A shell of water only 3 water molecules thick around a phospholipid molecule is enough for energy transfer within 1 ps, researchers of the Max-Born-Institute at Berlin, have discovered. For reference, 1 ps = 10-12 second. They detailed how biomolecules transfer energy into extremely small water droplets in their environment in a recent issue of the Journal of Physical Chemistry.
A biochemical process occurs mostly in an aqueous environment. Particular groups of a biomolecule are embedded in a shell of water molecules, in a process known as hydration. The water shell stabilizes the biomolecular structure and enables an exchange of energy between the biomolecule and its environment. One examples of this is the double helix of DNA, the carrier of basic genetic information, in an aqueous medium, another is the membranes of living cells which consist of phospholipids. The molecular mechanisms, the speed and the efficiency of energy exchange between the biomolecule and the water shell are only partially understood and are a topic of current basic research.
In their report, researchers say that 3 water molecules around the phosphate group of the phospholipid are sufficient for transferring the energy of vibrations from the phospholipid into this minimal water shell.
Extremely Efficient Mechanism
The transferred energy heats the water shell by 10 to 20 centigrades. The thermal energy is stored in tilting motions of water molecules, called librations, and leads to a weakening of the interaction between the water molecules, the so called hydrogen bonds. The overall molecular structure of the water shell remains practically unchanged. This extremely efficient mechanism of energy disposal allows for the transfer of even larger amounts of energy, protecting the biomolecule against damage by overheating.
In the above iilustration at the upper left you can a schematic of a reverse micelle consisting of phospholipid molecules. The phosphate groups of the lipid molecules (blue spheres) are arranged at the inner surface of the micelle. Nanodrop;ets, or water molecules, are located in the inner part of the micelle.
In the upper right of the picture is an enlarged view of the structure of a phospholipid molecule. Oxygen atoms are shown in red, hydrogen atoms in white, carbon atoms in grey, the nitrogen atom in blue, and the phosphorus atom in orange. The angled water molecules are arranged around the phosphate (PO4) group. The illustration’s lower part is the scheme of energy transfer. In the experiments, the asymmetric phosphate vibration is initially excited (red oxygen atoms). The energy released in the decay of the vibration is transferred to the surrounding water shell (see red H2O molecules) within 1 ps.