Formation and stability of calcium complexes of dimethyl sulfoxide in water

Dimethyl sulfoxide (DMSO) is widely utilized compound in biomedical and technological applications. Predominantly, it is used for biomembranes opening during a drug delivery procedure. The mechanism of DMSO membrane treatment is influenced by DMSO interactions with Ca 2+ as well as with water presence. In this study we investigate Ca 2+ interactions with DMSO in an aqueous environment using electrospray ionization mass spectrometry supported by density functional calculations. The number of DMSO ligands in [Ca(DMSO) x ] 2+ depends on the conditions during the electrospray and ion-transfer process. At capillary temperatures lower than 120 °C the complexes with a high number of DMSO are preferentially transferred to the gas phase (e.g. [Ca(DMSO) 6 ] 2+ ). With increasing temperature the number of DMSO ligands decreases while they are replaced with water molecules. Both the experiment and the calculations show that the bonding energy of DMSO to Ca 2+ is strongly influenced by the presence of water. The calculations also demonstrate that the bonding of DMSO to Ca 2+ is rather strong; the Ca 2+ -DMSO bonding energy was calculated to be two times higher than the Ca-H 2 O bonding energy.