Trimethylamine N-oxide (TMAO) stabilizes the native state of globular proteins and counteracts the destabilizing action of denaturants. However, at pH values lower than its pKa = 4.7, TMAO destabilizes the native state. We studied, via DFT calculations, the interactions of TMAO, its protonated form and isosteric tert-butanol with one, two and three water molecules in vacuo, PCM and SMD water. The energetic strength of the H-bonds the considered solutes make with three water molecules, coupled to a model of globular protein stability, allows us to propose a rationalization of the pH-dependent TMAO behavior toward globular proteins.
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