圆二色光谱Circular dichroism

手性分子的电子跃迁同时具有电偶极和磁偶极矩分量，导致对两种圆偏振光的吸收不同。CD 信号可正可负，对映体的 CD 谱互为镜像。在吸收带附近，旋光度随波长的异常变化称为科顿效应，CD 与旋光色散（ORD）通过克拉默斯-克勒尼希关系相联系。常用单位为摩尔椭圆度 [θ]（deg·cm²·dmol⁻¹）或 Δε。

Electronic transitions of chiral molecules have both electric- and magnetic-dipole components, so the two circular polarisations are absorbed differently. CD signals can be positive or negative, and enantiomers give mirror-image spectra. The anomalous change of optical rotation with wavelength near an absorption band is the Cotton effect, and CD is related to optical rotatory dispersion (ORD) through the Kramers–Kronig relations. Results are reported as molar ellipticity [θ] in deg·cm²·dmol⁻¹ or as Δε.

1847 年海丁格在紫水晶中观察到圆二色性。1895 年法国物理学家艾梅·科顿在酒石酸铜和铬溶液中系统研究了圆二色性和旋光色散的异常（科顿效应）。1960 年代商用 CD 光谱仪出现，此后在生物化学中广泛应用。

Haidinger observed circular dichroism in amethyst in 1847. In 1895 the French physicist Aimé Cotton studied CD and anomalous optical rotatory dispersion systematically in solutions of copper and chromium tartrates, the Cotton effect. Commercial CD spectrometers appeared in the 1960s and spread widely in biochemistry.

远紫外 CD（190–250 nm）用于快速估计蛋白质二级结构：α 螺旋在 208 和 222 nm 处有负峰、193 nm 处有正峰，β 折叠在约 218 nm 处有负峰；近紫外 CD 反映芳香氨基酸环境和三级结构；还用于监测蛋白质热变性、配体结合，测定手性药物和天然产物的绝对构型（结合量子化学计算的电子圆二色 ECD 和振动圆二色 VCD），以及 DNA 构象（B 型、Z 型）。

Far-UV CD (190–250 nm) quickly estimates protein secondary structure: α-helices give negative bands at 208 and 222 nm and a positive band at 193 nm, and β-sheets a negative band near 218 nm; near-UV CD reflects aromatic side-chain environments and tertiary structure. CD also monitors thermal unfolding and ligand binding, establishes the absolute configuration of chiral drugs and natural products (electronic CD and vibrational CD combined with quantum calculation), and distinguishes DNA conformations such as B and Z.