12 - Conclusion: H-Bond, Water Molecule and Life

Abstract

H-bonds have three fundamental properties that give them an up-to-now unsuspected importance: they are directional and have enthalpies of formation of the order of nkT at room temperature, with n < 10 for weak H-bonds. The result is that they play a unique role in everyday life, even if this role has hardly been emphasized. Their directionality allows them to be at the origin of well-defined molecular structures, in much the same way as covalent bonds allow atoms to assemble and build well-defined molecules. Covalent bonds have enthalpies of formation one order of magnitude greater than H-bonds. They are strong bonds that make most molecules stable but rigid at room temperature, allowing only small amplitude vibrations of atoms around their equilibrium positions. Variations of temperature have almost no effect on them. Stable assemblies of H-bonded molecules are conversely flexible and evolutive, as a modification of their structures requires energies that are within reach of thermal fluctuations. The DNA double helix is a good illustration of these properties of stability, flexibility, and possibility of evolution that all biomacromolecules have to possess.