Physical and Chemical Properties

Dimethyl sulfoxide-d6 is a colourless, low-odour liquid, as shown in Figure 1. Its molecular formula is (CD?)?SO, obtained by replacing all six hydrogen atoms in dimethyl sulphoxide with deuterium (2H). This isotopic modification confers unique physicochemical properties on dimethyl sulfoxide-d6, which has a molecular weight of 84.17 g/mol. Its boiling point is as high as 189 °C, which is one of the key advantages of dimethyl sulfoxide-d6, as it offers a wide temperature range of applicability. However, its melting point is close to room temperature, meaning that the solvent may solidify at temperatures slightly below room temperature and must be gently heated prior to use. Furthermore,dimethyl sulfoxide-d6 is hygroscopic and readily absorbs moisture from the air; consequently, it requires specific storage and handling conditions.

Figure 1. Dimethyl sulfoxide-d6

Synthesis Routes

The synthesis of dimethyl sulfoxide-d6 is primarily achieved through a hydrogen-deuterium exchange reaction. A typical method involves heating dimethyl sulphoxide in heavy water (D?O) and carrying out the reaction in the presence of a basic catalyst, such as calcium oxide. The reaction is essentially a hydrogen-deuterium isotope exchange reaction catalysed by a base; the core reaction is represented by the equation (CH?)?SO + 6D?O ? (CD?)?SO + 6HDO. As this is a reversible equilibrium reaction, to drive the equilibrium towards the fully deuterated product, the water produced must be repeatedly removed whilst simultaneously replenishing the deuterium-labelled water.

?Another method involves carrying out a nucleophilic substitution reaction using deuterated iodomethane. This must also be conducted under anhydrous conditions and under the protection of an inert atmosphere, such as nitrogen or argon, to prevent interference from moisture and to ensure the isotopic purity of the product.

Applications

The primary application of dimethyl sulfoxide-d6 is as a solvent for nuclear magnetic resonance (NMR) spectroscopy. In 1H NMR, because the resonance frequency of the deuterium nucleus (2H) differs significantly from that of the proton (1H), signals from the deuterated solvent are virtually undetectable; this prevents the sample signals from being overwhelmed by the abundant proton signals present in ordinary solvents ^[1]. Furthermore, the deuterium signal in the solvent can be utilised for the spectrometer’s ‘field-locking’ operation, enabling real-time monitoring and compensation for magnetic field drift, thereby ensuring the stability and resolution of the spectrum.

Furthermore, as this substance is compatible with both aqueous biochemical buffers and protein-based assay methods, as well as with many hydrophobic small-molecule drug candidates, a single compound stock solution can be used across a variety of analytical platforms, including biochemical assays, surface plasmon resonance (SPR) and nuclear magnetic resonance (NMR) spectroscopy targeting ligands or proteins, This reduces inter-batch variability and ensures consistency in the data generated across different analytical modalities.

Dimethyl sulfoxide-d6 also facilitates the detection of reactive hydrogen atoms, such as hydroxyl (-OH), amine (-NH) and carboxyl (-COOH) protons. In protic deuterated solvents (such as methanol-d? or heavy water), these active protons undergo rapid chemical exchange with the solvent, leading to signal broadening or even disappearance ^[2]. The proton chemical shifts in dimethyl sulfoxide-d6 can be used to estimate the dissociation constants (pKa′s) of oximes ^[3]. Furthermore, this compound can be used to study the binding modes of mercury(II) with nucleosides and thionucleosides.

Dimethyl sulfoxide-d6 possesses strong solubilising properties; it can be formulated with glycerol or glycerol-d8 to create a high-viscosity solvent system capable of dissolving complex bioactive compounds ^[4]. For example, it can dissolve 1-allyloxy-2-hydroxypropyl starch (AHP-starch), thereby enabling the acquisition of its high-resolution 1H NMR spectrum. Furthermore, when used in combination with pyridine-d5, it enables two-dimensional NMR analysis of the solution state of ball-milled plant cell wall gels in plant cell wall research ^[5].

The applications of dimethyl sulfoxide-d6 extend beyond traditional NMR analysis and now encompass industrial processing and pharmaceutical synthesis.For example, in drug development, it can be used for solvent extraction during the manufacture of active pharmaceutical ingredients (APIs); in biotechnology, it can be utilised for protein characterisation and drug discovery. Furthermore, methods combining 1H quantitative NMR (qNMR) with dimethyl sulfoxide-d6 can, for instance, be used to simultaneously determine the content of ferulic acid, ligustrene and pinobanksic acid in Angelica sinensis ^[6].

Analysis

The quality of dimethyl sulfoxide-d6 is primarily controlled by three parameters: deuteration purity, chemical purity and water content. Deuteration purity determines the level of residual solvent background signal and is directly related to the quality of the NMR spectrum; water content affects the observation of reactive hydrogen atoms and experiments sensitive to moisture. Both Fourier transform infrared spectroscopy and NMR can be used to assess the purity of dimethyl sulfoxide-d6.

References

[1]Esturau N , Espinosa JF.Optimization of diffusion-filtered NMR experiments for selective suppression of residual nondeuterated solvent and water signals from 1H NMR spectra of organic compounds[J].Journal of Organic Chemistry, 2006, 71(11):4103-4110.doi: 10.1021/jo060229i.

[2]Nerantzaki A A, Tsiafoulis C G, Charisiadis P, et al. Novel determination of the total phenolic content in crude plant extracts by the use of 1H NMR of the –OH spectral region[J]. Analytica Chimica Acta, 2011, 688(1): 54-60.https://doi.org/10.1016/j.aca.2010.12.027.

[3]Kurtz AP, D'Silva TD. Estimation of dissociation constants (pKa's) of oximes from proton chemical shifts in dimethyl sulfoxide solution[J]. Journal of Pharmaceutical Sciences,1987, 76(8): 599-610. DOI: 10.1002/jps.2600760805.

[4] Lameiras P , Nuzillard J M .Highly Viscous Binary Solvents: DMSO-d6/Glycerol and DMSO-d6/Glycerol-d8 for Polar and Apolar Mixture Analysis by NMR[J].Analytical Chemistry, 2016:4508-4515.DOI:10.1021/acs.analchem.6b00481.

[5] Kim H , Ralph J .Solution-state 2D NMR of ball-milled plant cell wall gels in DMSO-d6/pyridine-d5[J].Bioenergy Research, 2010, 8(3):576-591.DOI:10.1039/b916070a.

[6]Qiu D, Li Q, Feng Y, et al. Multi-Index Quantitative Evaluation of Angelicae sinensis Radix Based on 1H-qNMR[J]. Journal of AOAC International, 2020, 103(6): 1633-1638. DOI: 10.1093/jaoacint/qsaa054.