Structure of Aqueous Solutions of Trimethylaminoxide, Urea, and Their Mixture

Aqueous solutions of natural osmolytes (trimethylaminoxide (TMAO), urea, and their mixture) at relatively small (biologically relevant) concentrations are analyzed by the all-atom molecular dynamics simulation. In the recent work (Smolin N. et al. PCCP. 2017. 19. P. 6345) it has been noted that in the protein hydration shell the fraction of TMAO molecules is much smaller than that of urea. The urea addition causes a further decrease in the TMAO fraction in the protein hydration shell. This work shows that in binary solutions urea fraction at urea molecules is always larger than the bulk urea concentration. At the same time, the TMAO fraction near TMAO is the same as in the bulk. In ternary solutions, TMAO and urea behave the same as the binary ones, i.e. they do not noticeably affect each other. This means that the behavior of TMAO and urea molecules in the protein hydration shell is associated with protein rather than their interaction with each other.