Rethinking The Concept Of Biological Water From Numerical Simulations

Simulation box containing water molecules (red) confined by a phospholipid membrane.

Biological water is the thin layer of water molecules that, by surrounding biological systems such as, e.g., proteins, organelles, and cell membranes, affects their fluidity, phase behavior and, ultimately, proper functioning. As a counter effect, the biological molecules interact with water molecules slowing down their molecular rotations and diffusion.

The dynamical properties of the bulk water are recovered beyond the thin hydration shell, i.e., at a distance of around 1.5 nm away from the biological molecules. Because of this interconnected effect, it is still not clear if water affects the biological entities or vice versa.

In our study, we have simulated the interaction of liquid water with a biological membrane modeled by a phospholipid bilayer.

Biological membranes provide a limiting structure that separates the interior and exterior of cells and organelles. Being selectively permeable, membranes control the flow of substances in and out of the cell, which permits the regulation of the cell composition and communication between cells through signaling. Membranes are also involved in the capture and release of energy.

Biological membranes are composed of many biomolecules, including proteins, sugars, cholesterol, and phospholipids. Among these components, phospholipids provide structure to biological membranes. This is due to their spontaneous self-assembly, arising from the hydrophobic effect and resulting in the formation of bilayers. For this reason, phospholipid membranes are used as models to investigate the fundamental properties of biological membranes, both experimentally and theoretically.

We have initially quantified the degree of dynamical slow down of water molecules. We have shown that approaching the phospholipid surface, the water molecules rotate and diffuse at a pace comparable with bulk liquid water at deeply undercooled conditions, i.e., at 240 K.

We have then probed structural properties of water molecules using a new, sensitive order metric developed by some of the authors. We have demonstrated that phospholipid membranes have an effect on the structure of the surrounding water that propagates well beyond the thin layer of biological water (at least 2.5 times further), much further than previously hypothesized.

This intriguing result could help the scientific community to rethink the concept of biological water to better understand the role of water in biological systems. We are currently simulating other systems in order to understand if the same effect we observe in this article occurs also with other biological systems.

These findings are described in the article entitled Structural properties of water confined by phospholipid membranes, published in the journal Frontiers of Physics. This work was led by Fausto Martelli from Princeton University.

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