A Toroidally Folded Monomolecular DNA Chain : From the perspective of colloidal physics(Poster session 2, New Frontiers in Colloidal Physics : A Bridge between Micro- and Macroscopic Concepts in Soft Matter)

Abstract

DNAは荷電高分子として振る舞い静電相互作用を通して溶液中の様々なイオンとの間に複合体を作っている。溶液条件によってはこのような複合体は分子全体を覆いDNAのセグメント同士が凝集したグロビュールと呼ばれる構造を作ることがある。T4DNAの場合、典型的なグロビュールはトロイド構造をしており、このことから、グロビュールへの折りたたみ転移は分子鎖上でのループ形成とループ構造同士の凝集という二つの過程を通して理解できる。本発表では折りたたみ転移の平衡論における一般論を展開し、また、wormlike chainモデルに基づいてトロイド構造の持つべき自由エネルギーを導く。これらより得られた理論的予想は実際の実験におけるDNAの折りたたみ転移挙動をよく記述するものとなっている。

DNA is a double-helical polyelectrolyte molecule that has an entire length ranging from several nanometers to centhneters. In principle, DNA is a very stiff molecule, and the chain shorter than its persistence length (≃50nm) behaves as a stiff rod. On the other hand, most of natural DNA molecules have the sizes much longer than this length, and they exhibit a nature of soft matter even in the level of a monomolecular chain. For example, a single T4 DNA (57μm) chain undergoes the transition from a fluctuating coiled state to a compact condensed state (a coil-globule transition) in the presence of various ionic species. Such a compact state is, after all, an aggregate of a number of DNA segments that are chemically linked with each other one-dimentionally. Typically, a condensed structure of T4 DNA exhibits a toroidally folded ring structure. In this case, the coil-globule transition of DNA may be characterized by the following two processes; the formation of loops and their aggregation along a single chain.