Greg Grason (polymer science & engineering): soft matter theory/polymer physics Filamentous Assemblies: How do small, flexible molecules pack themselves? mitotic spindle growth cone of axon -Filaments are both nanoscopic (~1-10 nm diameter) and microscopic (~1m) length -Filaments have well defined structure, chemistry and mechanical properties -Thermal forces and packing constraints gives rise to a host of complex assembly properties How does Nature generate such robust & functional assemblies?? bundles of protein filaments polymer-based nano-rods ordered assembly How can one design/functionalize synthetic filaments to achieve desired assembly?? Moon & McCarthy, Macromolecules (2003).
Statistical mechanics: melting to states of intermediate order high-temperature, low-density columnar liquid crystal low-temperature, high-density crystal phase behavior or rod-like fd virus inhomogenous structure along filament backbone longitudinal thermal fluctuations -density variation locks into registry -breaks symmetry along backbone direction -non-zero shear modulus -thermal fluctuations along filaments to slide freely -fluid symmetry -shear modulus vanishes • Questions: • what are critical properties? • how does melting influence mechanical properties of assemblies? Grelet, Phys. Rev. Lett. (2008).
Geometry & frustration: 2D packings of flexible screws bundles of filamentous actin Biological filaments (like DNA) have helical, screw-like structure… …and molecular screws exert a mutual twist, inducing relative tilt Claessens, Semmrich, Ramos & Bausch, PNAS (2007). • Questions: • how does local twist packing dictate assembly properties (size) of bundles? • other twisted assembly motifs? writhing bundles? planar assemblies of short filaments? Bundles of helical filaments want twist & 2D order Can they have both?
Better filament packing with defects: Questions: What is state of densely packed DNA? Crystalline or liquid crystalline? What type of defect structures accommodate packaging? 5-fold disclinations drive twisting writhing conformations DNA in bacteriophage capsid (EM) Leforestier & Livolant, PNAS (2009). DNA in bacteriophage capsid (simulation) Petrov, Locker & Harvey, PRE (2009).
currently seeking graduate student Raw materials: espresso Badel: postdoc Homin: postdoc Stephanie: PSE Apparatus: “belafonte” Wei: PSE www.pse.umass.edu/ggrason/ firstname.lastname@example.org