Olsthoorn C S, Bostelaar L J, De Rooij J F, Van Boom J H, Altona C
Eur J Biochem. 1981 Apr;115(2):309-21. doi: 10.1111/j.1432-1033.1981.tb05240.x.
The temperature dependence of the circular dichroism (CD) spectra of a series of deoxyadenylates (dA)n, n = 2, 3, 6, 9, 12, infinity, in aqueous solution was studied. The data were interpreted on the basis of a new conformational model for the stacked state suggested by our previous proton NMR studies on (dA)2 and (dA)3 [C. S. M. Olsthoorn, L. J. Bostelaar, J. H. van Boom & C. Altona (1980) Eur. J. biochem. 112, 95-110]. In this model the stacked regions of the single-stranded oligomers consist of residues taking up a geometry resembling that of the B-DNA genus of structures (all sugars S or C2'-endo) except those residues at the 3' end that do not 'feel' a following stacking interaction. The deoxyribose rings in the latter residues retain (or regain when melting out removes a stacking interaction somewhere along the chain) the conformational freedom (S in equilibrium N, N = C3'-endo) that these rings possess in the monomers 2'-deoxyadenosine 5'-methylphosphate or in 2'-deoxyadenosine 3',5'-bis(methylphosphate), as the case may be. It is shown that this model allows (a) construction of the CD spectra of (dA)n, n = 3, 6, 9, 12, from those of the dimer and the polymer; (b) the separation of the weak CD displayed by the regular S-S stacking mode and the far stronger CD exhibited by the 3'-end S-N stacking (the latter CD resembles that of the A-DNA genus of structures); (c) delineation of the thermodynamics of stacking. The melting temperature remains constant and independent of chain length (about 50 degrees C) whereas delta H degrees and delta S degrees show a slight increase in absolute values on increasing n from 2 to infinity owing to small cooperativity effects. Near 0 degrees C the dimer occurs for about 90% in the stacked form, the oligomers attain even higher conformational purities. It is suggested that premelting phenomena observed in the CD spectra of double-helical DNAs may also involve local transitions from the normal B-like ----S-S-s---- stacking mode to an A-like ----S-S-N---- stacking geometry.
研究了一系列脱氧腺苷酸(dA)n（n = 2、3、6、9、12、∞）在水溶液中的圆二色性(CD)光谱随温度的变化。根据我们之前对(dA)2和(dA)3的质子核磁共振研究提出的一种新的堆积态构象模型对数据进行了解释[C. S. M. Olsthoorn, L. J. Bostelaar, J. H. van Boom & C. Altona (1980) Eur. J. biochem. 112, 95 - 110]。在该模型中，单链寡聚物的堆积区域由占据类似于B - DNA结构类几何形状的残基组成（所有糖为S型或C2'-内型），除了3'端那些没有“感受到”后续堆积相互作用的残基。后一类残基中的脱氧核糖环保留（或当链上某处的堆积相互作用因解链而消除时重新获得）这些环在单体2'-脱氧腺苷5'-甲基磷酸酯或2'-脱氧腺苷3',5'-双（甲基磷酸酯）中所具有的构象自由度（S型与N型平衡，N = C3'-内型），视具体情况而定。结果表明，该模型允许：(a) 根据二聚体和聚合物的CD光谱构建(dA)n（n = 3、6、9、12）的CD光谱；(b) 区分由规则的S - S堆积模式显示的弱CD和由3'端S - N堆积表现出的强得多的CD（后者的CD类似于A - DNA结构类）；(c) 描绘堆积的热力学性质。解链温度保持恒定且与链长无关（约50℃），而ΔH°和ΔS°由于小的协同效应，在n从2增加到∞时绝对值略有增加。在0℃附近，二聚体约90%以堆积形式存在，寡聚物具有更高的构象纯度。有人提出，在双链DNA的CD光谱中观察到的预解链现象可能也涉及从正常的类似B型的----S - S - s----堆积模式到类似A型的----S - S - N----堆积几何形状的局部转变。
