生命体系中极性逆转与同性相吸的物理机制研究

Adsorption of polyelectrolytes on charged surfaces plays a key role in numerous technological applications and is also relevant to many biological and biomedical processes. Despite notable achievements in experimental and theoretical aspects for polyelectrolyte adsorption at charged interfaces, much remains to be disclosed on the molecular mechanisms underlying both surface charge inversion due to the interfacial adsorption of oppositely charged polyelectrolytes and the counterintuitive attraction between polyelectrolytes and a like-charged interface. The objective of the proposed project is to investigate the nature of single-stranded DNA adsorption onto both oppositely and likely charged surfaces within the context of biological systems by means of computer simulations, in an attempt to reveal the physical origins of surface charge inversion and like-charge attraction from the points of view of both charge and spatial correlations. Our main focus is to develop a much deeper understanding of how DNA interacts with biological interfaces on the coarse-graining scales by monitoring the monomer trait of the DNA molecule, dielectric mismatch between different phases, electrolyte concentration, counterion size and valency as well as shape, the character of surface charge distribution, and the strength of electrostatic coupling as well. The obtained results will provide novel insights that have significant implications for guiding future physical experiments and theoretical developments and be vital to elucidate biological processes at cell surfaces as well. At the same time, the progress in this direction will yield new clues for applications in gene delivery and biotechnology as well as be potentially used to demonstrate possible routes for controlling polyelectrolyte layer-by-layer self-assembly.

聚电解质在带电界面上的吸附不仅对许多的技术应用起着至关重要的作用，而且也密切相关于大量的生物及生物医学过程。尽管在该领域内理论和实验方面取得了显著进展，但是由聚电解质凝聚导致的界面有效电荷极性逆转和同种电荷带电体之间反常吸引现象的驱动机制还远没有被国内外学者所理解。本项目拟在前期工作的基础上，利用计算机模拟详细研究生理条件下单链DNA与生物界面相互作用的物理本质，通过探索DNA单体特性、界面介电常数失配、电解液浓度、补偿离子形状、尺寸与化合价、表面电荷分布特点以及静电耦合强度等因素的组合影响，力图从电荷关联（能量）和空间关联（熵）的角度系统地揭示极性逆转与同性相吸的物理根源。相关成果不仅有望用于指导未来的物理实验和理论发展，也必将有助于深入理解细胞表面重要的生物过程，而且可为基因输运及生物技术应用提供新的线索，亦可为聚电解质逐层自组装展示潜在的控制途径。

带电聚合物广泛用于修饰衬底表面或胶体颗粒悬浮液的性质。由此产生的聚电解质和带电表面之间的相互作用在胶体界面科学领域的诸多实际应用中起着重要作用。尽管如此，但许多问题仍然悬而未决，特别是聚电解质在类电荷表面上的反常吸附和聚电解质吸附反转衬底表面电荷极性的分子起源一直饱受争议。..本项目研究了生命体系中单链DNA分子与带电膜表面之间相互作用引起的同性相吸和极性逆转两种反常现象的物理机制。基于粗粒化模型的蒙特卡洛模拟：生物聚合物吸附在类电荷均匀带电表面上需要溶液中多价补偿离子的辅助，这类同性相吸现象是由衬底表面上吸附的多价补偿离子以及聚电解质电荷单体与多价补偿离子之间的离子配对所引起。另外，即便溶液中不涉及多价补偿离子，聚电解质也能吸附在带有正负电荷混合基团的类电荷衬底表面，这类同性相吸现象要求聚电解质具有高度的柔韧性来响应表面电荷的分布特点。相反，即使在溶液中存在多价离子的情况下，粗粒化模拟结果表明：纯粹的静电关联效应不会由于吸附的反电荷聚电解质而引起衬底表面电荷的极性发生改变。然而，在考虑溶液中仅含有生理盐分的条件下，由于多价补偿离子与界面离散基团之间的静电耦合，表面电荷的极性不仅能发生改变，而且其强度在很大程度上取决于界面两相之间介电常数的失配度和溶液中微观离子尺寸引起的熵效应。这些发现不仅可为实验物理研究提供互补的微观视角，而且能为发展理论模型奠定基础。..考虑到碱基是DNA的基本组成部分，迄今已有许多研究强调了衬底表面与碱基相互作用的重要性，以广泛探索其绑定机制、自组装特性和生物传感应用。为此，寻找碱基吸附研究的新材料在胶体界面科学领域具有持续的兴趣和科学意义。本项目拓展了相关的研究内容，运用密度泛函理论和从头分子动力学模拟探索了几种新材料的电磁特性，这为课题组日后研究DNA碱基在二维材料薄片上的吸收和发射性能提供了很好的前期基础。