What Is a Hydrogen Bond? A Weak Attraction That Holds Biology Together

When hydrogen is bonded to a strongly electron-attracting atom, generally oxygen, nitrogen or fluorine, the shared electrons sit closer to that atom and leave the hydrogen with a partial positive charge and very little electron cloud around it. That exposed positive region is attracted to a region of negative charge on another atom, typically a lone pair of electrons on another oxygen or nitrogen, producing an attraction that is directional and considerably stronger than the general attractions between molecules while being roughly a twentieth the strength of a covalent bond. That intermediate strength is the whole point, since a structure held together by many such attractions is stable enough to persist and weak enough to be assembled and disassembled at ordinary temperatures without breaking anything permanently.

Nearly every unusual property of water follows from these attractions:

• •A boiling point far higher than molecules of similar size, which would otherwise be a gas at everyday temperatures
• •A very high heat capacity, since energy goes into breaking the attractions rather than into raising the temperature, which moderates climate
• •High surface tension, which supports insects and drives water up narrow tubes
• •Ice being less dense than liquid, since the solid arrangement is an open framework held at fixed angles
• •Its ability to dissolve a wide range of substances, since it surrounds and separates charged particles
• •A high heat of vaporisation, which makes evaporative cooling effective and drives weather systems

The structures of life depend on these attractions at every level. The double helix is held together by them between the paired bases, with two or three per pair, and the number of bonds is why one pairing is stronger than the other and why the helix can be unzipped for copying without breaking any covalent bonds. Protein folding depends on them extensively, with the regular helical and sheet arrangements stabilised by them along the backbone, and the overall folded shape is held by a combination including them. Enzymes recognise their substrates partly through them, which supplies the specificity. Cellulose fibres are held together by them, which is why plant material is strong. And the fact that all of this is held by attractions weak enough to break at moderate temperatures is why heat destroys biological structure so readily.

The existence of these attractions was inferred from anomalies before anything could be seen directly. Water and several related compounds have boiling points far above the trend their molecular size predicts, which was noticed in the nineteenth century and required an explanation involving association between molecules. The term and the modern understanding developed in the 1920s and 1930s, with Pauling doing much to establish the concept and to apply it to structure. X-ray crystallography made the geometry visible, revealing the characteristic near-linear arrangement and the distances involved, and the structure of DNA was determined partly by working out which pairings allowed the bonds to form with the right geometry. Neutron diffraction later located the hydrogen atoms themselves, which X-rays see poorly. The concept has been refined repeatedly, including a formal redefinition in 2011 broadening what counts.

The attraction accounts for a range of everyday properties. Adhesion between materials involves it, and several adhesives work partly by forming such bonds across a joint. Fibres including cotton, wool and silk owe much of their behaviour to them, which is why they change when wet as water competes for the attractions holding the fibre together. Paper is held together by them between cellulose fibres, which is why it weakens when wet and why it can be made without glue. Cooking depends on them, since denaturing proteins means disrupting the attractions holding a folded shape. Pharmaceutical design considers them carefully, since a drug binds its target partly through them and a molecule with the wrong arrangement of donors and acceptors will not fit. And the properties of several polymers including nylon follow from them directly.

Hydrogen attached to oxygen, nitrogen or fluorine is left with an exposed positive charge that attracts electrons on another such atom, giving an attraction a twentieth the strength of a real bond. That weakness is the point, since structures held by many of them assemble and come apart at ordinary temperatures. Water's boiling point, heat capacity, surface tension and floating ice all follow from them.