Wu H M, Dattagupta N, Crothers D M
Proc Natl Acad Sci U S A. 1981 Nov;78(11):6808-11. doi: 10.1073/pnas.78.11.6808.
We report transient electric dichroism studies of short double-helical DNA and poly[d(G-C)] fragments in alcohol/water mixtures. The limiting reduced dichroism and the rotational correlation time changed abruptly in the alcohol concentration range expected for the DNA B-to-A and poly[d(G-C)] B-to-Z transitions. The Z form of poly[d(G-C)] was also induced by mitomycin C crosslinking in aqueous solution. The rotational correlation times observed for A- and Z-DNA were approximately consistent with dimensions determined by crystallographic and fiber diffraction analysis: the estimated rise per base pair was 2.8 A for A-DNA and 3.7 A for Z-DNA in solution. In addition, the observed limiting reduced dichroism values for A- and Z-DNA were close to the theoretical limit of -1.5, requiring a structure in which the base transition moments are effectively perpendicular to the double-helix axis. This is the result expected for any DNA double helix having dyad symmetry in which the base pairs are flat and the base transition moments lie predominantly in the short axis of the base pair and therefore close to a helix dyad axis. Only B-DNA deviates from this rule, strongly reinforcing our earlier conclusion that the base pairs in B-DNA are not flat but are propeller twisted, either statically or as a dynamic average.
我们报告了在酒精/水混合物中短双链DNA和聚[d(G-C)]片段的瞬态电二色性研究。在预期DNA从B型到A型以及聚[d(G-C)]从B型到Z型转变的酒精浓度范围内，极限还原二色性和旋转相关时间发生了突然变化。聚[d(G-C)]的Z型也可通过丝裂霉素C在水溶液中的交联诱导产生。观察到的A-DNA和Z-DNA的旋转相关时间与通过晶体学和纤维衍射分析确定的尺寸大致一致：溶液中A-DNA每碱基对的估计上升高度为2.8 Å，Z-DNA为3.7 Å。此外，观察到的A-DNA和Z-DNA的极限还原二色性值接近理论极限-1.5，这需要一种碱基跃迁矩有效地垂直于双螺旋轴的结构。对于具有二元对称的任何DNA双螺旋，这都是预期的结果，其中碱基对是扁平的，碱基跃迁矩主要位于碱基对的短轴上，因此接近螺旋二元轴。只有B-DNA偏离了这一规则，有力地强化了我们早期的结论，即B-DNA中的碱基对不是扁平的，而是呈螺旋桨扭曲，无论是静态的还是动态平均的。
