2994

CEST effect of dimethyl sulfoxide (DMSO) at negative offset frequency
Haoyun Su1,2, Jianpan Huang1, and Kannie W.Y. Chan1,2,3,4,5
1Department of Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong, 2Hong Kong Centre for Cerebro-Cardiovascular Health Engineering (COCHE), New Territories, Hong Kong, 3Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, United States, 4City University of Hong Kong Shenzhen Research Institute, Shenzhen, China, 5Tung Biomedical Sciences Centre, City University of Hong Kong, Kowloon, Hong Kong

Synopsis

Keywords: CEST & MT, Molecular Imaging

CEST MRI can detect mM range of endogenous and exogenous molecules, and bound small molecules such as glycogen and lactate via rNOE. Here we reported for the first time that DMSO and its structural analogs in aqueous solution had distinctive CEST peaks at the negative offsets of Z-spectrum. CEST effect of DMSO was dependent on saturation power and concentration, and less sensitive to tested temperature. Alcohols, acetone, acetonitrile, acetic acid and N,N-dimethylformamide also showed observable CEST peaks at the negative offsets. Interaction between DMSO and water molecule, such as hydrogen bonding, could contribute to the observed CEST effect.

Introduction

Dimethyl sulfoxide (DMSO) is a common organic solvent, pharmacological agent and cryoprotectant. Understanding its interaction with biomolecules in aqueous environment is essential for the DMSO usage in biomedical applications. We reported for the first time that DMSO in aqueous solution was detectable with Chemical Exchange Saturation Transfer Magnetic Resonance Imaging (CEST MRI) at around -2 ppm. The positive offset frequencies have been widely studies, including amide proton transfer (APT) at 3.5 ppm 1, creatine at 1.8 ppm 2, and glucose at 1-3 ppm 3. For the negative side of Z-spectrum, relayed nuclear Overhauser effect (rNOE) where non-exchangeable aliphatic protons transfer magnetization to neighboring dipolar-coupled protons which then exchange with water, is known to indicate macromolecules such as glycogen 4 and lipid contents 5, and bound small molecules such as lactate 6 and caffeine7. It is a new finding to observe the CEST effect of DMSO at a negative offset, which could be attributed to the intermolecular hydrogen bonding between DMSO and water8,9. We performed CEST experiments on DMSO and structurally similar polar organic solvents to investigate the origin of the observed CEST peak.

Materials and methods

ACS grade DMSO, N,N-dimethylformamide (DMF) and acetic acid were purchased from Sigma-Aldrich. ACS grade acetone, methanol, and acetonitrile were purchased from Anaqua. Ethanol and isopropyl alcohol were purchased from UNI-Chem and Oriental Chemicals and Lab Supplies.
Phantom solutions were prepared by mixing DMSO or structural analogs in deionized water to final concentrations of 2, 5, 10, 20 v/v%. Samples then were transferred to 0.5 mL tubes for image acquisition. Unless otherwise specified, CEST image acquisition was performed at 37 °C using a horizontal bore 3T preclinical scanner (Bruker, Ettlingen, Germany) with continuous-wave pulse, RARE factor = 32, TR = 9000 ms, TE = 75 ms, FOV = 35x35 mm2, saturation duration (Tsat) = 4 s, and image size = 64x64. Z-spectra were acquired at B1 amplitudes of 0.4, 0.8, 1.2, 1.5, and 2.0 μT, with a saturation offset range of -10 to 10 ppm. Offset step size of 0.1 ppm was used between -2 and 2 ppm and 0.2 ppm step size for rest of the offset ranges. Z-spectrum was processed with direct water saturation removal by Lorentzian fitting, using our customized Matlab (Mathworks, Massachusetts, USA) codes. Exchange rate (kex) was calculated with quantification of exchange rate using varying saturation power (QUESP) experiment10,11.

Results

DMSO solutions at various volume ratios were imaged. 10 v/v% DMSO showed a distinctive dip at around -2 ppm away from water with ~6% CEST contrast (Fig. 1A). The maximum CEST contrast of ~13.5% was observed at 20 v/v% with B1=1.2 μT (Fig. 1B). It was dependent on B1 amplitude and DMSO concentration, and was less dependent on the tested temperature (25-44 °C) (Fig 1C-E). QUESP fit for 10 v/v% DMSO at 37 °C was plotted in Fig. 1F, with estimated kex around 60 Hz. Structural analogs of DMSO, such as alcohols, acetone, acetic acid, and N,N-dimethylformamide, also showed noticeable CEST peaks at negative frequency offsets, with B1 and concentration dependencies (Fig. 2 and Fig. 3). CEST contrast were observed at -1.4 ppm for methanol, at -1.1 ppm and -3.6 ppm for ethanol, and -3.6 ppm for isopropanol (Fig. 2). Moreover, DMF, acetone, acetonitrile and acetic acid all generated CEST contrast at a range of -1.8 ppm to -2.8 ppm (Fig 3).

