Water-Ion-Biopolymer Interactions

组织功能特性产生于细胞和亚细胞长度尺度上的过程。因此，组织的物理行为（如渗透和机械性能、水合状态、电荷密度）必须在100nm以下的距离尺度上进行表征。了解聚电解质与离子的相互作用有助于阐明离子结合的基本物理学以及影响大量生物过程的物理机制。在生物学中，渗透压在调节和调解各种生理过程中起着特别重要的作用。为了帮助理解生物分子和生物分子组装中物理/化学相互作用的本质，我们开发了一种实验方法，通过结合宏观渗透膨胀压测量和小角散射测量，同时研究它们的结构（形态）和热力学性质作为长度尺度（空间分辨率）的函数。膨胀压力测量在大尺度范围内探测系统，从而提供有关整体热力学响应的信息。小角中子散射（SANS）和小角x射线散射（SAXS）使我们能够在自然环境中研究生物聚合物分子，并将环境条件（如离子浓度、离子价、pH值、温度）的变化与分子构象和渗透压等物理性质联系起来。SANS和SAXS同时提供了不同结构元素的大小及其各自对渗透性能的贡献的信息。结合这些测量，我们既可以将热力学浓度波动引起的散射强度与大型静态上层结构（如聚集体）散射的强度分开，也可以确定控制宏观热力学性质的长度尺度。这种热力学和结构信息是其他技术无法获得的。

Tissue functional properties arise from processes occurring at cellular and subcellular length scales. Therefore, the physical behavior of tissues (e.g., osmotic and mechanical properties, state of hydration, charge density) must all be characterized on distance scales below 100 nm. Understanding the interaction of polyelectrolytes with ions could help clarify the basic physics of ion binding as well as physical mechanisms affecting a large number of biological processes. In biology, osmotic pressure is particularly important in regulating and mediating various physiological processes. To help understand the nature of physical/chemical interactions in biomolecules and biomolecular assemblies, we have developed an experimental approach to study their structure (morphology) and thermodynamic properties simultaneously as a function of the length scale (spatial resolution) by combining macroscopic osmotic swelling pressure measurements and small-angle scattering measurements. Swelling pressure measurements probe the system in the large length scale range, thus providing information on the overall thermodynamic response. Small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) allow us to investigate biopolymer molecules in their natural environment and to correlate the changes in the environmental conditions (e.g., ion concentration, ion valance, pH, temperature) with physical properties such as molecular conformation and osmotic pressure. SANS and SAXS simultaneously provide information about the size of different structural elements and their respective contribution to the osmotic properties. Combining these measurements allows us both to separate the scattering intensity arising from thermodynamic concentration fluctuations from the intensity scattered by large static superstructures (e.g., aggregates), and to determine the length scales governing the macroscopic thermodynamic properties. This thermodynamic and structural information cannot be obtained by other techniques. Specifically, we have applied this approach to study the effect of multivalent cations, particularly calcium ions, on the structure of various model systems mimicking soft tissues. Divalent cations are ubiquitous in the biological milieu, yet existing theories do not adequately explain their effect on and interactions with charged macromolecules. Moreover, experiments to study these interactions are difficult to perform, particularly in solution, because above a low ion concentration threshold multivalent cations generally cause phase separation or precipitation of charged molecules. Since macroscopic phase separation does not occur in cross-linked gels, we have overcome this limitation by cross-linking our biopolymers, extending the range of ion concentrations over which the system remains stable and can be studied. In pilot studies, this new non-destructive procedure has been used to investigate cross-linked gels of a model synthetic polymer, polyacrylic acid, and different biopolymers such as DNA and hyaluronic acid to determine the size of the structural elements that contribute to the osmotic concentration fluctuations. We have combined SANS and SAXS to estimate the osmotic modulus of hyaluronic acid solutions in the presence of monovalent and divalent counterions. We studied the diffusion processes in these solutions by dynamic light scattering and determined the osmotic modulus from the relaxation response. We developed an experimental procedure to determine the distribution of counterions around charged biopolymer molecules using anomalous small-angle X-ray scattering measurements. We analyzed a series of simple models of network elasticity that address essential physical aspects of rigid chain biopolymer systems. The extracellular matrix of cartilage provides a good example of such a rigid chain biological network. We found that in compression (or indentation which is essentially a compressive process) strain stiffening was dominant. The knowledge of physical-chemical interactions among the macromolecular constituents of biological tissues is essential for understanding tissue properties and function. These interactions have important consequences for the macroscopic mechanical and osmotic properties of the tissue, such as the compressive resistance or load-bearing ability of cartilage. A better understanding of these interactions may contribute to a more complete model of cartilage biomechanical properties and may help to develop novel strategies for treatment of cartilage disease, such as osteoarthritis. We developed a novel method to quantify the stability and intracellular mode-of-action of DNA based nanoparticles. Atomic force microscopy reveals that these polyplexes are pathogen-like particles (having a size and shape resembling spherical viruses that naturally evolved to deliver nucleic acids to the cells). We found that optical-absorption measurements provide a useful determination of the structural stability, as well as biological activity, relevant to the ability of the nanoparticles to escape from the endosome and release the DNA at the nucleus. Salt, pH and temperature influence both the shelf-life and intracellular stability of the nanoparticles. This approach should facilitate the development of diverse polyplex nanomedicines, where the delivered DNA-expressed antigens induce immune responses against chronic diseases.