<unk>  is an aldopentose (a monosaccharide containing five carbon atoms) that, in its open chain form, has an aldehyde functional group at one end. In the conventional numbering scheme for monosaccharides, the carbon atoms are numbered from C1' (in the aldehyde group) to C5'. The deoxyribose derivative found in DNA differs from<unk>  by having a hydrogen atom in place of the hydroxyl group at C2'. This hydroxyl group performs a function in RNA splicing. The "-" in the name<unk>  refers to the stereochemistry of the chiral carbon atom farthest away from the aldehyde group (C4'). In<unk> , as in all -sugars, this carbon atom has the same configuration as in -glyceraldehyde. File:Alpha-D-Ribopyranose numbered.α--Ribopyranose File:Beta-D-Ribopyranose numbered.β--Ribopyranose File:Alpha-D-Ribofuranose numbered.α--Ribofuranose File:Beta-D-Ribofuranose Numbered.β--Ribofuranose Relative abundance of forms of<unk>  in solution: β--ribopyranose (59%), α--ribopyranose (20%), β--ribofuranose (13%), α--ribofuranose (7%) and open chain (0.1%). For<unk>  residues in nucleosides and nucleotide, the torsion angles for the rotation encompassing the bonds influence the configuration of the respective nucleoside and nucleotide. The secondary structure of a nucleic acid is determined by the rotation of its 7 torsion angles. Having a large amount of torsion angles allows for greater flexibility. In closed ring<unk> s, the observed flexibility mentioned above is not observed because the ring cycle imposes a limit on the number of torsion angles possible in the structure. Conformers of closed form<unk> s differ in regards to how the lone oxygen in the molecule is positioned respective to the nitrogenous base (also known as a nucleobase or just a base) attached to the<unk> . If a carbon is facing towards the base, then the<unk>  is labeled as endo. If a carbon is facing away from the base, then the<unk>  is labeled as exo. If there is an oxygen molecule attached to the 2' carbon of a closed cycle<unk> , then the exo confirmation is more stable because it decreases the interactions of the oxygen with the base. The difference itself is quite small, but when looking at an entire chain of RNA the slight difference amounts to a sizable impact. File:2' endo.2' endo File:2' endo 3' exo.2' endo 3' exo File:3' endo 2' exo.3' endo 2' exo File:3' endo.3' endo A<unk>  molecule is typically represented as a planar molecule on paper. Despite this, it is typically non-planar in nature. Even between hydrogen atoms, the many constituents on a<unk>  molecule cause steric hindrance and strain between them. To relieve this crowding and ring strain, the ring puckers, i.e. becomes non-planar. This puckering is achieved by displacing an atom from the plane, relieving the strain and yielding a more stable configuration. Puckering, otherwise known as the sugar ring conformation (specifically<unk>  sugar), can be described by the amplitude of pucker as well as the pseudorotation angle. The pseudo-rotation angle can be described as either "north (N)" or "south (S)" range. While both ranges are found in double helices, the north range is commonly associated with RNA and the A form of DNA. In contrast, the south range is associated with B form DNA. Z-DNA contains sugars in both the north and south ranges. When only a single atom is displaced, it is referred to as an "envelope" pucker. When two atoms are displaced, it is referred to as a "twist" pucker, in reference to the zigzag orientation. In an "endo" pucker, the major displacement of atoms is on the β-face, the same side as the C4'-C5' bond and the base. In an "exo" pucker, the major displacement of atoms is on the α-face, on the opposite side of the ring. The major forms of<unk>  are the 3'-endo pucker (commonly adopted by RNA and A-form DNA) and 2'-endo pucker (commonly adopted by B-form DNA). These ring puckers are developed from changes in ring torsion angles; there are infinite combinations of angles so therefore, there is an infinite number of transposable pucker conformations, each separated by disparate activation energies.
Ribose
Lactose
Fructosamine
Glucuronic acid
Methyl cellulose
Gluten
Glycogen
Inositol