Side chain
In organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 and biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, a side chain is a chemical group
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

 that is attached to a core part of the molecule called "main chain" or backbone
Backbone chain
In polymer science, the backbone chain or main chain of a polymer is the series of covalently bonded atoms that together create the continuous chain of the molecule....

. The placeholder R is often used as a generic placeholder for alkyl group side chains in chemical structure diagrams. To indicate other non-carbon groups in structure diagrams, X, Y, or Z is often used. The R, historically, is derived from radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...

or rest or random.

In polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 science, the side chain or pendant chain is oligomer
In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...

ic or polymeric offshoot extends from the backbone chain
Backbone chain
In polymer science, the backbone chain or main chain of a polymer is the series of covalently bonded atoms that together create the continuous chain of the molecule....

 of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its crystallinity and density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

. An oligomeric branch may be termed a short-chain branch and a polymeric branch may be termed a long-chain branch. Side groups
Pendant group
A pendant group or side group is a group of molecules attached to a backbone chain of a long molecule. Usually, this molecule would be a polymer...

 are different from side chains; they are neither oligomeric nor polymeric.

In protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s (composed of amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s) the side chains are attached to the alpha-carbon atoms of the amide backbone.
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