In organic chemistry
, a side chain is a chemical group
that is attached to a core part of the molecule called the "main chain" or backbone
. The side chain is a hydrocarbon branching element of a molecule that is attached to a larger hydrocarbon backbone. It is one factor in determining a molecule's properties and reactivity. A side chain is also known as a pendant chain, but a pendant group
(side group) has a different definition.
The placeholder R is often used as a generic placeholder for alkyl
(saturated hydrocarbon) group side chains in chemical structure diagram
s. To indicate other non-carbon groups in structure diagrams, X, Y, or Z are often used.
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").
In polymer science
, the side chain of an oligomer
ic or polymer
ic offshoot extends from the backbone chain
of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its crystallinity
. An oligomeric
branch may be termed a short-chain branch, and a polymeric
branch may be termed a long-chain branch. Side group
s are different from side chains; they are neither oligomeric nor polymeric.
s, 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 charge
of the amino acid. The amino acid side chains are also responsible for many of the interactions that lead to proper protein folding
Amino acids with similar polarity are usually attracted to each other, while nonpolar and polar side chains usually repel each other. Nonpolar/polar interactions can still play an important part in stabilizing the secondary structure due to the relatively large amount of them occurring throughout the protein. Spatial positions of side-chain atoms can be predicted based on protein backbone geometry using computational tools for side-chain reconstruction.
*Branching (polymer chemistry)