Podgornik R, Strey H H, Gawrisch K, Rau D C, Rupprecht A, Parsegian V A
Laboratory of Structural Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
Proc Natl Acad Sci U S A. 1996 Apr 30;93(9):4261-6. doi: 10.1073/pnas.93.9.4261.
By equilibrating condensed DNA arrays against reservoirs of known osmotic stress and examining them with several structural probes, it has been possible to achieve a detailed thermodynamic and structural characterization of the change between two distinct regions on the liquid-crystalline phase diagram: (i) a higher density hexagonally packed region with long-range bond orientational order in the plane perpendicular to the average molecular direction and (ii) a lower density cholesteric region with fluid-like positional order. X-ray scattering on highly ordered DNA arrays at high density and with the helical axis oriented parallel to the incoming beam showed a sixfold azimuthal modulation of the first-order diffraction peak that reflects the macroscopic bond-orientational order. Transition to the less-dense cholesteric phase through osmotically controlled swelling shows the loss of this bond orientational order, which had been expected from the change in optical birefringence patterns and which is consistent with a rapid onset of molecular positional disorder. This change in order was previously inferred from intermolecular force measurements and is now confirmed by 31P NMR. Controlled reversible swelling and compaction under osmotic stress, spanning a range of densities between approximately 120 mg/ml to approximately 600 mg/ml, allow measurement of the free-energy changes throughout each phase and at the phase transition, essential information for theories of liquid-crystalline states.
通过将凝聚的DNA阵列与已知渗透压的储库平衡，并使用几种结构探针进行检测，得以对液晶相图上两个不同区域之间的变化进行详细的热力学和结构表征：（i）在垂直于平均分子方向的平面内具有长程键取向有序的较高密度六方堆积区域，以及（ii）具有类流体位置有序的较低密度胆甾相区域。在高密度且螺旋轴平行于入射束的高度有序DNA阵列上进行的X射线散射显示，一阶衍射峰存在六重方位调制，这反映了宏观键取向有序。通过渗透压控制的膨胀转变为密度较低的胆甾相，显示出这种键取向有序的丧失，这从光学双折射图案的变化中可以预期，并且与分子位置无序的迅速出现一致。这种有序性的变化先前是从分子间力测量中推断出来的，现在通过31P NMR得到了证实。在渗透压下进行可控的可逆膨胀和压实，密度范围在约120 mg/ml至约600 mg/ml之间，使得能够测量每个相以及相变时的自由能变化，这是液晶态理论的重要信息。
