Hydrogen bonds or ‘H-bonds’ are polar, non-covalent bonds or interactions between a hydrogen atom (H) attached to a more electronegative atom, such as oxygen (O) or nitrogen (N), which partially pulls the electron cloud away from the H, leaving it electropositive—with another electronegative atom, such as O or N from a different molecule or from a different part of the same molecule. H-bond interactions play a huge role in the biochemistry of living processes, and in the structures and interactions of biological molecules, with each other and with different molecules including water. Nature's natural solvent, water, is itself a dynamic H-bonded polar structure, which strongly affects solubility and, as (dynamic) water of hydration, interactions between molecules. Compared with covalent and ionic bonds, H-bonds are individually much weaker (<20 kJ/mol), which make them ideal for molecular recognition phenomena. When many H-bonds come together they can form strong insoluble structures such as cellulose and the impermeable derivative of cellulose known as chitin, or helical structures with intra-chain stabilizing H-bonds such as the α-helix. Perhaps the most important H-bonded structure of them all is DNA.