Ramaswamy N, Bansal M, Gupta G, Sasisekharan V
Proc Natl Acad Sci U S A. 1982 Oct;79(20):6109-13. doi: 10.1073/pnas.79.20.6109.
Earlier, we showed that, for the D form (n = 8 and h = 3.03 A, where n is number of nucleotide units per turn and h is height per nucleotide unit) of poly[d(A-T)], both right- and left-handed double helical models are stereochemically satisfactory and give good agreement with the observed fiber diffraction data. It was also noted that the conformations of the right- and left-handed D-DNA models are very similar to those of the right- and left-handed B-DNA models. This observation was consistent with the D leads to B transition in the solid phase. As a continuation of our earlier studies, we have carried out similar experiments with poly[d(I-C)]. We could obtain a crystalline D-form pattern (n = 8, h = 3.13 A) of the fiber at 75% relative humidity (r.h.); the hydrated (r.h. approximately equal to 95%) form of the same fiber gave the classical B-form pattern (n = 10, h = 3.40 A). In the present report, we show that both right- and left-handed double-helical models are consistent with the fiber diffraction data of poly[d(I-C)] in the D-form. Theoretical energy calculations also suggest that the right- and left-handed B- and D-DNA models are almost equally stable. Hence, we conclude that the right- and left-handed double-helical models of poly[d(I-C)] in a given form (B or D) are equally likely and that the fiber diffraction data do not permit discrimination.
之前，我们表明，对于聚[d(A-T)]的D型（n = 8且h = 3.03 Å，其中n是每圈核苷酸单元的数量，h是每个核苷酸单元的高度），右手和左手双螺旋模型在立体化学上都是令人满意的，并且与观察到的纤维衍射数据吻合良好。还注意到右手和左手D-DNA模型的构象与右手和左手B-DNA模型的构象非常相似。这一观察结果与固相中的D向B转变是一致的。作为我们早期研究的延续，我们用聚[d(I-C)]进行了类似的实验。在75%相对湿度（r.h.）下，我们可以获得该纤维的结晶D型图案（n = 8，h = 3.13 Å）；同一纤维的水合形式（r.h.约等于95%）给出了经典的B型图案（n = 10，h = 3.40 Å）。在本报告中，我们表明右手和左手双螺旋模型都与聚[d(I-C)]的D型纤维衍射数据一致。理论能量计算也表明右手和左手B型及D型DNA模型几乎同样稳定。因此，我们得出结论，聚[d(I-C)]在给定形式（B或D）下的右手和左手双螺旋模型同样可能，并且纤维衍射数据无法进行区分。
