The Penn Science Teacher Institute: A Proven Model

The University of Pennsylvania’s Master of Chemistry Education (MCE) program graduated five cohorts of approximately twenty teachers between 2002 and 2006. One year after the teachers in the last cohort earned their degrees, the Penn Science Teacher Institute (Penn STI) initiated a follow-up study to ascertain if the goals of the MCE program had been sustained. For example, were the teachers incorporating updated content knowledge into their lessons and were their students learning more chemistry? A total of seventy-four of the eighty-two graduates participated in some aspect of this study. Because baseline data were not available for the MCE teachers and their students, baseline data from a comparable group of chemistry teachers enrolled in the first cohort of the Penn STI program and their students were used in some analyses. Among other findings, the data indicate that MCE met its goals: 1) to improve the chemistry content knowledge of its teacher participants; 2) to increase the use of research-based instruction in their classrooms; and, 3) to improve student achievement in chemistry (students of MCE graduates scored significantly higher than the comparison group)

Stabilizing Salt-Bridge Enhances Protein Thermostability by Reducing the Heat Capacity Change of Unfolding

Most thermophilic proteins tend to have more salt bridges, and achieve higher thermostability by up-shifting and broadening their protein stability curves. While the stabilizing effect of salt-bridge has been extensively studied, experimental data on how salt-bridge influences protein stability curves are scarce. Here, we used double mutant cycles to determine the temperature-dependency of the pair-wise interaction energy and the contribution of salt-bridges to ΔCp in a thermophilic ribosomal protein L30e. Our results showed that the pair-wise interaction energies for the salt-bridges E6/R92 and E62/K46 were stabilizing and insensitive to temperature changes from 298 to 348 K. On the other hand, the pair-wise interaction energies between the control long-range ion-pair of E90/R92 were negligible. The ΔCp of all single and double mutants were determined by Gibbs-Helmholtz and Kirchhoff analyses. We showed that the two stabilizing salt-bridges contributed to a reduction of ΔCp by 0.8–1.0 kJ mol−1 K−1. Taken together, our results suggest that the extra salt-bridges found in thermophilic proteins enhance the thermostability of proteins by reducing ΔCp, leading to the up-shifting and broadening of the protein stability curves

Planar liquid-like arrangement of photopigment molecules in frog retinal receptor disk membranes

Low-angle X-ray diffraction arising from 40 to 50 A particles within wet frog retinal receptor disk membranes at 26 [deg]C was not consistent with a planar crystalline lattice of the particles within the disk membranes. The nature of the diffraction suggested the possibility of a planar liquid-like arrangement of the particles. Such an arrangement is supported by the observation that the planar ordering of the particles is easily altered by their interaction with globular protein molecules non-specifically adsorbed to the disk membranes. In view of the above, we obtained diffraction patterns from our wet disk membrane preparations at several temperatures between 4.5 and 42.5 [deg]C, and applied a Fourier analysis to the diffracted intensities appropriate for a planar liquid-like arrangement of the 40 to 50 A particles. The analysis gave the planar radial distribution function description of the supposed planar liquid-like arrangement of the particles. These radial distribution functions, derived from the diffracted intensities, were examined in terms of their shape and variation with temperature, and compared with the known predictions from statistical mechanics for a liquidlike arrangement of particles whose pair potential contains both attractive and repulsive terms. This comparison for the derived radial distribution functions demonstrated that the observed diffraction data from the 40 to 50 A particles were indeed consistent with a planar liquid-like arrangement of these particles within the disk membrane.Our radial distribution function analysis allowed model scattering factors for the particles to be tested. It was found that only hard sphere cross-sectional electron densities for the particle with diameters of 40 to 44 A or reasonably hard, soft-sphere cross-sectional electron densities, with a core of uniform electron density 38 to 40 A in diameter and a total diameter of 44 to 46 A, gave good agreement.A similar analysis was applied to the diffracted intensities arising from the antirhodopsin molecules adsorbed to the wet disk membranes which had been treated with our antirhodopsin serum and is discussed relative to the preceding paper (Blasie, Worthington & Dewey, 1969). A comparison of the radial distribution functions for the antirhodopsin molecules adsorbed to the antirhodopsin serum treated disk membranes and the 40 to 50 A particles of the untreated disk membranes at identical temperatures showed the particles to be the photopigment molecules.The mathematical derivation of the planar radial distribution function and a critical evaluation of the errors involved are presented in the Appendices.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33022/1/0000406.pd

