Side chain

In organic chemistry and biochemistry, a side chain or pendant chain (not to be confused with pendant group or side group) is a chemical group that is attached to a core part of the molecule called the "main chain" or backbone. It is one factor in determining a molecule's properties and reactivity.

Conventions

The placeholder R is often used as a generic placeholder for side chains in structural formulae (see R group (chemistry)). Otherwise, X, Y, or Z are often used, X particularly for halogens.

History

The R symbol was introduced by 19th-century French chemist Charles Frédéric Gerhardt, who advocated its adoption on the grounds that it would be widely recognizable and intelligible given its correspondence in multiple European languages to the initial letter of "root" or "residue": French racine ("root") and résidu ("residue"), these terms' respective English translations along with radical (itself derived from Latin radix below), Latin radix ("root") and residuum ("residue"), and German Rest ("remnant" and, in the context of chemistry, both "residue" and "radical").

Usage

Biochemistry

In proteins, which are composed of amino acid residues, the side chains are attached to the alpha-carbon atoms of the amide backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining properties such as charge and polarity, that then are responsible for many of the interactions which lead to proper protein folding and function. Spatial positions of side-chain atoms can be predicted based on protein backbone geometry using computational tools for side-chain reconstruction.

See also

• Alkyl
• Backbone-dependent rotamer library
• Backbone chain
• Branching (polymer chemistry)
• Functional group
• Pendant group
• Residue (chemistry)
• R group (chemistry)
• Substituent

References