High resolution mechanics of viruses studied by Atomic Force Microscopy

Schmidt, C.F. [Promotor]Wuite, G.J.L. [Copromotor

Anisotropic colloids through non-trivial buckling

We present a study on buckling of colloidal particles, including experimental, theoretical and numerical developments. Oil-filled thin shells prepared by emulsion templating show buckling in mixtures of water and ethanol, due to dissolution of the core in the external medium. This leads to conformations with a single depression, either axisymmetric or polygonal depending on the geometrical features of the shells. These conformations could be theoretically and/or numerically reproduced in a model of homogeneous spherical thin shells with bending and stretching elasticity, submitted to an isotropic external pressure.Comment: submitted to EPJ

Elastic properties of hollow colloidal particles

The elastic properties of micrometer-sized hollow colloidal particles obtained by emulsion templating are probed by nanoindentation measurements in which point forces are applied to solvent-filled particles supported on a flat substrate. We show that the shells respond linearly up to forces of 7–21 nN, where the indentation becomes of the order of the shell thickness 20–40 nm . In the linear region, the particle deformation is reversible. The measured Young’s modulus 200 MPa is comparable to values for stiff rubbers or soft polymers. At larger applied force, we observe a crossover into a nonlinear regime, where the shells assume a buckled shape. Here, the force increases approximately as the square root of the indentation, in agreement with the theory of elasticity of thin shells. We also observe permanent deformation of the shells after probing them repetitively beyond the linear regime. Finally, the measured elastic properties of the shells nicely explain their spontaneous buckling in solution and due to drying

Probing the impact of loading rate on the mechanical properties of viral nanoparticles

The effects of changes in the loading rate during the forced dissociation of single bonds have been studied for a wide variety of interactions. Less is known on the loading rate dependent behaviour of more complex systems that consist of multiple bonds. Here we focus on viral nanoparticles, in particular the protein shell (capsid) that protects the viral genome. As model systems we use the well-studied capsids of the plant virus Cowpea Chlorotic Mottle Virus (CCMV) and of the bacteriophages φ29 and HK97. By applying an atomic force microscopy (AFM) nanoindentation approach we study the loading rate dependency of their mechanical properties. Our AFM results show very diverse behaviour for the different systems. In particular, we find that not only the breaking force, but also the spring constant of some capsids depend on the loading rate. We describe and compare the measured data with simulation results from the literature. The unexpected complex loading rate dependencies that we report present a challenge for the current theoretical considerations aimed at understanding the molecular level interactions of highly ordered protein assemblies. © 2012 Elsevier Ltd

Effects of Salts on Internal DNA Pressure and Mechanical Properties of Phage Capsids

Based on atomic force microscopy nanoindentation measurements of phage λ, we previously proposed a minimal model describing the effect of water hydrating DNA that strengthens viral capsids against external deformation at wild-type DNA packing density. Here, we report proof of this model by testing the prediction that DNA hydration forces can be dramatically decreased by addition of multivalent ions (M

Failure of viral shells

We report a combined theoretical and experimental study of the structural failure of viral shells under mechanical stress. We find that discontinuities in the force-indentation curve associated with failure should appear when the so-called Föpplâ€"von Kármán (FvK) number exceeds a critical value. A nanoindentation study of a viral shell subject to a soft-mode instability, where the stiffness of the shell decreases with increasing pH, confirms the predicted onset of failure as a function of the FvK number. © 2006 The American Physical Society