Tuning bilayer twist using chiral counterions

From seashells to DNA, chirality is expressed at every level of biological structures. In self-assembled structures it may emerge cooperatively from chirality at the molecular scale. Amphiphilic molecules, for example, can form a variety of aggregates and mesophases that express the chirality of their constituent molecules at a supramolecular scale of micrometres (refs 1-3), Quantitative prediction of the large-scale chirality based on that at the molecular scale remains a largely unsolved problem. Furthermore, experimental control over the expression of chirality at the supramolecular level is difficult to achieve(4-7): mixing of different enantiomers usually results in phase separation(18). Here we present an experimental and theoretical description of a system in which chirality can be varied continuously and controllably ('tuned') in micrometre-scale structures. we observe the formation of twisted ribbons consisting of bilayers of gemini surfactants (two surfactant molecules covalently linked at their charged head groups). We find that the degree of twist and the pitch of the ribbons can be tuned by the introduction of opposite-handed chiral counterions in various proportions. This degree of control might be of practical value; for example, in the use of the helical structures as templates for helical crystallization of macromolecules(8,9).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62619/1/399566a0.pd

Steel and bone: Mesoscale modeling and middle-out strategies in physics and biology

Mesoscale modeling is often considered merely as a practical strategy used when information on lower-scale details is lacking, or when there is a need to make models cognitively or computationally tractable. Without dismissing the importance of practical constraints for modeling choices, we argue that mesoscale models should not just be considered as abbreviations or placeholders for more “complete” models. Because many systems exhibit different behaviors at various spatial and temporal scales, bottom-up approaches are almost always doomed to fail. Mesoscale models capture aspects of multi-scale systems that cannot be parameterized by simple averaging of lower-scale details. To understand the behavior of multi-scale systems, it is essential to identify mesoscale parameters that “code for” lower-scale details in a way that relate phenomena intermediate between microscopic and macroscopic features. We illustrate this point using examples of modeling of multi-scale systems in materials science (steel) and biology (bone), where identification of material parameters such as stiffness or strain is a central step. The examples illustrate important aspects of a so-called “middle-out” modeling strategy. Rather than attempting to model the system bottom-up, one starts at intermediate (mesoscopic) scales where systems exhibit behaviors distinct from those at the atomic and continuum scales. One then seeks to upscale and downscale to gain a more complete understanding of the multi-scale systems. The cases highlight how parameterization of lower-scale details not only enables tractable modeling but is also central to understanding functional and organizational features of multi-scale systems

Cooperative Adaptation to Establishment of a Synthetic Bacterial Mutualism

To understand how two organisms that have not previously been in contact can establish mutualism, it is first necessary to examine temporal changes in their phenotypes during the establishment of mutualism. Instead of tracing back the history of known, well-established, natural mutualisms, we experimentally simulated the development of mutualism using two genetically-engineered auxotrophic strains of Escherichia coli, which mimic two organisms that have never met before but later establish mutualism. In the development of this synthetic mutualism, one strain, approximately 10 hours after meeting the partner strain, started oversupplying a metabolite essential for the partner's growth, eventually leading to the successive growth of both strains. This cooperative phenotype adaptively appeared only after encountering the partner strain but before the growth of the strain itself. By transcriptome analysis, we found that the cooperative phenotype of the strain was not accompanied by the local activation of the biosynthesis and transport of the oversupplied metabolite but rather by the global activation of anabolic metabolism. This study demonstrates that an organism has the potential to adapt its phenotype after the first encounter with another organism to establish mutualism before its extinction. As diverse organisms inevitably encounter each other in nature, this potential would play an important role in the establishment of a nascent mutualism in nature

Diacetylenic lipid tubules:experimental evidence for a chiral molecular architecture

Molecular self-assembly is of key importance for the rational design of advanced materials. To investigate the causal relation between molecular structure and the consequent self-assembled microstructure, self-assembled tubules of diacetylenic lipids were studied. Circular-dichroism studies give experimental evidence that the formation of tubules is driven by chiral molecular packing, in agreement with recent theories of tubules. On the basis of these results, a molecular mechanism for the formation of tubules is proposed