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The canonical Watson-Crick base pairs, G:C and A:T/U as well as most of the non-canonical ones are stabilized by two or more (e.g. 3 in the case of G:C cWW) hydrogen bonds. Justifiably, a significant amount of research on non-canonical base pairs has been carried out towards bench-marking their strengths (interaction energies) and (geometric ...
An additional type of bond is a "non-bond", indicated with ., to indicate that two parts are not bonded together. For example, aqueous sodium chloride may be written as [Na+].[Cl-] to show the dissociation. An aromatic "one and a half" bond may be indicated with :; see § Aromaticity below.
In this manner, two nucleobases, one on each strand, can be held together by hydrogen bonds in the major groove. A Hoogsteen base pair applies the N7 position of the purine base (as a hydrogen bond acceptor) and C4 amino group (as a donor), which bind the Watson–Crick (N3–C4) face of the pyrimidine base.
An ubiquitous example of a hydrogen bond is found between water molecules. In a discrete water molecule, there are two hydrogen atoms and one oxygen atom. The simplest case is a pair of water molecules with one hydrogen bond between them, which is called the water dimer and is often used as a model system. When more molecules are present, as is ...
Non-covalent hydrogen bonds between the bases are shown as dashed lines. The wiggly lines stand for the connection to the pentose sugar and point in the direction of the minor groove. Hydrogen bonding is the chemical interaction that underlies the base-pairing rules described above.
Purines form hydrogen bonds to pyrimidines, with adenine bonding only to thymine in two hydrogen bonds, and cytosine bonding only to guanine in three hydrogen bonds. This arrangement of two nucleotides binding together across the double helix (from six-carbon ring to six-carbon ring) is called a Watson-Crick base pair.
The A–T pairing is based on two hydrogen bonds, while the C–G pairing is based on three. In both cases, the hydrogen bonds are between the amine and carbonyl groups on the complementary bases. Nucleobases such as adenine, guanine, xanthine , hypoxanthine , purine, 2,6-diaminopurine , and 6,8-diaminopurine may have formed in outer space as ...
Canonical bases may have either a carbonyl or an amine group on the carbons surrounding the nitrogen atom furthest away from the glycosidic bond, which allows them to base pair (Watson-Crick base pairing) via hydrogen bonds (amine with ketone, purine with pyrimidine). Adenine and 2-aminoadenine have one/two amine group(s), whereas thymine has ...