Cyclitol
A cyclitol is a cyclic polyol; that is, a cycloalkane that has at least three hydroxyl groups, each attached to a different ring carbon atom. Cyclitols are cyclic analogues of carbohydrates, but lack the carbonyl group that defines carbohydrates. They are a subclass of polyols and can be regarded as cyclic forms of open-chain polyols like sorbitol or mannitol.
Nomenclature:
The naming of cyclitols follows specific conventions. The parent cycloalkane is named first (e.g., cyclohexane, cyclopentane), and then the suffix "-itol" is added. The hydroxyl groups are then numbered in such a way as to give the lowest possible numbers to the carbon atoms bearing them. If there are stereoisomers, appropriate prefixes (e.g., cis, trans, epi, myo, scyllo, chiro) are used to define the stereochemistry of the hydroxyl groups relative to the ring. The myo- prefix, for example, signifies that the hydroxyl group on carbon-2 is cis to the hydroxyl groups on carbon-1 and carbon-3, which themselves are trans to each other.
Occurrence and Significance:
Cyclitols are found widely in nature, in both plants and microorganisms. They play various biological roles.
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Signaling Molecules: Some cyclitols, like inositol phosphates derived from myo-inositol, are important signaling molecules in eukaryotic cells, involved in processes such as cell growth, apoptosis, and insulin signaling.
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Plant Defense: Certain cyclitols act as plant protectants, helping them to withstand environmental stresses like drought, salinity, and pathogen attack.
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Osmoprotectants: Some cyclitols act as osmoprotectants, helping organisms maintain osmotic balance in response to changing environmental conditions.
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Precursors: Cyclitols are biosynthetic precursors to other important metabolites, including plant hormones like abscisic acid.
Examples:
Some common examples of cyclitols include:
- Inositol: A six-carbon cyclitol with the formula C6H12O6, which exists in nine stereoisomeric forms. myo-Inositol is the most abundant isomer and is biologically significant.
- Quinic acid: A cyclitol derivative involved in plant metabolism.
- Shikimic acid: A cyclitol precursor to aromatic amino acids in plants and microorganisms. It is also the starting material for the synthesis of the antiviral drug oseltamivir (Tamiflu).