Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P2: New Techniques for Biological Structure Determination |
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Sponsoring Units: DCMP Chair: Janos Kirz, SUNY Stony Brook Room: LACC 151 |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P2.00001: Organic-inorganic templates in biomineralization of shells, bone, teeth, and bacterial biofilms Invited Speaker: Recent experiments with the Spectromicroscope for PHotoelectron Imaging of Nanostructure with X-rays (SPHINX)[1] on the biofilm formed by Fe-oxidizing bacteria in fresh, ground water, demonstrated that microbially extruded polysaccharide filaments provide the precipitation site for amorphous FeOOH filaments [2]. Upon aging the mineralized filaments crystallize to ferrihydrite (2-line FeOOH), with one curved pseudo-single crystal of akaganeite $\beta$-FeOOH), at the core of each filament. The crystals are only 2 nm wide and up to 10 micron long (aspect ratio 1:1000:1), and their structure and morphology is unprecedented. Furthermore, akaganeite should not form in fresh water, therefore a templation mechanism was hypothesized, and supported by SPHINX analysis of carbon XANES. The results indicate that after formation of the crystal fiber, the polysaccharide structure is also altered, and C1s spectra suggest that the COO$^{-}$ group is involved in the templation mechanism. This was the first successful attempt to understand the organic-inorganic chemical interface in a biomineralized system. Many more templated biomineral systems can and will now be analyzed with this new approach. \begin{enumerate} \item Ultramicroscopy \textbf{99}, 87-94 (2004). \item Science \textbf{303}, 1656-1658 (2004). \end{enumerate} [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:27PM |
P2.00002: X-Ray Tomography Generates 3-D Reconstructions of the Invited Speaker: Carolyn Larabell Soft X-ray microscopy is an emerging new technique that can image whole, hydrated, biological specimens with a spatial resolution 5-10 times better than that obtained with light microscopy. X-ray imaging at photon energies below the K- absorption edge of oxygen exploits the strong natural contrast for organic material embedded in a mostly water matrix. With a transmission X-ray microscope using Fresnel zone plate optics, specimens up to 10 microns thick can be examined. The highest X-ray transmission in hydrated samples is obtained at a wavelength of 2.34 nm but, due to the low numerical aperture of zone plate lenses operated in first order diffraction mode (NA$\sim $0.1), the structures resolved are much larger than the X-ray wavelength. To date, soft X-ray microscopy has been used to resolve 30 nm structures in frozen hydrated specimens. Because of the low NA of X-ray lenses, combined with the effect of polychromatic illumination and a wavelength dependant focal length, the effective depth of field is large (6-10 microns). In this paper, we show tomographic reconstructions of rapidly frozen live cells in a 10 um diameter glass capillary (200 nm wall thickness). The image sequences span 180 degrees and consist of either 45 or 90 images spaced by 4 or 2 degrees, respectively. Computed tomographic reconstructions generate 3-D images of whole cells at better than 50 nm isotropic resolution. Using the x-ray linear absorption coefficient, quantitative information is obtained from the reconstructed data. Data sets containing 180 images, made possible by our new fully automated cryo-rotation stage, will generate images at resolution approaching 30 nm. This stage also enables collection of data in less than 3 minutes, making soft x-ray tomography the first high-throughput, high-resolution imaging technique for biological specimens. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 1:03PM |
P2.00003: Biological x-ray microscopy: from biochemical mapping to lensless imaging Invited Speaker: Cell structure has been very succesfully studied using light and electron microscopy. However, x rays ofer new insights, by imaging whole cells at 20-40 nm resolution using zone plate lenses, and in particular by combining this with spectroscopic sensitivity to organic functional groups. While spectra of single compounds can provide exquisite information on electronic states, a cell is much more complex. Pattern recognition algorithms provide a way to deal with this complexity and obtain insights into biochemical organization at a fine spatial scale, as illustrated in an ongoing study of the correlation of morphology with biochemical content in sperm. Another approach to biological imaging is to abandon the use of lenses and their resolution limits. The purest form of measurement is to collect x rays scattered by a cell with no optics-imposed losses. By using iterative phasing algorithms, this diffraction data can be phased to deliver a real-space image of a complex cell (at present, 30 nm resolution in studies of freeze-dried yeast) with a possible ultimate extension to atomic resolution imaging of proteins using x-ray free electron lasers. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:39PM |
P2.00004: 3D Diffraction Microscope Provides a First Deep View Invited Speaker: Jianwei Miao When a coherent diffraction pattern is sampled at a spacing sufficiently finer than the Bragg peak frequency (i.e. the inverse of the sample size), the phase information is in principle encoded inside the diffraction pattern, and can be directly retrieved by using an iterative process. In combination of this oversampling phasing method with either coherent X-rays or electrons, a novel form of diffraction microscopy has recently been developed to image nanoscale materials and biological structures. In this talk, I will present the principle of the oversampling method, discuss the first experimental demonstration of this microscope, and illustrate some applications in nanoscience and biology. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 2:15PM |
P2.00005: In Vivo Microtesla Magnetic Resonance Imaging Invited Speaker: We have developed a magnetic resonance imaging (MRI) system which operates at magnetic fields of 132 microtesla, corresponding to proton Larmor frequencies of 5.6 kHz. The main advantages of performing MRI at low magnetic fields ($<$ 10 mT) are the reduced costs compared to conventional high- field MRI, and the reduction of nuclear magnetic resonance line broadening caused by inhomogeneous magnetic fields and susceptibility variations in the sample. Our technique combines prepolarization of the nuclear spins in a magnetic field up to 300 mT and signal detection at 132 microtesla using an untuned superconducting input circuit coupled to a superconducting quantum interference device (SQUID) to achieve a signal amplitude independent of the measurement field. We employ a standard spin-echo pulse sequence to acquire three-dimensional images in less than 6 minutes. Using encoding gradients of about 100 $\mu$T/m we obtain images of bell peppers and water phantoms with a resolution of 2 mm x 2 mm x 8 mm. Three- dimensional images of a human forearm were acquired at 132 microtesla with an average prepolarization field of 50 mT showing a signal-to-noise ratio (SNR) of 10 and an in-plane resolution of 3 mm x 3 mm. We have shown that for certain materials the longitudinal relaxation time (T$_1$) contrast is greatly enhanced at low magnetic fields. This enhancement is expected to lead to novel applications in specialized clinical imaging of human subjects, for example, low-cost tumor screening. To make such applications feasible further improvements of the SNR and resolution of the system are necessary. By employing a SQUID detector with a lower magnetic field noise and by raising the maximum polarizing field, an improvement of the SNR by an order of magnitude should be possible. [Preview Abstract] |
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