Kumar Gopinatha Suresh, Das Suman, Bhadra Kakali, Maiti Motilal
Biophysical Chemistry Laboratory, Indian Institute of Chemical Biology, 4, Raja S.C. Mullick Road, 700 032, Kolkata, India.
Bioorg Med Chem. 2003 Nov 17;11(23):4861-70. doi: 10.1016/j.bmc.2003.09.028.
The pH -induced structural changes on the conformation of homo- and hetero-polymers of guanosine-citydine (G.C) sequences were investigated using spectrophotometric and circular dichroic techniques. At pH 3.40, 10 mM [Na(+)] and 10 degrees C both polynucleotides adopted a unique and stable structural conformation different from their respective B-form structures. The protonated hetero-polymer is established as left-handed structure with Hoogsteen base pairing (H(L)-form) while the homo-polymer favored Watson-Crick base pairing with different stacking arrangements from that of B-form structure as evident from thermal melting and circular dichroic studies. The interaction of berberine, a naturally occurring protoberberine group of plant alkaloid, with the protonated structures was studied using various biophysical techniques. Binding of berberine to the H(L)-form structure resulted in intrinsic circular dichroic changes and generation of extrinsic circular dichroic bands with opposite sign and magnitude compared to its B-form structure while with the homo-polymer of G.C no such reversal of extrinsic circular dichroic bands was seen indicating different stacking arrangement of berberine at the interaction site. Scatchard analysis of the binding data, however, indicated non-cooperative binding to both the protonated forms similar to that of their respective B-form structure. Fluorescence spectral studies, on the other hand, showed remarkable increase in the intrinsic fluorescence of the alkaloid in presence of the protonated forms compared to their respective B-form structure. These results suggest that berberine could be used as a probe to detect the alteration of structural handedness due to protonation and may potentiate its use in regulatory roles for biological functions.
利用分光光度法和圆二色技术研究了鸟苷 - 胞苷（G.C）序列的同聚物和杂聚物构象在pH值诱导下的结构变化。在pH 3.40、10 mM [Na⁺] 和10℃条件下，两种多核苷酸均采用了一种独特且稳定的结构构象，不同于它们各自的B型结构。质子化的杂聚物被确定为具有Hoogsteen碱基配对的左手结构（H(L)型），而同聚物则倾向于Watson - Crick碱基配对，其堆积排列与B型结构不同，这从热变性和圆二色性研究中可以明显看出。使用各种生物物理技术研究了小檗碱（一种天然存在的原小檗碱类植物生物碱）与质子化结构的相互作用。小檗碱与H(L)型结构结合导致了内在圆二色性变化，并产生了与B型结构相比具有相反符号和大小的外在圆二色性带，而对于G.C同聚物，未观察到这种外在圆二色性带的反转，这表明小檗碱在相互作用位点的堆积排列不同。然而，结合数据的Scatchard分析表明，与各自的B型结构类似，小檗碱与两种质子化形式的结合均为非协同结合。另一方面，荧光光谱研究表明，与各自的B型结构相比，在质子化形式存在下生物碱的内在荧光显著增加。这些结果表明，小檗碱可作为一种探针来检测由于质子化引起的结构手性变化，并可能增强其在生物功能调节作用中的应用。
