Westhof E, Sundaralingam M
Institut de Biologie Moleculaire et Cellulaire, Centre National de la Recherche Scientifique, Strasbourg, France.
J Biomol Struct Dyn. 1984 Aug;2(1):159-64. doi: 10.1080/07391102.1984.10507554.
The interaction of proflavine hemisulfate with the sodium salt of poly(rC-rA) in solution (unbuffered) yields an inverted (mirror-like) circular dichroism (CD) spectrum to that of the free poly(rC-rA). Simultaneously, an induced negative Cotton effect appears in the proflavine band region with a maximum at 467 nm and a slight shoulder at 420 nm. This observation may be explained as resulting from the formation of a poly(rC-rA).proflavine complex with the polynucleotide existing as a right-handed parallel chain duplex with the proflavine intercalated between the CpA sequence and not the ApC sequence. The intercalation geometry here is expected to be analogous to that found in the crystal structure of the dinucleotide CpA.proflavine complex (Westhof et al. J. Mol. Biol., 1981) which forms a miniature right-handed helix. Although normally an inverted spectra could be attributed to a reversal in the helix handedness, the similarity in the 31P nuclear magnetic resonance spectra between the free and proflavine bound poly(rC-rA) indicates that their handedness is the same. The inverted CD spectrum may be a result of the different stacking orientation between the intercalated proflavine and the A-A base-pair on one hand and the triply hydrogen bonded protonated C-C base-pair on the other.
硫酸原黄素半硫酸盐与溶液中（未缓冲）的聚（rC-rA）钠盐相互作用，产生与游离聚（rC-rA）相反（镜像）的圆二色性（CD）光谱。同时，在原黄素谱带区域出现诱导的负科顿效应，在467 nm处有最大值，在420 nm处有轻微肩峰。这一观察结果可以解释为形成了一种聚（rC-rA）·原黄素复合物，其中多核苷酸以右手平行链双链体形式存在，原黄素插入CpA序列而非ApC序列之间。这里的插入几何结构预计与二核苷酸CpA·原黄素复合物晶体结构中发现的结构类似（韦斯托夫等人，《分子生物学杂志》，1981年），该复合物形成一个微型右手螺旋。虽然通常情况下，反转光谱可归因于螺旋手性的反转，但游离的和与原黄素结合的聚（rC-rA）之间的31P核磁共振光谱相似性表明它们的手性是相同的。反转的CD光谱可能一方面是由于插入的原黄素与A-A碱基对之间的堆积方向不同，另一方面是由于三重氢键质子化的C-C碱基对之间的堆积方向不同。
