Most
land-living organisms regularly experience dehydration. In
nature, one commonly applied strategy to protect against this osmotic
stress is to introduce small polar molecules with low vapor pressure,
commonly called osmolytes. Two examples of naturally occurring small
polar compounds are urea and trimethylamine <i>N</i>-oxide
(TMAO), which are known to have counteracting effects on protein stability.
In this work, we investigate the effects of urea and TMAO on lipid
self-assembly at varying water contents, focusing on dehydrated conditions.
By using complementary experimental techniques, including sorption
microcalorimetry, NMR, and X-ray scattering, together with molecular
dynamics simulations in model systems composed of phosphatidylcholine
lipids, water, and solute, we characterize interactions and self-assembly
over a large range of hydration conditions. It is shown that urea
and TMAO show qualitatively similar effects on lipid self-assembly
at high water contents, whereas they have clearly different effects
in dehydrated conditions. The latter can be explained by differences
in the molecular interactions between the solutes and the lipid headgroups.
TMAO is repelled from the bilayer interface, and it is thereby expelled
from lipid lamellar systems with low water contents and narrow inter-bilayer
regions. In these conditions, TMAO shows no effect on the lipid phase
behavior. Urea, on the other hand, shows a slight affinity for the
lipid headgroup layer, and it is present in the lipid lamellar system
at all water contents. As a result, urea may exchange with water in
dry conditions and thereby prevent dehydration-induced phase transitions.
In nature, urea and TMAO are sometimes found together in the same
organisms and it is possible that their combined effect is to both
protect lipid membranes against dehydration and still avoid denaturation
of proteins