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Threose is a four-carbon monosaccharide with molecular formula C4H8O4. It has a terminal aldehyde group, rather than a ketone, in its linear chain and so is considered part of the aldose family of monosaccharides. The threose name can be used to refer to both the - and - and more generally to the racemic mixture (/L-, equal parts D- and L-) as well as to the more generic threose structure (absolute stereochemistry unspecified).
The prefix "threo-" which derives from threose (and "erythro-" from a corresponding diastereomer erythrose) offer a useful way to describe general organic structures with adjacent chiral centers, where "the prefixes... designate the relative configuration of the centers". Formulas Using Other Configurational Notations, W. Rausch, accessed 1 March 2011 As is depicted in a Fischer projection of -threose, the adjacent substituents will have a Syn addition orientation in the isomer referred to as "threo", and are Anti addition in the isomer referred to as "erythro".Prof. Rausch helpfully notes that the prefixes "may be applied to racemic compounds, as well as pure enantiomers and meso compounds", and that when depicted in the common "zig-zag" representation, adjacent "substituents may lie on the same side of the carbon chain... syn or on opposite sides... anti", which is opposite of their depiction in a Fischer projection.
Although often inconsequential, threose in aqueous solution mainly exists as the hydrate owing to the following equilibrium:
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