Department of Biology, Johns Hopkins University, Baltimore, Maryland.
Biophysics Laboratories, School of Biology, University of Portsmouth, Portsmouth, United Kingdom.
Biophys J. 2018 Jan 9;114(1):15-20. doi: 10.1016/j.bpj.2017.11.003.
Investigation of folding/unfolding DNA duplexes of various size and composition by superprecise calorimetry has revised several long-held beliefs concerning the forces responsible for the formation of the double helix. It was established that: 1) the enthalpy and the entropy of duplex unfolding are temperature dependent, increasing with temperature rise and having the same heat capacity increment for CG and AT pairs; 2) the enthalpy of AT melting is greater than that of the CG pair, so the stabilizing effect of the CG pair in comparison with AT results not from its larger enthalpic contribution (as expected from its extra hydrogen bond), but from the larger entropic contribution of the AT pair that results from its ability to fix ordered water in the minor groove and release it upon duplex unfolding; 3) the translation entropy, resulting from the appearance of a new kinetic unit on duplex dissociation, determines the dependence of duplex stability on its length and its concentration (it is an order-of-magnitude smaller than predicted from the statistical mechanics of gases and is fully expressed by the stoichiometric correction term); 4) changes in duplex stability on reshuffling the sequence (the "nearest-neighbor effect") result from the immobilized water molecules fixed by AT pairs in the minor groove; and 5) the evaluated thermodynamic components permit a quantitative expression of DNA duplex stability.
通过超精密量热法研究各种大小和组成的折叠/展开 DNA 双链，修正了几个关于形成双螺旋的作用力的长期存在的观念。研究结果表明：1）双链展开的焓和熵随温度升高而增加，CG 和 AT 对的热容增量相同；2）AT 熔解的焓大于 CG 对，因此 CG 对与 AT 相比的稳定性增强并非来自其更大的焓贡献（如预期的那样，由于其额外的氢键），而是来自 AT 对的更大的熵贡献，因为它能够固定在小沟中的有序水，而在双链展开时释放它；3）双链解离时出现新的动力学单元导致的翻译熵决定了双链稳定性与其长度和浓度的依赖性（它比气体统计力学预测的小一个数量级，并且完全由化学计量学校正项表示）；4）通过改变序列重排（“最近邻效应”）改变双链稳定性的原因是 AT 对在小沟中固定的固定水分子；5）评估的热力学分量允许对 DNA 双链稳定性进行定量表达。
