Chirality and Entropy in Self-Assembly of Biopolymers

智力优势：该提案有四个实验目标，围绕熵和手性的共同主题，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位，这对细丝部分是由熵（耗散）力结合在一起的。目的是确定熵涨落对细丝间吸引力的贡献。将采用两种可供选择的方法。其中一种是用激光镊子将一对细丝分开，并结合荧光显微镜同时对系统成像。引力可以直接从这个实验中提取出来。另一种方法是分析以发夹形式自行折叠的孤立细丝的图像。通过认识到它与储存在折叠丝中的弹性能量相平衡来确定粘附能，从而确定发夹环的大小。第二个目标是研究单分散非棒状病毒在胶体膜上的自组装，这是最近在PI实验室观察到的一种现象。这些病毒颗粒没有表现出大多数膜形成者的极化正反两亲性特征，因此假设这种膜组装过程是由耗竭力驱动的。第三个实验使用手性和非手性丝状病毒检查手性对胶体膜和扭曲带形成的影响。这项工作探讨了一个假设，即带状的形成是由组成杆的分子手性驱动的。最后的项目考察了排除体积驱动的棒状粒子过渡到向列相在适度的棒状密度理论的扩展。在这里，计划检查手性在杆状体中的作用，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，主要的实验工具是一个多模显微镜，可以在微分干涉对比和荧光模式下获得图像。这与激光镊子相结合，对胶体珠或其他具有高折射率的结构施加力，并在微秒时间尺度上以纳米精度确定珠的位置。更广泛的影响：该提案本质上是高度跨学科的，将为组装复杂的生物材料提供新的设计原则。PI正在开发一门新的定量生物学仪器高级实验课程，该课程将侧重于现代显微镜的设计和应用。这门课程也是布兰代斯大学一项更大努力的核心，该努力部分由霍华德休斯医学研究所和NSF-IGERT资助，旨在建立一个跨学科的定量生物学研究生项目。该项目将为两名研究生提供培训平台。奖金中还包括每年支持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.