In organic chemistry and

biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

, a side chain is a

chemical group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...

that is attached to a core part of the molecule called the "main chain" or

backbone The backbone is the vertebral column of a vertebrate. Arts, entertainment, and media Film * ''Backbone'' (1923 film), a 1923 lost silent film starring Alfred Lunt * ''Backbone'' (1975 film), a 1975 Yugoslavian drama directed by Vlatko Gilić ...

. 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 In IUPAC nomenclature of chemistry, a pendant group (sometimes spelled pendent) or side group is a group of atoms attached to a backbone chain of a long molecule, usually a polymer. Pendant groups are different from pendant chains, as they are ...

(side group) has a different definition.

Conventions

The placeholder R is often used as a generic placeholder for

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloal ...

(saturated hydrocarbon) group side chains in

chemical structure diagram The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bond ...

s. To indicate other non-carbon groups in structure diagrams, X, Y, or Z are often used.

History

The ''R'' symbol was introduced by 19th-century French

chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe th ...

Charles Frédéric Gerhardt Charles Frédéric Gerhardt (21 August 1816 – 19 August 1856) was a French chemist, born in Alsace and active in Paris, Montpellier, and his native Strasbourg. Biography He was born in Strasbourg, which is where he attended the gymnasium (an ...

, 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

Organic chemistry

polymer science Polymer science or macromolecular science is a subfield of materials science concerned with polymers, primarily synthetic polymers such as plastics and elastomers. The field of polymer science includes researchers in multiple disciplines includ ...

, the side chain of an oligomeric or polymeric offshoot extends from the

backbone chain In polymer science, the polymer chain or simply backbone of a polymer is the main chain of a polymer. Polymers are often classified according to the elements in the main chains. The character of the backbone, i.e. its flexibility, determines the ...

of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its

crystallinity Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusio ...

and density. An

oligomeric In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relative ...

branch may be termed a short-chain branch, and a polymeric branch may be termed a long-chain branch.

Side group In IUPAC nomenclature of chemistry, a pendant group (sometimes spelled pendent) or side group is a group of atoms attached to a backbone chain of a long molecule, usually a polymer. Pendant groups are different from pendant chains, as they are n ...

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

Biochemistry

In proteins, which are composed of

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

residues, the side chains are attached to the

alpha-carbon In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule. Numeric locants The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of n ...

atoms of the

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...

backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining

charge Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * '' Charge!!'', an album by The Aqu ...

and

polarity Polarity may refer to: Science *Electrical polarity, direction of electrical current * Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of or ...

of the amino acid. The amino acid side chains are also responsible for many of the interactions that lead to proper protein folding and function. 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. AAs_table

References