The Beckman Institute Graduate Student Seminar Series presents the work of outstanding graduate students working in Beckman research groups. The seminar starts at Noon in Beckman Institute Room 1005 and is open to the public. Lunch will be served.

Curvature-Inducing Properties of Light-Harvesting Complex II (LH2)
Danielle Chandler

The photosynthetic apparatus of purple bacteria is contained within organelles called chromatophores, which form as extensions of the cytoplasmic membrane.  The shape of these chromatophores can be spherical (as in Rb. sphaeroides), lamellar (as in Rps. acidophila and Ph. molischianum), or tubular (as in certain Rb. sphaeroides mutants).  Chromatophore shape is thought to be influenced by the integral membrane proteins Light Harvesting Complexes I and II (LH1 and LH2), which pack tightly together in the chromatophore.  It has been suggested that the shape of LH2, together with its close packing in the membrane, induces membrane curvature.  The mechanism of LH2-induced curvature is explored via molecular dynamics simulations of multiple LH2 complexes in a membrane patch.  LH2s from three species - Rb. sphaeroides, Rps. acidophila, and Ph. molischianum - were simulated in different packing arrangements.  In each case, the LH2s pack together and tilt with respect to neighboring LH2s in a way that produces an overall curvature.  This curvature appears to be driven by electrostatic forces that are modulated by the conjunction of LH2 shape and the presence of well-conserved cytoplasmic charged residues, the removal of which inhibits LH2 curvature.  The interaction of LH2s and an LH1 monomer is also explored, and suggests that curvature is diminished by the presence of LH1 monomers.  The implications of our results for chromatophore shape are discussed.

Spiropyrans as Mechanochemical Damage Sensors
Doug Davis

Bulk polymers generally react to large external forces by undergoing random mechanochemical chain scission which leads to damage or failure of the material.  The incorporation of mechanically activated molecules called mechanophores into the polymer matrix allows these forces to induce useful chemical reactions that can be utilized to produce unique polymer properties such as damage sensing or self-healing.  Force is transferred to the cleavable bonds of the mechanophores by covalently linking polymer chains on either side of the desired bonds.  We currently utilize spiropyran mechanophores which show a dramatic change in color when they undergo an electrocyclic ring opening reaction involving the cleavage of the central, spiro carbon-oxygen bond.  These spiropyrans are useful as damage sensors because the location and intensity of this color change can be correlated with the stress distribution of the sample.  We have also demonstrated that this mechanochromic effect can be activated in a variety of elastomeric and glassy polymers such as poly(methyl acrylate), poly(methyl methacrylate), polyurethane, and polystyrene.

Focusing Ultrasound with Acoustic Metamaterial Network
Shu Zhang

We present the first experimental demonstration of focusing ultrasound waves through a flat acoustic metamaterial lens composed of a planar network of subwavelength Helmholtz resonators. We observed a tight focus of half-wavelength in width at 60.5 KHz by imaging a point source. This result is in excellent agreement with the numerical simulation by transmission line model in which we derived the effective mass density and compressibility. This metamaterial lens also displays variable focal length at different frequencies. Our experiment shows the promise of designing compact and light-weight ultrasound imaging elements.