Sodium affinity of caffeine and adenine: the effect of microsolvation and electrostatic field of solvent on the sodium affinity
The sodium affinity (SA) of caffeine (CAF), adenine (AD) and their microsolvated clusters containing one X molecule (CAF-X and AD-X; X = H2O, NH3, H2S and HF) has been calculated in the gas phase and water, separately. The density functional theory (DFT) employing CAM-B3LYP functional has been used for all of the calculations in this work. The solvent was modelled by the polarised continuum model (PCM) which considers the electrostatic field of solvent on solute. The calculated SA of [CAF-X] and [AD-X] was higher than that of CAF and AD in the gas phase, respectively, which showed that the microsolvation of molecules in the gas phase could be used for changing the tendency of molecules for binding to Na+. Also, it was observed that the electrostatic effect of solvent decreases the SA of the species compared to the gas phase, considerably. The symmetry adapted perturbation theory (SAPT) calculations were also used to interpret the change in the SA of CAF and AD due to the clustering with one X molecule in the gas phase. In addition, there is a detailed study on the position of Na+ relative to AD and CAF structures in different conditions including gas phase, microsolvation and electrostatic field of solvent in this work.