The phenomenon where compounds have the same molecular formula but exist in different spatial forms that cannot readily interchange with each other.
From 'alloisomer' + '-ism' suffix; a chemistry term combining Greek 'allo-' (other) with the concept of isomerism established in 19th-century organic chemistry.
Alloisomerism is why some drugs work but their mirror-image versions don't—your body's proteins are like left-hand gloves, so a right-hand molecule just doesn't fit properly, even though chemically it's identical!
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