Chirality and Entropy in Self-Assembly of Biopolymers

学术价值：该提案有四个实验目标，围绕着共同的主题--熵和手性，以及它们如何影响生物聚合物之间的相互作用和自组装。第一个目标将集中在测量一对肌动蛋白细丝之间的介观电势，这些肌动蛋白细丝部分地由熵(耗尽)力保持在一起。目标是确定熵涨落对细丝之间引力的贡献。将使用两种替代方法。在一种方法中，使用激光镊子将一对细丝拉开，并结合荧光显微镜同时对系统进行成像。吸引力可以直接从这个实验中提取出来。另一种方法是分析孤立细丝的图像，这些细丝以发夹的方式折叠起来。粘合能是通过认识到它与存储在折叠细丝中的弹性能量平衡来确定发夹环的大小来确定的。第二个目的是研究单分散非棒状病毒自组装成胶体膜的现象，这是最近在PI的实验室观察到的现象。这些病毒颗粒不表现出大多数膜形成者的两极化头尾亲水性特征，因此假设这种膜组装过程是由耗竭力驱动的。第三个实验考察了手性对使用手性和非手性丝状病毒形成胶体膜和扭曲带的影响。这项工作探索了一种假设，即条带的形成是由组成棒的分子手性驱动的。最后一个项目考察了棒状粒子在中等棒密度下从排除体积驱动转变为向列相的理论的扩展。在这里，计划研究手性在棒中的作用，以解释在这些系统中经常观察到的手性向列相(胆甾相)。在所有这些研究中，主要的实验工具是多模显微镜，它可以在不同的干涉、对比度和荧光模式下获取图像。它与激光镊子结合在一起，对胶体微珠或其他高折射率结构施加力，并在微秒级以纳米精度确定微珠的位置。BROADER影响：该提议具有高度跨学科的性质，将为组装复杂的生物材料提供新的设计原则。该研究所正在开发一门新的定量生物仪器高级实验室课程，该课程将专注于现代显微镜的设计和应用。这门课程也是Brandeis更大的努力的核心，部分由Howard Hughes医学研究所和NSF-IGERT赠款支持，以建立一个跨学科的定量生物学研究生课程。该项目将为培养两名研究生提供平台。该奖项还包括每年资助两名本科生参与该项目的资金。

INTELLECTUAL MERIT: The proposal has four experimental aims organized around the common themes of entropy and chirality and how these affect interactions between, and self-assembly of, biopolymers. The first aim will focus on measuring the mesoscopic potential between a pair of actin filaments held together in part by entropic (depletion) forces. The goal is to determine the contribution of entropic fluctuations to the attraction between the filaments. Two alternative approaches will be used. In one, a laser tweezer is used to pull a pair of filaments apart in combination with a fluorescence microscope to image the system simultaneously. The attractive forces can be extracted directly from this experiment. An alternative approach analyzes images of isolated filaments which fold on themselves in hairpin fashion. The adhesion energy is determined by recognizing that it is balanced against the elastic energy stored in the folded filament to determine the size of the hairpin loop. The second aim is to study the self-assembly of monodisperse non-rodlike viruses into colloidal membranes, a phenomenon recently observed in the PI's laboratory. These virus particles do not exhibit the polarized head-tail amphilicity characteristic of most membrane formers, and this membrane assembly process is thus hypothesized to be driven by depletion forces. The third experiment examines the effects of chirality on the formation of colloidal membranes and twisted ribbons using both chiral and achiral filamentous viruses. This work explores the hypothesis that formation of ribbons is driven by the molecular chirality of the constituent rods. The final project examines extensions of the theories of the excluded-volume-driven transition of rodlike particles to a nematic phase at modest rod densities. Here it is planned to examine the role of chirality in the rods to explain the observed chiral nematic (cholesteric) phase often observed in these systems. In all of these studies the primary experimental tool is a multimode microscope that can acquire images in differential interference contrast and fluorescence modes. This is coupled with a laser tweezer to exert force on colloidal beads or other structures with high refractive index and determine the position of the beads with nanometer precision on a microsecond timescale.BROADER IMPACTS: The proposal is highly interdisciplinary in nature and will provide new design principles for assembling complex biomaterials. The PI is developing a new advanced laboratory course in quantitative biology instrumentation, which will focus on the design and application of modern microscopy. This course is also the centerpiece of a larger effort at Brandeis, partially supported by the Howard Hughes Medical Institute and an NSF-IGERT grant, to establish an interdisciplinary graduate program in Quantitative Biology. The project will provide a platform for training of two graduate students. Funds are also included in the award to support two undergraduate students per year on the project.