The N-acetylneuraminic acid (NANA) molecule plays an important role in intercellular interactions and in cell interactions with drugs. Therefore, when developing drugs where ultrafine silica (UFS) is the carrier, it is important to fix NANA on its surface. But this sialic acid is not adsorbed on the hydroxylated of UFS surface from aqueous solution. Thus it is important to find such UFS surface modifiers which, on the one hand, should ensure the fixation of NANA on the UFS surface, and on the other be biocompatible with the plasma membrane of the cell. Such requirements are met by saccharides but as it is known from literature, monosaccharides are not adsorbed on the surface of UFS, while the adsorption of disaccharides occurs from aqueous solution.
Therefore, the first part of the work demonstrates the modification of the UFS surface by adsorption of sucrose and lactose from aqueous solution. It was established that the filling of the surface with adsorbate molecules goes with their placement of molecules in a chain variant. The obtained adsorption isotherm, according to Giles' classification, belongs to class S2.
The results of temperature-programmable desorption mass spectrometry found that during thermal decomposition of sucrose samples in both condensed and adsorption states on the UFS surface, the corresponding spectra contain lines inherent in both states (m / z 17, 28, 44, 97). This demonstrates the adsorption of carbohydrate on the surface of silica. A similar conclusion holds for lactose.
It was found that the thermal decomposition of NANA samples in the condensed state and the state adsorbed on the modified UFS surface has three stages. The first of them begins at 90, the second - at 230, and the third - 350 С.
The calculation results in the approximation of DFT (B3LYP, 6-31 G (d, p) energy of dimerization and formation of adsorption complexes with silica monosaccharides (glucose and fructose) and NANA for gas phase conditions indicate the interaction of substances in a mixture of NANA with monosaccharides compared to interaction of individual substances.
Molecules of disaccharides, N-acetylneuraminic acid and silica form triple complexes due to intermolecular hydrogen bonds between polar functional (mainly -OH groups) molecules of the studied substances. Sucrose is better adsorbed on the surface of silica (∆Eads = –211.7 kJ/mol) compared to the adsorption of lactose ( 154.9 kJ/mol). The most energy is released (245.2 kJ/mol) when interacting with a silica cluster of the intermolecular complex of N-acetylneuraminic acid and sucrose where silica and the sucrose molecule are in direct contact.
The quantum chemical calculations of the equilibrium spatial structure and energy of formation of disaccharides adsorption complexes with the influence of solvent in supermolecular and continuum approximations taken into account, showed that their adsorption is thermodynamically possible and does not depend on the size of the hydrating cluster. Thus Eads for sucrose and lactose is −33.0 and −24.5 kJ/mol, respectively. The adsorption energy from the aqueous phase for glucose and fructose has positive values (+9.8 and + 2.7 kJ/mol). For N-acetylneuraminic acid molecule, taking into account the effect of the solvent for Eads is −1.3 and 0.9 kJ/mol for the reaction with five and eight molecules of water, respectively.
It was found that in the aqueous solution in the presence of sucrose on the surface of silica, the hydration energy decreases (i.e. it is easier to replace the N acetylneuraminic acid molecule with a cluster of water from the surface of the modified adsorbent) thereby contributing to NANA adsorption on the silica surface. I.e. here we can see the mutual influence of substances in the mixture of NANA with carbohydrates on the interaction with silica in comparison with the interaction of substances with silica separately.
Key words: N-acetylneuraminic acid, monosaccharides, disaccharides, silica surface, adsorption, TPD MS, water cluster, method of density functional theory
