Effects of co-solutes on the hydrogen bonding structure and dynamics in aqueous N-methylacetamide solution: a molecular dynamics simulations study

The effects of trimethylamine-N-oxide (TMAO), urea and tetramethyl urea (TMU) on the hydrogen bonding structure and dynamics of aqueous solution of N-methylacetamide (NMA) are investigated by classical molecular dynamics simulations. The modification of the water's hydrogen bonding structure and interactions is calculated in presence of these co-solutes. It is observed that the number of four-hydrogen-bonded water molecules in the solution decreases significantly in the presence of TMAO rather than urea and TMU. The lifetime and structural relaxation time of water-water and NMA-water hydrogen bonds show a strong increase with the addition of TMAO and TMU in the solution, whereas the change is nominal in case of urea solution. It is also found that the translational and rotational dynamics of water and NMA slowdown with increasing the concentration of these osmolytes. The slower dynamics of water and NMA is more pronounced in case of TMAO and TMU solution, as these co-solutes strengthen the average hydrogen bond energies between water-water and NMA-water, whereas urea has a little effect on the hydrogen bonding structure and dynamics of aqueous NMA solution. The calculated self-diffusion coefficient values for water and these co-solutes are in similar pattern with experimental observations.