Crowding Stabilizes DMSO-Water Hydrogen-Bonding Interactions

Up to 40% of intracellular water is confined due to the dense packing of macromolecules, ions, and osmolytes. Despite the large body of work concerning the effect of additives on the biomolecular structure and stability, the role of crowding and heterogeneity in these interactions is not well understood. Here, infrared spectroscopy and molecular dynamics simulations are used to describe the mechanisms by which crowding modulates hydrogen bonding interactions between water and dimethyl sulfoxide (DMSO). Specifically, we use formamide and dimethylformamide (DMF) as molecular crowders and show that the S=O hydrogen bond populations in aqueous mixtures are increased by both amides. These additives increase the amount of water within the DMSO first solvation shell through two mechanisms: (a) directly stabilizing water DMSO hydrogen bonds; (b) increasing water exposure by destabilizing DMSO-DMSO self-interactions. Further, we quantified the hydrogen bond enthalpies between the different components: DMSO-water (61 kJ/mol) > DMSO-formamide (32 kJ/mol) > water-water (23 kJ/mol) >> formamide-water (4.7 kJ/mol). Spectra of carbonyl stretching vibrations show that DMSO induces the dehydration of amides as a result of strong DMSO water interactions, which has been suggested as the main mechanism of protein destabilization.