title: Folding transition of a single semiflexible polyelectrolyte chain through toroidal bundling of loop structures.
abstract: We consider how the DNA coil-globule transition progresses via the formation of a toroidal ring structure. We formulate a theoretical model of this transition as a phenomenon in which an unstable single loop generated as a result of thermal fluctuation is stabilized through association with other loops along a polyelectrolyte chain. An essential property of the chain under consideration is that it follows a wormlike chain model. A toroidal bundle of loop structures is characterized by a radius and a winding number. The statistical properties of such a chain are discussed in terms of the free energy as a function of the fraction of unfolded segments. We also present an actual experimental observation of the coil-globule transition of single giant DNA molecules, T4 DNA (165.5 kbp), with spermidine (3+), where intrachain phase segregation appears at a NaCl concentration of more than 10 mM. Both the theory and experiments lead to two important points. First, the transition from a partially folded state to a completely folded state has the characteristics of a continuous transition, while the transition from an unfolded state to a folded state has the characteristics of a first-order phase transition. Second, the appearance of a partially folded structure requires a folded structure to be less densely packed than in the fully folded compact state.
authors: Okayama Institute for Quantum Physics, Kyoyama 1-9-1, Okayama 700-0015, Japan. takafumi_iwaki@pref.okayama.jp
authors: Takafumi
authors: T
authors: Iwaki
authors: Naoko
authors: N
authors: Makita
authors: Kenichi
authors: K
authors: Yoshikawa
mesh_terms: D004247:DNA / Q000737:chemistry* / Q000378:metabolism*; D004573:Electrolytes / Q000737:chemistry* / Q000378:metabolism*; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D011108:Polymers / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D004573:Electrolytes; D011108:Polymers; D004247:DNA