THE ORIGIN OF THE A TO B TRANSITION IN DNA FIBERS AND FILMS
THE ORIGIN OF THE A TO B TRANSITION IN DNA FIBERS AND FILMS
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DOI：
10.1002/bip.360270610
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发表时间：
1988-06-01
期刊：
影响因子：
2.9
通讯作者：
RUPPRECHT, A
中科院分区：
文献类型：
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作者：
LINDSAY, SM;LEE, SA;RUPPRECHT, A
我们研究了Na-DNA和Li-DNA纤维和薄膜的水合作用，测量了水含量、X射线纤维衍射图案、低频拉曼光谱（低于100 cm-1）、高频拉曼光谱（600-1000 cm-1）和溶胀，作为相对湿度的函数。大多数样品在一天内达到重量平衡（尽管不是构象平衡）。当DNA水合时，碱基对所占据的体积（从X射线和溶胀数据获得）显示出在A型发生的区域中Na-DNA的情况下的异常。我们的拉曼和X-射线再现了已知的已建立的构象转变的特征，但是我们在拉曼光谱和光学性质中发现了在低于40%相对湿度的Na-DNA中可能是无序B样构象的转变的证据。我们研究了结晶度对A到B转变的影响。我们发现，过渡到B-型是阻碍在高度结晶的样品。在大多数样品中，转变发生在三天内（将样品置于92%相对湿度后），但在高度结晶的样品中，转变可能需要数月。通过比较高度有序和无序的薄膜的高频拉曼光谱，我们表明，结晶度的程度控制的A-DNA形成的量时，乙醇被用来包裹的薄膜。我们表明，快速脱水（激光加热）不会导致B到A的过渡。产生A型X射线反射的纤维可能在非晶区含有类B物质。在Na-和Li-DNA中，低频拉曼光谱由约25 cm-1处的带主导。另一条带在Na-DNA中35 cm-1附近，在湿度下样品为A型。与以前的报道相比，我们发现，拉曼强度不依赖于相对于散射矢量的纤维取向。“35 cm-1”带在很大程度上是去极化的（即垂直极化入射和水平极化散射，VH，或反之亦然，HV），而“25 cm-1”带出现在VV、VH和HV极化中。这些谱带在HH偏振中都较弱。“25-cm-1”条带可能是由于磷酸盐及其相关抗衡离子的剪切运动，而“35-cm-1”条带可能是A-DNA微晶的特征。我们认为质量加载，弛豫耦合到水化壳层，软化原子间的相互作用势可能的解释所观察到的软化的低频拉曼频带上的水化。弛豫数据表明，所添加的水结合紧密（在这些时间尺度上）和质量加载模型占所观察到的软化相当好。我们的结论是，A到B转变不是由“25 cm-1”带的软化驱动的。相反，它很可能是晶体堆积力的结果，当这些力很强时，晶体中更规则的A型更受欢迎。
We have studied the hydration of Na-DNA and Li-DNA fibers and films, measuring water contents, x-ray fiber diffraction patterns, low-frequency Raman spectra (below 100 cm-1), high-frequency Raman spectra (600-1000 cm-1), and swelling, as a function of relative humidity. Most samples gain weight equilibrium (though not conformational equilibrium) in one day. The volume occupied by a base pair as the DNA is hydrated (obtained from the x-ray and swelling data) shows anomalies for the case of Na-DNA in the region where the A-form occurs. Our Raman and x-ray reproduce the well-known features of the established conformational transitions, but we find evidence in the Raman spectra and optical properties of a transition to what may be a disorder B-like conformation in Na-DNA below 40% relative humidity. We have studied the effects of crystallinity on the A to B transition. We find that the transition to the B-form is impeded in highly crystalline samples. In most samples, the transition occurs in three days (after putting the sample at 92% relative humidity) but in highly crystalline samples, the transition may take months. By comparing the high-frequency raman spectra of highly ordered and disordered films, we show that the extent of crystallinity controls the amount of A-DNA formed when ethanol is used to dehydrate the films. We show that rapid dehydration (by laser heating) does not result in a B to A transition. A fiber that gives A-type x-ray reflections probably contains B-like material in noncrystalline regions. The low-frequency Raman spectrum is dominated by a band at about 25 cm-1 in both Na- and Li-DNA. Another band is seen near 35 cm-1 in Na-DNA at humidities where the sample is in the A-form. In contrast to earlier reports, we find that the Raman intensity does not depend on fiber orientation relative to the scattering vector. The "35 cm-1" band is largely depolarized (i.e. vertical polarization incident and horizontal polarization scattered, VH, or vice versa, HV) while the "25-cm-1" band appears in both VV, VH and HV polarizations. These bands are all weaker in HH polarization. The "25-cm-1" band may be due to a shearing motion of the phosphates and their associated counterions, while the "35-cm-1" band may be characteristic of A-DNA crystallites. We consider mass-loading, relaxational coupling to the hydration shell, and softening of interatomic potentials as possible explanations of the observed softening of the low-frequency Raman bands on hydration. Relaxation data suggest that the added water binds tightly (on these time scales) and a mass-loading model accounts for the observed softening rather well. We conclude that the A to B transition is not driven by softening of the "25-cm-1" band. Rather, it is most probably a consequence of crystal-packing forces, with the more regular A-form favored in crystals when these forces are strong.