手性Chirality (chemistry)

分子没有任何反轴 Sₙ（包括镜面 σ = S₁ 和对称中心 i = S₂）时才是手性的，因此手性分子只属于 Cₙ、Dₙ、T、O、I 点群。最常见的来源是连有四个不同基团的手性碳，但轴手性（联芳烃、丙二烯）、面手性和螺旋手性的分子并不含手性中心。含多个手性中心的分子若有内部镜面则为内消旋，不具手性。

A molecule is chiral only if it has no improper axis Sₙ, which includes mirror planes (σ = S₁) and inversion centres (i = S₂), so chiral molecules belong only to the Cₙ, Dₙ, T, O and I point groups. The commonest source is a carbon bearing four different groups, but molecules with axial (biaryls, allenes), planar or helical chirality need no stereocentre. A molecule with several stereocentres and an internal mirror plane is meso and achiral.

1815 年毕奥发现某些有机物溶液能使偏振光旋转。1848 年巴斯德在显微镜下用镊子把外消旋酒石酸钠铵晶体分成左右两种，各自溶液旋光方向相反，首次揭示分子的“不对称性”。1874 年范特霍夫与勒贝尔独立提出碳的四面体构型，解释了光学异构。“手性”一词由开尔文勋爵在 1894 年引入。

In 1815 Biot found that solutions of some organic substances rotate polarised light. In 1848 Pasteur used tweezers under a microscope to sort crystals of racemic sodium ammonium tartrate into two mirror-image forms whose solutions rotated light in opposite directions, the first evidence of molecular dissymmetry. In 1874 van 't Hoff and Le Bel independently proposed the tetrahedral carbon, explaining optical isomerism. Lord Kelvin introduced the word chirality in 1894.

绝对构型用 CIP 规则标为 R 或 S；糖和氨基酸常沿用费歇尔的 D/L 标记；旋光方向用 (+)/(−) 表示，它与 R/S 没有必然联系。1951 年比伊沃特用 X 射线反常散射首次测定了酒石酸的绝对构型，证实费歇尔当年的任意假定恰好正确。

Absolute configuration is labelled R or S by the CIP rules; sugars and amino acids still often use Fischer's D/L labels; the sign of rotation is written (+) or (−) and has no fixed relation to R/S. In 1951 Bijvoet used anomalous X-ray scattering to determine the absolute configuration of tartaric acid for the first time, showing that Fischer's arbitrary assumption had been right.

生物体几乎只使用 L-氨基酸和 D-糖，这种“同手性”的起源仍是研究课题。药物的两个对映体活性可能完全不同：沙利度胺的悲剧促使监管机构要求分别评价对映体。不对称催化（2001 年诺尔斯、野依良治、夏普莱斯获诺贝尔奖）和有机小分子催化（2021 年李斯特、麦克米伦获奖）让单一对映体药物的工业生产成为常态。

Living things use almost exclusively L-amino acids and D-sugars, and the origin of this homochirality is still debated. The two enantiomers of a drug can act quite differently; the thalidomide tragedy led regulators to require each enantiomer to be evaluated. Asymmetric catalysis (Nobel Prize 2001 to Knowles, Noyori and Sharpless) and organocatalysis (2021 to List and MacMillan) have made single-enantiomer drug manufacture routine.