DFT study of the adsorption of microsolvated glycine on a hydrophilic amorphous silica surface

Abstract

Density functional theory (DFT) periodic ab initio molecular dynamics calculations are used to study the adsorption of gaseous and microsolvated glycine on a hydroxylated, hydrophilic silica surface. The silica model is presented and the interaction of water with surface silanols is studied. The heat of interaction of water is higher with the associated silanols (be they terminal or geminal ones) studied here than with isolated silanols presented in past works. Glycine is stabilized in a parallel mode on the hydroxylated surface. Terminal silanols do not allow the stabilization of the zwitterionic form, whereas geminal silanols do. Molecular dynamics (MD) first-principle calculations show that microsolvated zwitterion glycine directly binds through the carboxylate function to a surface silanol rather than through water molecules. The adsorption mode, whether with or without additional water molecules, is parallel to the surface. The ammonium function does not interact directly with the silanol groups but rather through water molecules. Thus, the carboxylate and ammonium functions exhibit two different reactivities towards silanols. The calculated free energies, taking into account the chemical potentials of water and glycine in the gas phase, suggest the existence of a thermodynamic domain in which the glycine is present in the gas phase as well as strongly adsorbed on specific sites of the surface.

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…Drawn with data from Ref. 641 In conclusion, silica-water interface represents a complex system to be studied because of the subtle interplay between complementary/competing forces (hydroxyl-hydroxyl network vs water-hydroxyls interactions) on one hand, and intrincate H-bond patterns occurring at the amorphous silica surfaces, on the other hand. One clear point is that Q 2 and Q 3 develop the same attitude to H-bond with adsorbed water molecules and can be considered to exhibit the same Brönsted acidity.…”

“…For this case, two H-bonds present on the pristine surface were destroyed in order to form the four H-bonds that bind Gly onto the surface. In a more recent work, Costa et al 641 computed the interaction of Gly on a highly hydroxylated amorphous silica surface (7.66 OH per nm 2 ) particularly dominated by geminal SiOH groups (5.1 per nm 2 , with a ratio of about 66% of the total surface SiOH). Calculations were based on ab initio molecular dynamics simulations carried out at T= 300K using the PBE density functional method.…”

“…As mentioned, these theoretical results only addressed the interaction of Gly with isolated SiOH groups and not with geminal ones, which due to their more acidic behavior are expected to be different. Because of that, Costa et al 641 and Zhao et al 323 studied the adsorption of Gly on silica surfaces rich in geminal silanol groups in the presence of water, both works using first-principles molecular dynamics simulations. In the work of Costa et al 641 the adsorption was simulated considering a microsolvated environment constituted by 5 water molecules (enough to ensure the stabilization of Gly-zwitt over Gly-can).…”

“…The dominant configuration obtained from several ab initio molecular dynamics simulations (PBE method and T= 300K) for Gly interacting with a silica surface model rich in geminal silanol groups (drawn with data from Ref. 641 ). The computed adsorption energy (E ads , in kJ mol -1 ) was obtained by performing a geometry optimization onto this configuration at the B3LYP/6-311++G(d,p) level.…”

“…510 A more favorable energy of adsorption is calculated for water adsorption on silanol nests: a study of discrete water molecules adsorption on amorphous silica was performed by Costa et al 641 The amorphous silica surface model is inspired on the model of Masini and Bernasconi. 507 This surface presents a high silanol density of 7.66 OH per nm 2 , partitioned in 66% geminal silanols, and 34% terminal-associated ones.…”