We modify and extend the recently developed statistcal mechanicla theory or chain molecules having noncovalent double-stranded conformationsw,as in RNA or single-stranded DNA,and beta sheets in protein,to the force-stretched case as in typical single-molecule experiment.The conformations of double-stranded regions of the molecules are calculated based on polymer graph-theoretic approach[S.-J.Chen and K.A.Dill,J.Chem.Phys.109,4602(1998)],while the unpaired single-stranded regions are treated as self-avoiding walks.Two classes of conformations-the hairpin conformations and RNA secondary structure-are explored.For the hairpin conformations,all possible end-to-end distances corresponding to the differetn types of double-stranded regions are enumerated exactly.For the RNA secondary structures,a recrusive formula incorporating the secondary structure and end-to-end distribution has been derived.The sequence dependence and excluded volume interaction are taken into account explicitly.Using the extended theory,we inverstigate the extension-force and force-extension curves,distribution of the extensions at fixes forces,and reentering phenomenon,respectively.We find that the mechanical behaviors of homogeneous chains of haripin conformations and secondary structures are quite different:the unfolding of the hairpin case is two state,while the unfolding of the latter is one stte.In addition,a reentering transition is observed in hairpin conformations.
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机译：我们修改并扩展了最近发展的统计力学理论或具有非共价双链构象的链分子（如RNA或单链DNA和蛋白质的β折叠），并扩展到了典型的单分子实验中的受力拉伸情况。基于聚合物图论方法计算分子的双链区的构象[S.-J.Chen and KADill，J.Chem.Phys.109,4602（1998）]，而未配对的单链区探索了两类构象：发夹构象和RNA二级结构。对于发夹构象，精确列举了与双链区的不同类型相对应的所有可能的端对端距离对于RNA二级结构，推导了一个包含二级结构和端到端分布的推导式。明确考虑了序列依赖性和排除的体积相互作用。在扩展理论上，我们分别对延伸力和力-延伸曲线进行了反演，分别对固定力处的延伸分布和再进入现象进行了反演。我们发现，哈利平构象和二级结构的均相链的力学行为有很大不同：发夹盒的展开是两种状态，而发夹盒的展开是一种状态。此外，在发夹构象中观察到重新进入的过渡。
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