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Hydrogen Bonding

A hydrogen bond is the attractive force

between the hydrogen attached to an

electronegative atom of one molecule and an

electronegative atom of a different molecule.

Usually the electronegative atom is oxygen,

nitrogen, or fluorine, which has a partial

negative charge. The hydrogen then has the

partial positive charge

Strength.

Hydrogen bonds are classified as weak bonds because they are easily and rapidly formed and broken under normal biological conditions. The typical hydrogen bond is stronger than van der Waals forces, but weaker than covalent or ionic bonds.

Bond length

The length of hydrogen bonds depends on bond strength, temperature, and pressure. The bond strength itself is dependent on temperature, pressure, bond angle, and environment (usually characterized by local dielectric constant). The typical length of a hydrogen bond in water is 1.97 Å

Some examples :

Hydrogen bonding in water

Each water molecule is hydrogen bonded to four others.

The hydrogen bonds that form between water molecules account for some of the essential and unique properties of water.

The attraction created by hydrogen bonds keeps water liquid over a wider range of temperature than is found for any other molecule its size.
The energy required to break multiple hydrogen bonds causes water to have a high heat of vaporization; that is, a large amount of energy is needed to convert liquid water, where the molecules are attracted through their hydrogen bonds, to water vapor, where they are not.

Two outcomes of this:

The evaporation of sweat, used by many mammals to cool themselves, achieves this by the large amount of heat needed to break the hydrogen bonds between water molecules

The hydrogen bond has only 5% or so of the strength of a covalent bond. However, when many hydrogen bonds can form between two molecules (or parts of the same molecule), the resulting union can be sufficiently strong as to be quite stable.

Multiple hydrogen bonds

hold the two strands of the DNA double helix together
hold polypeptides together in such secondary structures as the alpha helix and the beta conformation;
help enzymes bind to their substrate;
help antibodies bind to their antigen
help transcription factors bind to each other;
help transcription factors bind to DNA

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