pyranose
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Pyranose is a collective term for
saccharide A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. There may be other carbons external to the ring. The name derives from its similarity to the oxygen heterocycle
pyran In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the ...

pyran
, but the pyranose ring does not have
double bond In chemistry, a double bond is a covalent bond A covalent bond is a chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, ...

double bond
s. A pyranose in which the anomeric OH at C(l) has been converted into an OR group is called a pyranoside.


Formation

The pyranose ring is formed by the reaction of the
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the ...

hydroxyl
group on carbon 5 (C-5) of a sugar with the
aldehyde Chemically, an aldehyde is a compound containing a functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same ...

aldehyde
at carbon 1. This forms an intramolecular
hemiacetal A hemiacetal or a hemiketal have the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol In , alcohol is an that carries at least one (−OH) bound to a ...

hemiacetal
. If reaction is between the C-4 hydroxyl and the aldehyde, a
furanose A furanose is a collective term for carbohydrates is a disaccharide found in animal milk. It consists of a molecule of galactose, D-galactose and a molecule of glucose, D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () is a bio ...
is formed instead. The pyranose form is thermodynamically more stable than the furanose form, which can be seen by the distribution of these two cyclic forms in solution.


History

Hermann Emil Fischer Hermann Emil Louis Fischer Royal Society of London, FRS Fellow of the Royal Society of Edinburgh, FRSE FCS (; 9 October 185215 July 1919) was a German chemist and List of Nobel laureates in Chemistry, 1902 recipient of the Nobel Prize in Chemis ...

Hermann Emil Fischer
won the
Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "MD ...
(1902) for his work in determining the structure of the D-
aldohexose In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...
s. However, the linear, free-aldehyde structures that Fischer proposed represent a very minor percentage of the forms that hexose sugars adopt in solution. It was
Edmund Hirst Sir Edmund Langley Hirst CBE Royal Society, FRS FRSE (21 July 1898 – 29 October 1975), was a British chemist. Life Hirst was born in Preston, Lancashire on 21 July 1898 the son of Elizabeth (née Langley) and Rev Sim Hirst (1856-1923) a Baptist ...
and Clifford Purves, in the research group of
Walter Haworth Sir Walter Norman Haworth Fellow of the Royal Society, FRS (19 March 1883 – 19 March 1950) was a British chemist best known for his groundbreaking work on ascorbic acid (vitamin C) while working at the University of Birmingham. He received the ...
, who conclusively determined that the hexose sugars preferentially form a pyranose, or six-membered, ring. Haworth drew the ring as a flat hexagon with groups above and below the plane of the ring – the
Haworth projection 275px, Haworth projection of the structures for α-D-glucopyranose and L-Glucose, α-L-glucopyranose. A Haworth projection is a common way of writing a structural formula to represent the cyclic molecular geometry, structure of monosaccharides with ...
. A further refinement to the conformation of pyranose rings came when Sponsler and Dore (1926) realized that Sachse’s mathematical treatment of six-membered rings could be applied to their
X-ray structure X-ray crystallography (XRC) is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract into many specific directions. By mea ...

X-ray structure
of
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
. It was determined that the pyranose ring is puckered, to allow all of the carbon atoms of the ring to have close to the ideal tetrahedral geometry.


Conformations

This puckering leads to a total of 38 distinct basic pyranose conformations: 2 chairs, 6 boats, 6 skew-boats, 12 half-chairs, and 12 envelopes. These conformers can interconvert with one another; however, each form may have very different relative energy, so a significant
barrier A barrier or barricade is a physical structure which blocks or impedes something. Barrier may also refer to: Places * Barrier, Kentucky, a community in the United States * Barrier, Voerendaal, a place in the municipality of Voerendaal, in southeas ...

barrier
to interconversion may be present. The energy of these conformations can be calculated from
quantum mechanics Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with ...
; an example of possible
glucopyranose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucopyranose
interconversions is given. The conformations of the pyranose ring are superficially similar to that of the
cyclohexane Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is non-polar. Cyclohexane is a colorless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cycloh ...

cyclohexane
ring. However, the specific nomenclature of pyranoses includes reference to the ring oxygen, and the presence of hydroxyls on the ring have distinct effects on its conformational preference. There are also conformational and stereochemical effects specific to the pyranose ring.


Nomenclature

To name conformations of pyranose, first the conformer is determined. The common conformers are similar to those found in
cyclohexane Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is non-polar. Cyclohexane is a colorless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cycloh ...

cyclohexane
, and these form the basis of the name. Common conformations are chair (C), boat (B), skew (S), half-chair (H) or envelope (E). The ring atoms are then numbered; the
anomer An anomer is a type of geometric variation found at certain atoms in carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallog ...
ic, or hemiacetal, carbon is always 1. Oxygen atoms in the structure are, in general, referred to by the carbon atom they are attached to in the acyclic form, and designated O. Then: *Position the ring so that, if looking at the top face, the atoms are numbered clockwise. *In the chair and skew conformations, the reference plane should be selected. In the chair conformation, the reference plane is chosen such that the lowest-numbered atom (usually C-1) is exoplanar. In the skew conformation, the plane contains three adjacent atoms and one other with the atom with the lowest possible number exoplanar. *Atoms above the plane are written before the conformer label, as a superscript *Atoms below the plane are written following the conformer label, as a subscript


NMR spectroscopy

As shown by the relative structure energies in the diagram above, the chair structures are the most stable carbohydrate form. This relatively defined and stable conformation means that the hydrogen atoms of the pyranose ring are held at relatively constant angles to one another. Carbohydrate NMR takes advantage of these
dihedral angle A dihedral angle is the angle between two intersecting planes or half-planes. In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, ele ...

dihedral angle
s to determine the configuration of each of the hydroxyl groups around the ring.


See also

*
Furanose A furanose is a collective term for carbohydrates is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide found in a ...
*
Mutarotation Mutarotation is the change in the ''optical rotation Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization (waves), polarization about the optical axis of l ...
*
Monosaccharide Monosaccharides (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
*
Carbohydrate conformation Carbohydrate conformation refers to the overall three-dimensional structure adopted by a carbohydrate (saccharide) molecule as a result of the through-bond and through-space physical forces it experiences arising from its molecular structure. The p ...


References

{{Carbohydrates   Carbohydrate chemistry