Effects of urea on the aqueous solvation structure and dynamics of cis/ trans-N-methylformamide: An insight from molecular dynamics simulation study

We have performed classical molecular dynamics simulation of aqueous cis/trans-N-methylformamide (NMF) systems in the presence of urea at 298 K by considering the Smith and Duffy models of urea with varying concentrations, ranging from 0 to 13.26 m. It is found that irrespective of urea models, the donating ability of the hydrogen atom of NMF to the oxygen of urea is higher than the oxygen of water, while it is comparatively higher in Duffy model. The overall urea-water interaction is significant for the Smith model, whereas the Duffy model favors the urea-urea interactions. The center of mass radial distribution functions [gCM(r)] of cis/trans-NMF suggest water is preferred in the near vicinity over urea in the solvation shell of these two conformers. A shoulder around 3.50 angstrom in gCM(r) of trans-NMF and water is mainly due to the hydrogen bonding contribution from the water molecules. The translational and rotational dynamics of cis/trans-NMF are slower with the addition of urea, but the slowing down is significant in trans-NMF. A similar behavior is also observed for water and urea. The higher dipole orientational relaxation time of urea for the Duffy model indicates its aggregation behavior. The hydrogen bond lifetime of Otrans...Hwat is higher compared to Ocis...Hwat for both the urea models. It is also observed that a significant increase in water-water hydrogen bond lifetime with varying concentrations of the Smith model of urea. It is also found that the stability of all these hydrogen bonds comparatively higher in Smith model.