Eur. Phys. J. E (2019) 42: 53
https://doi.org/10.1140/epje/i2019-11816-9

Regular Article

Hydrogen isotope replacement changes hydration and large scale structure, but not small scale structure, of agarose hydrogel networks

¹ Faculty of Science and Technology, Materials and Life Sciences, Sophia University, 7-1 Kioicho, Chiyoda, 102-8554, Tokyo, Japan
² School of Natural Sciences and Health, Tallinn University, Narva mnt 29, 10120, Tallinn, Estonia
³ Glyn O. Phillips Hydrocolloids Research Centre, School of Food and Pharmaceutical Engineering, Faculty of Light Industry, Hubei University of Technology, 430068, Wuchang, Wuhan, China
⁴ Nanoscale Organisation and Dynamics Group, School of Science and Health, Western Sydney University, 2751, Penrith, NSW, Australia
⁵ Graduate School of Science and Technology, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-Ku, 108-8477, Tokyo, Japan

^* e-mail: physicalchemistrytom@gmail.com

Received: 27 November 2018
Accepted: 20 March 2019
Published online: 7 May 2019

Abstract

Agarose samples of low (Ag1) and high (Ag2) O -methyl content on position 6 of the galactose residue were studied in H₂O and D₂O. Differential scanning calorimetry, turbidity and rheological measurements showed a 2 ^° C shift in the coil-to-helix transition temperature, indicating higher helix stability in D₂O. The differential scanning calorimetry data could be superimposed using a temperature shift factor, suggesting similar extents of helix aggregation in both solvents. Small angle X-ray scattering of H₂O and D₂O gels were essentially identical, indicating no change in the small scale ( 0.05-20 nm) network structure on isotopic exchange. Larger ( 1 μm) scale heterogeneities were more pronounced in deuterium gels. The ¹HT₂ relaxation times were measured at different H/D ratios. These relaxation times were analyzed using a model assuming regular solution mixing of H₂O, HDO and D₂O between the solvent and gel phases. The fit results suggested that H₂O has higher affinity for the agarose network than HDO and D₂O. The difference, however, was much larger for the Ag2 sample. This finding implies that the higher hydrophobic effect observed in D₂O affects the hydration state much more strongly for the more hydrophobic (and more polarizable) agarose sample Ag2. As a consequence, Ag2 (but not Ag1) gels retained more H₂O than D₂O. In contrast, the bulk rheology of either hydrogel was not affected by the isotopic exchange.