Computer modeling of adsorption on an activated charcoal surface

The molecular modeling program SYBYL was used to simulate the adsorption of various barbiturates by an activated charcoal surface. The compounds barbituric acid (BA), barbital (B), phenobarbital (PB), mephobarbital (M), and primidone (Pr) were modeled, and their structures were energetically minimized. These structures agreed with literature reports for the conformations of these molecules in dimethyl sulfoxide-d6, methanol-d4, and chloroform-d. The activated charcoal surface was modeled using graphitic crystallites which had either no oxygen-containing functional group, a C-OH functional group, or a C = O functional group. The presence of the C-O (presumably C-OH) and C = O functional states on activated charcoal surfaces had been previously determined by X-ray photoelectron spectroscopy. It was assumed that the crystallite was locally flat. Upon docking, conformational changes were observed for barbital, phenobarbital, mephobarbital, and primidone. Estimates for the heat of adsorption ranged from -62.3 kJ/mol for barbituric acid to -91.1 kJ/mol for mephobarbital on the hydroxylated surface. Allowance for the heat of desorption of the required number of water molecules from the surface, also determined by SYBYL, gave heat of displacement values of -19.4 kJ/mol for barbituric acid and -32.6 kJ/mol for mephobarbital. These values compared well to the heat of displacement values obtained by isoperibol calorimetry, which were -20.3 kJ/mol for barbituric acid and -31.6 kJ/mol for mephobarbital. Previous laboratory studies had demonstrated the greater importance of the C-O functional state for barbiturate adsorption compared to the C = O functional state. The computer-modeled system predicted the same result.