Zacharias W, Martin J C, Wells R D
Biochemistry. 1983 May 10;22(10):2398-405. doi: 10.1021/bi00279a015.
Circular dichroism and laser Raman spectroscopy reveal that the synthetic DNA polymer (dG-dC)n X (dG-dC)n undergoes a cooperative transition induced by sodium acetate from a right-handed B-form to a left-handed Z-type conformation with a midpoint at 2.05 M. However, at concentrations only slightly higher than the end point of this transition (above approximately 2.2 M) and up to approximately 2.65 M, the Z-form is not stable in solution but aggregates to form highly condensed DNA. A manyfold increase of positive ellipticity in the range 340-250 nm is observed which is indicative of a psi (+)-type structure. At even higher concentrations (greater than or equal to 2.7 M), the Z-form is stable without condensation, and there is no change in the inverted CD spectrum. All structural transitions are reversible except that it is not possible to redissolve the highly condensed psi (+)-form by further increasing the salt concentration to greater than or equal to 2.7 M. The very high cooperativity of these transitions enables the DNA polymer to adopt three distinctly different structures (B-, Z-, and psi-forms) within a narrow range of sodium acetate concentration (approximately 200 mM). The Raman spectra of the condensed form and the Z-form in very concentrated sodium acetate show that the psi (+)-type state forms without substantial changes of the secondary conformation of the DNA. This indicates that the left-handed Z-helix of (dG-dC)n X (dG-dC)n can form psi-type aggregates with an ordered superstructure similar to those observed for natural right-handed DNA helices.
圆二色光谱和激光拉曼光谱显示，合成的DNA聚合物(dG-dC)n X (dG-dC)n在醋酸钠诱导下发生协同转变，从右手B型构象转变为左手Z型构象，中点浓度为2.05 M。然而，在仅略高于此转变终点的浓度下（约高于2.2 M），直至约2.65 M，Z型在溶液中不稳定，会聚集形成高度浓缩的DNA。在340 - 250 nm范围内观察到正椭圆率增加了许多倍，这表明存在psi(+)型结构。在更高浓度（大于或等于2.7 M）时，Z型稳定且无凝聚现象，反转圆二色光谱也无变化。除了通过进一步提高盐浓度至大于或等于2.7 M无法使高度浓缩的psi(+)型重新溶解外，所有结构转变都是可逆的。这些转变的极高协同性使DNA聚合物能够在狭窄的醋酸钠浓度范围（约200 mM）内呈现三种截然不同的结构（B型、Z型和psi型）。在非常浓的醋酸钠中浓缩形式和Z型的拉曼光谱表明，psi(+)型状态的形成并未使DNA二级构象发生实质性变化。这表明(dG-dC)n X (dG-dC)n的左手Z螺旋可以形成具有类似于天然右手DNA螺旋所观察到的有序超结构的psi型聚集体。
