Saccharides being integral parts of some coenzymes and nucleic acids (DNA and RNA), constitute one of the most diverse and important classes of biomolecules in nature. Saccharides are important due to their hydroxyl (–OH) rich periphery, coordination ability and stereospecificity. Oligosaccharides found on the surface of cells as part of glycoproteins and glycolipids play key roles in the control of various normal and pathological processes in living organisms such as protein folding, cell-cell communication, bacterial adhesion, viral infection, masking of immunological epitopes, fertilization, neural development and cell proliferation and organization into specific tissues. A number of important polymers which occur in living cells are composed of glucose, hyaluronic acid and other saccharides [1-4].

In the pharmaceutical and biotechnological industries, sucrose and trehalose can be used as cryoprotectants against the destabilization and degradation of enzymes and drugs during lyophilization procedures. It has been shown that salts can be used to modulate the cryprotectant action of saccharides on biological macromolecules. This has been attributed to thermodynamic effects such as changes in saccharide hydration and changes in viscosity, diffusion and crystallization kinetics. Thus, experimental characterization of thermodynamic and transport properties of saccharide-salt aqueous mixtures is important not only for basic research, but also for applications in the biochemical and biotechnological fields [5-9].

The hydration characteristics of saccharides and their interactions with electrolytes/non-electrolytes in aqueous media are of significant biological and thermodynamic importance. The interaction between saccharides and electrolytes in aqueous solutions has been closely looked at since Angyal’s [10] pioneering studies. In spite that the conformational and configurational factors, which can affect the structures of saccharides in different solvents, are very complicated, these studies are useful for examining the characteristics of carbohydrates in body fluids. The study on the interactions of various saccharides/derivatives with ions in water is useful in rationalizing their hydration characteristics, and identifying the possible relationship between the thermodynamic parameters and the stereochemistry of solute molecules. The industrial and biological importance of phosphate systems has resulted in extensive investigations of physicochemical properties of aqueous solutions of phosphoric acid and phosphate salts.

The volumetric, viscometric and other thermodynamic data provide valuable information regarding solute-solvent/co-solute interactions. Calorimetry is a sensitive technique that gives useful thermodynamic data revealing various inter- and intra-molecular interactions. Compressibility, being a second derivative of free energy, is known to be sensitive to the nature of hydration and thus a very useful thermodynamic parameter for elucidating the behavior of a solute in solution [11-13].

The volumetric studies indicate that the hydrophilic-ionic type interactions between polar sites (–OH, –C=O, –O–) of the saccharide molecules and ions of the phosphate-based salts predominate over the hydrophobic-ionic interactions. The phosphate anions (H₂PO₄^–, PO₄^3–) have a great affinity for hydrogen bonding with the hydrophilic sites of the saccharide molecules. The ∆tV2° values are higher in tribasic (K₃PO₄) salt than monobasic (NH₄H₂PO₄, KH₂PO₄ and NaH₂PO₄) salts, indicating stronger interactions between the solutes and PO₄^3– than with H₂PO₄^–. This suggests a higher kosmotropicity of K₃PO₄ than KH₂PO₄, NaH₂PO₄ and NH₄H₂PO₄ phosphate salts. This effect is attributable not only to the salts anion charge but also to the changes in pH and ionic strength. The comparison of ∆_tV₂° values for saccharides   in   various   phosphate-based   salts  generally follows the order: K₃PO₄>NH₄H₂PO₄>KH₂PO₄>NaH₂PO₄ [14,15]. Based on the criterion used by Hepler [16]; (¶C°_P,2/¶P)_T= –T(¶²V₂°/¶T²)_P, positive (¶²V₂°/¶T²)_P values for the saccharides indicate their kosmotropicity.

The comparison of volumetric results for saccharides in phosphate-based salts with other electrolytes generally follows the order: C₆H₁₁O₇⁻3COO^–2PO₄^––4^3–4^2–4O₇^2–. This trend shows the strong influence of (1:2/1:3) salts in comparison to (1:1) salts, which is in accordance with the Hofmeister series.

The viscosity B-coefficients give information about the kosmotropic or chaotropic ability of solutes. The B-coefficients of the saccharides are positive and increase in the presence of salts indicating a kosmotropic nature of the saccharides in the phosphate salts. The B-coefficients also increase with the complexity of solutes from mono- to di- to tri-saccharides, suggesting a structural enhancement in the same order. The temperature dependence of the B-coefficients gives better information about the kosmo-/chaotropic effects of salts. The negative dB/dT coefficients for the saccharides in all the phosphate salts, suggest that saccharides and their derivatives act as ‘kosmotropes’ in phosphate salts [17].

Calorimetric studies of saccharides in aqueous phosphate-based salts reveal that the process of titration is exothermic and the exothermicity decreases with increase in concentration of the saccharide as well as with increase of temperature except for few cases. The limiting enthalpies of dilution (Δ_dilH°) of the saccharides are negative. The interactions between the hydrophilic (‒OH, ‒C=O, ‒O‒) sites of the saccharide and ions (M+/H₂PO₄^‒, PO₄^3–) of the cosolute contribute exothermicity to ΔdilH° while the interactions between the hydrophobic (R=CH, CH₂, CH₃) alkyl groups of the saccharide and ions of cosolute make ΔdilH° more positive (endothermicity). The observed negative Δ_dilH° values suggest that hydrophilic-ionic interactions between saccharide/derivative and co-solutes predominate over hydrophobic-ionic interactions and are responsible for the overall exothermicity. The exothermicity is greater in K₃PO₄ than monobasic salts solutions due to stronger interactions of PO₄^3– ions with saccharides. The calorimetric results also suggest that saccharides act as kosmotropes in aqueous phosphate-based inorganic salts. The hydration numbers determined from compressibility studies are positive which indicates that phosphate salts exhibit dehydration effects. K₃PO₄ has a stronger dehydration effect than the monobasic salts [18,19].

