetic Data Is Based

Chart 1

Chemical Structures with Carbon Position Numbers