At a stereocenter, exchanging two attached groups produces a different spatial configuration. The open-chain ribose and glucose examples have carbon stereocenters bearing four different groups; C2 of deoxyribose has two identical hydrogen substituents and is not a stereocenter. See Moss, Basic Terminology of Stereochemistry, IUPAC Recommendations 1996: stereogenic units.
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A stereocenter (or stereogenic center) is an atom in a molecule that is bonded to four different groups or atoms in such a way that the spatial arrangement of these groups creates stereoisomerism. This means that the arrangement of the groups around the stereocenter can lead to at least two distinct three-dimensional configurations, known as enantiomers, which are non-superimposable mirror images of each other.