= Galactose
<Galactose> is an <aldohexose>, a six-carbon <monosaccharide> whose open-chain reference form has an <aldehyde> group. It is the carbon-4 <epimer> of <glucose>: the corresponding structures differ at that one <stereocenter>. Its <molecular formula> is $\mathrm{C_6H_{12}O_6}$.<Galactose in ChEBI>[CHEBI:28260, Definition and Formula]{ref}
A <Fischer projection> makes the difference precise. With the aldehyde at the top, the carbon-2 through carbon-5 <hydroxyl group> pattern of D-<glucose> is right–left–right–right. Changing carbon 4 alone gives right–left–left–right for D-<galactose>. Reversing every stereocenter would instead make an <enantiomer>; the single-position change is an <epimerization>.
Neither the formula nor an accurate mass measurement distinguishes these two sugars, because moving a <hydroxyl group> from one side of a stereocenter to the other changes no atom counts. Their different spatial arrangements nevertheless present different binding surfaces to an <enzyme>. This is a useful example of why carbohydrate identification needs structural information in addition to elemental composition. The D designation fixes the reference stereochemistry; it does not itself select a single cyclic anomer or state the sign of <optical rotation>.
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