The effect of Solutes on the Molecular Structures of Water and Peptides

The structure and function of proteins and nucleic acids is controlled in
the solvation shell by enthalpic interactions of biomolecule-water-solute,
and their entropy. Historically, the entropic component focused on water
structure at the biological interface. In considering the Physics of Life, it is
noted that Earth is a terrestrial silicate planet with a veneer of saline liquid
water. In the Saturnian and Uranian systems there are worlds where water
and clathrate ices are major components of icy shells that overlie what might
be volatile rich sub-surface oceans. To explore the pre-biotic potential of
ocean worlds, I studied two mimetic aqueous systems; a terrestrial marine
model of a tripeptide in water with the denaturing and renaturing solutes,
urea and Trimethylamine-N-oxide (TMAO), and aqueous ammonia that
recreated an impact melt on Titan. I used neutrons as a diffraction probe,
and Empirical Potential Structure Refinement to model the results. The
environments studied are other-worldy but they share a common physical
process whereby solute effects are translated through the water network by
co-operative hydrogen bonding. In the terrestrial system, urea acts directly
on peptide molecules, and TMAO counteracts urea-induced protein
denaturation by incorporating urea into a hydrogen bond network in the
bulk-solution, partitioning urea away from the peptide surface.
Ammonia-addition leads to a highly ordered molecular symmetry drawn by
the translation of the strong interaction of water-ammonia hydrogen bond
into the hydration shell of the water molecule. The result brings to mind the
hypothesis of Frank and Evans, and Kauzmann, who argued that the
decrease in entropy of non-polar solutes in water was the result of ordered
water at the solute-water interface or what they described as ‘microscopic
icebergs’ around solute molecules.