Freier S M, Hill K O, Dewey T G, Marky L A, Breslauer K J, Turner D H
Biochemistry. 1981 Mar 17;20(6):1419-26. doi: 10.1021/bi00509a003.
The Raman laser temperature-jump technique has been used to measure the kinetics of the coil to helix reaction of poly(cytidylic acid) [poly(C)] in aqueous cosolvent mixtures. The rate of helix formation has a low activation energy and is proportional to reciprocal solvent viscosity. The observations suggest helix formation is rotationally diffusion controlled. The rate of coil formation in poly(C) has an activation energy of approximately 11 kcal/mol, presumably reflecting the electronic stacking interactions which stabilize the helix. Viscous cosolvents, glycerol or sucrose, slow down the rate of coil formation; acetonitrile and formamide at 5 mol % increase the rate relative to that in water. The polar cosolvents may specifically attack a cytosine stack. The absorbance vs. temperature profiles for poly(C) are analyzed with the one-dimensional Ising model. When only optical data are used, the cooperativity parameter, sigma, and the enthalpy, delta H, cannot be uniquely determined. A method is proposed that allows determination of sigma by combining spectroscopic and calorimetric data. The values of sigma derived for poly(C) are between 0.8 and 1.0, and delta H is about -9 kcal/mol of stack. An alternative method using integration of the excess heat capacity curve and extrapolation to fully stacked and random coil species yields a delta H of -7 kcal/mol of stack.
拉曼激光温度跃变技术已被用于测量聚（胞苷酸）[poly(C)]在水性共溶剂混合物中从无规线团到螺旋结构反应的动力学。螺旋结构形成的速率具有较低的活化能，并且与溶剂粘度的倒数成正比。这些观察结果表明螺旋结构的形成受旋转扩散控制。聚（C）中无规线团形成的速率具有约11千卡/摩尔的活化能，大概反映了稳定螺旋结构的电子堆积相互作用。粘性共溶剂甘油或蔗糖会减慢无规线团形成的速率；相对于在水中的速率，5摩尔%的乙腈和甲酰胺会提高其速率。极性共溶剂可能会特异性地攻击胞嘧啶堆积。用一维伊辛模型分析了聚（C）的吸光度与温度的关系曲线。当仅使用光学数据时，协同参数σ和焓ΔH不能唯一确定。提出了一种通过结合光谱和量热数据来确定σ的方法。聚（C）得到的σ值在0.8到1.0之间，ΔH约为-9千卡/摩尔堆积。另一种使用过剩热容曲线积分并外推到完全堆积和无规线团状态的方法得到的ΔH为-7千卡/摩尔堆积。
