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Abstract

Proteins of halophilic organisms, which accumulate molar concentrations of KCl in their cytoplasm, have a much
higher content in acidic amino acids than proteins of mesophilic organisms. It has been proposed that this excess is necessary to
maintain proteins hydrated in an environment with low water activity, either via direct interactions between water and the carboxylate
groups of acidic amino acids or via cooperative interactions between acidic amino acids and hydrated cations. Our simulation
study of five halophilic proteins and five mesophilic counterparts does not support either possibility. The simulations use
the AMBER ff14SB force field with newly optimized Lennard-Jones parameters for the interactions between carboxylate groups
and potassium ions. We find that proteins with a larger fraction of acidic amino acids indeed have higher hydration levels, as
measured by the concentration of water in their hydration shell and the number of water/protein hydrogen bonds. However,
the hydration level of each protein is identical at low (b_KCl = 0.15 mol/kg) and high (b_KCl = 2 mol/kg) KCl concentrations; excess
acidic amino acids are clearly not necessary to maintain proteins hydrated at high salt concentration. It has also been proposed
that cooperative interactions between acidic amino acids in halophilic proteins and hydrated cations stabilize the folded protein
structure and would lead to slower dynamics of the solvation shell. We find that the translational dynamics of the solvation shell is
barely distinguishable between halophilic and mesophilic proteins; if such a cooperative effect exists, it does not have that
entropic signature.