In chemistry, a molecule that has the same molecular formula and connectivity as another molecule but differs in the three-dimensional spatial arrangement of atoms in a way that isn't mirror images.
From Greek 'dia-' (different) + 'stereos' (solid/three-dimensional) + 'iso-' (equal) + 'meros' (part). The term emerged in 19th-century organic chemistry to distinguish from enantiomers (mirror-image isomers).
Diastereoisomers can have wildly different biological effects—one form might be a useful medicine while the other is toxic or inactive, which is why pharmaceutical companies care deeply about molecular geometry!
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