Discussion

Aliphatic protons in macromolecules can transfer saturation to nearby exchangeable protons (such as those of hydroxyl groups or bound water) both intermolecularly and intramolecularly through NOE and generate CEST contrast at negative offsets5,12,13. From our phantom experiment results, CEST peak of DMSO could have an origin similar to rNOE signals in the presence of methyl groups, leading to the observed CEST contrast ~ 6.5% at around -2 ppm for 10 v/v% DMSO (Fig 1). Spectroscopy and molecular dynamic studies indicate that hydrogen bonding between DMSO and water at low concentration could alter nearby water structure8,9. To further illustrate the concept, structural analogs of DMSO were selected since they all have methyl group and electronegative group. Polar protic solvents such as methanol, and polar aprotic solvents such acetone and acetonitrile are also known to have the capability to interact with neighboring water9. These solvents generated CEST contrast at a range of -1.1 ppm to -3.6 ppm, and also showed B1 and concentration dependencies (Fig. 2, Fig. 3). The intermolecular hydrogen bonding between these polar solvents and water might be one source of the detected peaks at negative offsets. For alcohols and acetic acid, CEST effects at negative side were also observed although having exchangeable hydroxyl groups in their structures. Further experiments for investigating the mechanism behind the observed peaks are underway.

Conclusion

DMSO its analogs generated CEST contrast at the negative offsets, which is conventionally known as NOE. Here, we demonstrated that in additional to aliphatic protons from macromolecules, small, organic polar molecules solvated in water at measured volume ratios also generated distinctive CEST signals at negative offsets. This could be due to the hydrogen bonding between these molecules and water. We are now examining the exchange properties and related contrast mechanisms of these common solvents.

Acknowledgements

Authors would like to acknowledge the funding supports from Research Grants Council (11102218, PDFS2122-1S01, 11200422, RFS2223-1S02, C1134-20G); City University of Hong Kong (7005433, 7005626, 9239070, 9609307, 9610560); National Natural Science Foundation China (81871409); Tung Biomedical Sciences Centre; Hong Kong Centre for Cerebro-cardiovascular Health Engineering

References

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[7] Yadav, Nirbhay N., Xing Yang, Yuguo Li, Wenbo Li, Guanshu Liu, and Peter van Zijl. "Detection of dynamic substrate binding using MRI." Scientific reports 7, no. 1 (2017): 1-7.

[8] Yang, Bo, Xianwen Cao, Chong Wang, Shenghan Wang, and Chenglin Sun. "Investigation of hydrogen bonding in Water/DMSO binary mixtures by Raman spectroscopy." Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 228 (2020): 117704.

[9] Vaisman, Iosif I., and Max L. Berkowitz. "Local structural order and molecular associations in water-DMSO mixtures. Molecular dynamics study." Journal of the American Chemical Society 114, no. 20 (1992): 7889-7896.

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Figures

Figure 1. CEST properties of DMSO. DMSO in water at 10 v/v% generated CEST signal at around -2 ppm and was dependent on power (A, D) and concentration (B, E). Lorentzian fitted spectra were plotted at the bottom of the Z-spectra. (C) showed signal variation with temperature from 25 to 44°C. Estimated kex for 10 v/v% DMSO at 37°C was around 60 Hz from QUESP (F)

Figure 2. CEST properties of alcohols. Methanol (A, D), ethanol (B, E), and isopropanol (C, F) respectively in water at 10% volume ratio had distinctive peaks at negative offset frequencies. Power and concentration dependencies were plotted in (G, H).

Figure 3. CEST properties of acetonitrile, acetone, acetic acid, and N,N-dimethylformamide (DMF). Acetonitrile, acetone, acetic acid and DMF had distinctive CEST peaks at negative offsets (A). Power and concentration dependency of their CEST signals were shown in (B, C).

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)
2994
DOI: https://doi.org/10.58530/2023/2994