The elastic properties of capsids of the cowpea chlorotic mottle virus have been examined at pH 4.8 by nanoindentation measurements with an atomic force microscope. Studies have been carried out on WT capsids, both empty and containing the RNA genome, and on full capsids of a salt-stable mutant and empty capsids of the subE mutant. Full capsids resisted indentation more than empty capsids, but all of the capsids were highly elastic. There was an initial reversible linear regime that persisted up to indentations varying between 20% and 30% of the diameter and applied forces of 0.6 -1.0 nN; it was followed by a steep drop in force that is associated with irreversible deformation. A single point mutation in the capsid protein increased the capsid stiffness. The experiments are compared with calculations by finite element analysis of the deformation of a homogeneous elastic thick shell. These calculations capture the features of the reversible indentation region and allow Young's moduli and relative strengths to be estimated for the empty capsids.atomic force microscopy ͉ cowpea chlorotic mottle virus ͉ finite element analysis ͉ biomechanics

The mechanical properties of materials depend strongly on crystal structure and defect configuration. Here we measure the strength of suspended single-crystal and bicrystal graphene membranes prepared by chemical vapour deposition. Membranes of interest are first characterized by transmission electron microscopy and subsequently tested using atomic force microscopy. Single-crystal membranes prepared by chemical vapour deposition show strengths comparable to previous results of single-crystal membranes prepared by mechanical exfoliation. Grain boundaries with large mismatch angles in polycrystalline specimens have higher strengths than their low angle counterparts. Remarkably, these large angle grain boundaries show strength comparable to that of single-crystal graphene. To investigate this enhanced strength, we employ aberration-corrected high-resolution transmission electron microscopy to explicitly map the atomic-scale strain fields in suspended graphene. The high strength is attributed to the presence of low atomic-scale strain in the carbon-carbon bonds at the boundary.

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Cellular membranes are a heterogeneous mix of lipids, proteins and small molecules. Special groupings enriched in saturated lipids and cholesterol form liquid-ordered domains, known as "lipid rafts," thought to serve as platforms for signaling, trafficking and material transport throughout the secretory pathway. Questions remain as to how the cell maintains small fluid lipid domains, through time, on a length scale consistent with the fact that no large-scale phase separation is observed. Motivated by these examples, we have utilized a combination of mechanical modeling and in vitro experiments to show that membrane morphology plays a key role in maintaining small domain sizes and organizing domains in a model membrane. We demonstrate that lipid domains can adopt a flat or dimpled morphology, where the latter facilitates a repulsive interaction that slows coalescence and helps regulate domain size and tends to laterally organize domains in the membrane.bilayer mechanics | lipid rafts | membrane morphology T he plasma and organelle membranes of cells are composed of a host of different lipids, lipophilic molecules, and membrane proteins (1). Together, they form a heterogeneous layer capable of regulating the flow of materials and signals into and out of the cell. Lipid structure and sterol content play a key role in bilayer organization, where steric interactions and energetically costly mismatch of lipid hydrophobic thickness result in a line tension that induces lateral phase separation (2). Saturated lipids and cholesterol are sequestered into liquid-ordered (L o ) domains, often known as "lipid rafts," distinct from an unsaturated liquid-disordered (L d ) phase (3-5). Domains whose lipids include saturated sphingolipids and cholesterol, with sizes in the range of ≈50-500 nm, have been implicated in a range of biological processes from lateral protein organization and virus uptake to signaling and plasma-membrane tension regulation (6-18). In the biological setting, maintenance of small domain size is thought to arise from a combination of lipid recycling and energetic barriers to domain coalescence (19)(20)(21) [potentially provided by transmembrane proteins (22)], ostensibly resulting in a stable distribution of domain sizes. These biological examples serve as a motivation to better understand the biophysical mechanisms that maintain small lipid domains over time and pose challenges to the classical theories of phase-separation and "domain ripening" [such as Cahn-Hilliard kinetics (23)].A simple physical model that describes the evolution of lipid domain size and position predicts that domains diffuse and coalesce, such that the number of domains constantly decreases, whereas the average domain size constantly increases (23). Indeed, models of 2D phase separation have been studied in detail for many physical systems (24-27), and where the phase boundary is unfavorable and characterized by an energy per unit length (28), the domain size grows continuously (23,29,30). However, membranes can adopt 3D morphologi...