Nordén B, Tjerneld F
Biophys Chem. 1976 Mar;4(2):191-8. doi: 10.1016/0301-4622(76)85009-0.
A stoichiometrical application of a sensitive method for linear dichroism (LD) detection is suggested for biochemical purposes. The complex formation between a binding site on a polynucleotide and a ligand may be studied with high precision if the following conditions are fulfilled: (1) The polymer can be given a fixed degree of orientation. (2) The site has a specific orientation with respect to the orientation axis of the polymer (e.g., intercalation). (3) The ligand has an anisotropic optical absorption property. The method was applied to studying the complex between DNA and ethidiumbromide, which was detected by LD with precision of +/- 0.5 X 10(-7) M in a 4 X 10(-4) M DNA solution, i.e., 0.1% occupation of the total site concentration can be detected. The complexation could be explained by a single type of site (n = 0.14 +/- 0.01 sites per nucleotide residue) and a stability constant K1 = (2.5 +/- 1) X 10(5) M-1 at 0.2 M ionic strength. From the specific LD an average angle 60 degrees was concluded between the helix axis and the long axis of the ethidiumbromide molecule. This value formally contradicts the Watson-Crick model or the intercalation model but may be explained by extension and deformation effects on the xhain by the flow.
为了生化研究目的，建议对线性二色性（LD）检测的灵敏方法进行化学计量学应用。如果满足以下条件，就可以高精度地研究多核苷酸上的结合位点与配体之间的复合物形成：（1）聚合物可以具有固定的取向度。（2）该位点相对于聚合物的取向轴具有特定的取向（例如，嵌入）。（3）配体具有各向异性的光吸收特性。该方法被应用于研究DNA与溴化乙锭之间的复合物，在4×10⁻⁴ M的DNA溶液中，通过LD检测，其精度为±0.5×10⁻⁷ M，即可以检测到占总位点浓度0.1%的情况。复合物的形成可以用单一类型的位点（每个核苷酸残基n = 0.14±0.01个位点）和在0.2 M离子强度下的稳定常数K1 =（2.5±1）×10⁵ M⁻¹来解释。从特定的LD可以推断出螺旋轴与溴化乙锭分子长轴之间的平均角度为60度。这个值在形式上与沃森-克里克模型或嵌入模型相矛盾，但可能是由于流动对链的延伸和变形效应来解释。
