1. Denoting a mixture of optically active compounds that is itself optically inactive, being composed of an equal number of dextro- and levorotatory substances, which are separable. Those compounds chracterized by two or more chiral centers and internal compensation (an internal line of symmetry), and are therefore not separable into d or l (or + and −) forms, receive the prefix meso-) (meso-cystine).
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