Non-monotonous Concentration Dependent Solvation of ATP Could Help to Rationalize Its Anomalous Impact on Various Biophysical Processes.

Adenosine triphosphate (ATP), one of the biologically most important molecules, offers certain anomalous behavior during folding and liquid-liquid phase separation of proteins and RNAs. ATP can act as a "biological hydrotrope", i.e., it solubilizes hydrophobic proteins or other biomolecules. However, upon exceeding the physiological concentration range (2-10 mM), aggregation of proteins and RNAs is promoted, an effect that is not understood yet. Here we present a time-domain and frequency-domain Terahertz (THz) spectroscopic investigation to understand the solvation of ATP with varying concentration in the range of 2-15 mM. Both time and frequency domain studies of the solvation of adenosine (Adn), sodium triphosphate (TPP), and ATP elucidate that both the adenosine as well as the triphosphate moiety contribute to nearly equal propensity towards the solvation structure of ATP at low concentrations. However, at higher concentrations (>10 mM), the effect of the adenosine moiety dominates, which leads to a more structured solvation shell followed by slower relaxation dynamics. This is due to the triphosphate-driven ATP aggregation with a reduced amount of water-exposed triphosphate groups, as revealed by molecular dynamics simulations. These observations could lead to an understanding of the complex role of ATP in different biological systems.