Hydrogen bond

A hydrogen bond forms when a hydrogen atom that is covalently bonded to a strongly electronegative atom, usually oxygen or nitrogen, is drawn toward another electronegative atom close by. The atom it is bonded to pulls the shared electrons toward itself, leaving the hydrogen with a slight positive charge, which is then attracted to the slight negative charge on a neighboring atom. Hydrogen bonds can link separate molecules or different parts of one large molecule.

A single hydrogen bond is weak, only a small fraction as strong as a covalent bond. Yet they form in enormous numbers, and together they shape much of biology. They hold water molecules to one another, giving water its high boiling point, its surface tension and its capacity to absorb heat, and they lock ice into an open lattice that is less dense than liquid water, so ice floats. In DNA, hydrogen bonds between bases pair adenine with thymine (two bonds) and guanine with cytosine (three bonds), holding the two strands together yet letting them part for copying. In proteins, hydrogen bonds between backbone atoms stabilize alpha helices and beta sheets.

Because they are weak and reversible, hydrogen bonds are usually the first to give way when proteins denature or when DNA strands are separated by heating, as in each cycle of PCR.