CONCLUSION

The study of the physicochemical properties, i.e., volumetric, viscometric, calorimetric and isentropic compressibility generally suggests that saccharides and their derivatives act as kosmotropes in aqueous phosphate-based salt solutions. Also, there is strong effect of highly basic and charged PO₄^3– anion in potassium phosphate tribasic salt than that of H₂PO₄^– anion in monobasic salts and these follow the order: K₃PO₄>NH₄H₂PO₄>KH₂PO₄>NaH₂PO₄.

ACKNOWLEDGEMENT

Author is thankful to Dr. Banipal PK and Dr. Banipal TS, Department of Chemistry, Guru Nanak Dev University, Amritsar for immense support and guidance in carrying out the research work.

1.       Cardoso MVC, Carvalho LVC, Sabadini E (2012) Solubility of carbohydrates in heavy water. Carbohydr Res 353: 57-61.

2.       Parfenyuk EV, Davydova OI, Lebedeva NS (2004) Interactions of D-maltose and sucrose with some amino acids in aqueous solutions. J Sol Chem 33: 1-10.

3.       Annunziata O, Miller DG, Albright JG (2010) Quarternary diffusion coefficients for the sucrose-NaCl-KCl-water system at 25°C. J Mol Liq 156: 33-37.

4.       Liu H, Chen Y, Zhang H, Lin R (2007) Studies on the interactions of L-Valine and L-Threonine with saccharides in aqueous solutions at 298.15 K. J Chem Eng Data 52: 609-612.

5.       Rudolph WW (2012) Raman spectroscopic measurements of the first dissociation constant of aqueous phosphoric acid solution from 5-301°C. J Sol Chem 41: 630-645.

6.       Goldberg RN, Schliesser J, Mittal A, Decker SR, Santos AFLOM, et al. (2015) A thermodynamic investigation of the cellulose allomorphs: Cellulose (am), Cellulose Iβ (cr), Cellulose II (cr) and Cellulose III (cr). J Chem Thermodyn 81: 184-226.

7.       Vaňková K, Gramblička M, Polakovič M (2010) Single-component and binary adsorption equilibrium of fructo-oligosaccharides, glucose, fructose and sucrose on a Ca-form cation exchanger. J Chem Eng Data 55: 405-410.

8.       Zafarani-Moattar MT, Sarmad S (2012) Apparent molar volumes, apparent isentropic compressibilities and viscosity B-coefficients of 1-ethyl-3-methylimidazolium bromide in aqueous di-potassium hydrogen phosphate and potassium di-hydrogen phosphate solutions at T=(298.15, 303.15, 308.15, 313.15 and 318.15) K. J Chem Thermodyn 54: 192-203.

9.       Ryshetti S, Gupta A, Tangeda SJ, Gardas RL (2014) Acoustic and volumetric properties of betaine hydrochloride drug in aqueous D(+)-glucose and sucrose solutions. J Chem Thermodyn 77: 123-130.

10.    Angyal SJ (1984) The composition of reducing sugars in solution. Adv Carbohydr Chem. Biochem 42: 15-68.

11.    Banipal PK, Singh V, Aggarwal N, Banipal TS (2015) Hydration behavior of some mono-, di-, tri-saccharides in aqueous sodium gluconate solutions at (288.15, 298.15, 308.15 and 318.15) K. Food Chem 168: 142-150.

12.    Banipal PK, Hundal AK, Aggarwal N, Banipal TS (2014) Studies on the interactions of saccharides and methyl glycosides with lithium chloride in aqueous solutions at (288.15 to 318.15) K. J Chem Eng Data 59: 2437-2455.

13.    Banipal PK, Sharma M, Aggarwal N, Banipal TS (2016) Investigations to explore interactions in (polyhydroxy solute+L-ascorbic acid+H₂O) solutions at different temperatures: Calorimetric and viscometric approach. J Chem Thermodyn 102: 322-332.

14.    Banipal PK, Aggarwal N, Banipal TS (2014) Study on interactions of saccharides and their derivatives with potassium phosphate monobasic (1:1 electrolyte) in aqueous solutions at different temperatures. J Mol Liq 196: 291-299.

15.    Banipal PK, Aggarwal N, Banipal TS (2015) Effects of phosphate-based monobasic and tribasic inorganic salts on hydration characteristics of saccharides and their derivatives. J Mol Liq 211: 78-89.

16.    Hepler LG (1969) Thermal expansion and structure in water and aqueous solutions. Can J Chem 47: 4613-4617.

17.    Banipal PK, Aggarwal N, Banipal TS (2016) Viscosities of some saccharides in aqueous solutions of phosphate-based inorganic salts. J Chem Eng Data 61: 1992-2001.

18.    Banipal PK, Aggarwal N, Banipal TS (2016) Calorimetric and isentropic compressibility studies on (saccharide/methyl glycoside/deoxy-derivative+KH₂PO₄+H₂O) solutions at different temperatures. J Mol Liq 215: 603-611.

19.    Aggarwal N, Sharma M, Banipal TS, Banipal PK (2019) Influence of phosphate based salts on enthalpy of dilution and isentropic compressibility properties of saccharides/derivatives in aqueous solutions. J Chem Eng Data 64: 517-528.