Angelini T E, Liang H, Wriggers W, Wong G C L
Department of Materials Science & Engineering, University of Illinois at Urbana-Champaign, IL 61801, USA.
Eur Phys J E Soft Matter. 2005 Apr;16(4):389-400. doi: 10.1140/epje/i2004-10097-9.
Attractions between like-charged polyelectrolytes have been observed in a variety of systems (W.M. Gelbart, R.F. Bruinsma, P.A. Pincus, V.A. Parsegian, Phys. Today 53, September issue, 38 (2000)). Recent biological examples include DNA, filamentous viruses, and F-actin. Theoretical investigations on idealized systems indicate that counterion correlations play a central role, but no experiments that specifically probe such correlations have been performed. Using synchrotron X-ray diffraction, we have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well-defined biological polyelectrolyte) and found an unanticipated symmetry-breaking collective counterion mechanism for generating attractions. Surprisingly, the counterions do not form a lattice that simply follows actin's helical symmetry; rather, the counterions organize into "frozen" ripples parallel to the actin filaments and form structures reminiscent of charge density waves. Moreover, these 1D counterion charge density waves form a coupled mode with twist deformations of the oppositely charged actin filaments. This counterion organization is not sensitive to thermal fluctuations in temperature range accessible to protein-based polyelectrolyte systems. Moreover, the counterion density waves are "pinned" to the spatial periodicity of charges on the actin filament even if the global filament charge density is varied, indicating the importance of charge periodicity on the polyelectrolyte substrate.
在多种体系中均观察到了带相同电荷的聚电解质之间的吸引力（W.M. 格尔巴特、R.F. 布鲁因斯马、P.A. 平卡斯、V.A. 帕尔塞吉安，《今日物理》53卷，9月号，38页（2000年））。近期的生物学实例包括DNA、丝状病毒和F - 肌动蛋白。对理想化体系的理论研究表明，反离子相关性起着核心作用，但尚未进行专门探究此类相关性的实验。利用同步加速器X射线衍射，我们直接观察了多价离子在细胞骨架丝状肌动蛋白（一种明确的生物聚电解质）上的组织情况，并发现了一种意想不到的对称破缺集体反离子机制来产生吸引力。令人惊讶的是，反离子并未形成简单遵循肌动蛋白螺旋对称性的晶格；相反，反离子组织成与肌动蛋白丝平行的“冻结”波纹，并形成让人联想到电荷密度波的结构。此外，这些一维反离子电荷密度波与带相反电荷的肌动蛋白丝的扭曲变形形成耦合模式。这种反离子组织对基于蛋白质的聚电解质体系可及温度范围内的热涨落不敏感。而且，即使全局丝电荷密度发生变化，反离子密度波仍“固定”在肌动蛋白丝上电荷的空间周期性上，这表明聚电解质底物上电荷周期性的重要性。