Molecular localization of frog retinal receptor photopigment by electron microscopy and low-angle X-ray diffraction

Low-angle X-ray diffraction patterns were obtained from ordered ultracentrifugal pellets of wet receptor disk membranes which had been either treated with antirhodopsin serum, normal rabbit serum, serum albumin, or untreated prior to sedimentation. A preliminary analysis of these patterns indicated: (a) differences between antirhodopsin serum treated and untreated preparations are due to conjugation of antirhodopsin molecules with their antigen in the disk membranes and not non-specific adsorption of other serum proteins to the disk membranes, (b) the planar ordering of the adsorbed antirhodopsin molecules over the surface of the disk membrane is nearly identical to that of the 40 to 50 A particles in the untreated disk membrane.A detailed Fourier analysis of these patterns in terms of a planar liquid-like arrangement of the 40 to 50 A particles in the untreated disk membranes and the antirhodopsin molecules adsorbed to the antirhodopsin serum treated disk membranes confirmed our preliminary analysis. The planar liquid-like arrangement of the 40 to 50 A particles is nearly identical to that of the adsorbed antirhodopsin molecules (3.0 and 3.1 nearest neighbors at a separation of 56 and 58 A respectively at 26 [deg] +/- 0.2 deg.C). Thus, the 40 to 50 A particles of the wet untreated disk membranes are the photopigment molecules.Electron micrographs of phosphotungstate negatively-stained disk membrane demonstrate particles ~40 A in diameter within the disk membrane. Optical transforms of these electron micrographs show that these particles appearing in the micrograph are arranged in a planar square array with a unit cell side of ~ 70 A. Correlation of these results with those obtained by low-angle X-ray diffraction in ultracentrifugal pellets of phosphotungstate stained and dried disk membranes as well as on wet pellets of untreated disk membranes before, during and after drying indicates the following: the ~ 40 A diameter particles seen in the electron micrographs are most likely the same 40 to 50 A particles giving rise to the observed low-angle X-ray diffraction from wet disk membranes. Hence the particles seen in the electron micrographs are most likely the nonpolar cores of the photopigment molecules.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33021/1/0000405.pd

Heme structure and orientation in single monolayers of cytochrome c on polar and nonpolar soft surfaces.

Polarized x-ray absorption fine structure (XAFS) spectroscopy has been performed in fluorescence mode under total external reflection conditions on frozen hydrated single monolayers of yeast cytochrome c (YCC). The protein molecules were vectorially oriented within the monolayer by tethering their naturally occurring and unique surface cysteine residues to the sulfhydryl-endgroups at the surface of a mixed organic self-assembled monolayer, itself covalently attached to an ultrapure silicon wafer. The sulfhydryl-endgroups were isolated by dilution with either methyl- or hydroxyl-endgroups, producing macroscopically nonpolar or uncharged-polar soft surfaces, respectively. Independent information on the heme-plane orientation relative to the monolayer plane was obtained experimentally via optical linear dichroism. The polarized XAFS data have been analyzed both qualitatively and by a global mapping approach limited to systematically altering the various iron-ligand distances within a model for the local atomic environment of the heme prosthetic group, and comparing the theoretically generated XAFS spectra with those obtained experimentally. A similar analysis of unpolarized XAFS data from a frozen solution of YCC was performed using either the heme environment from the NMR solution or the x-ray crystallographic data for YCC as the model structure. All resulting iron-ligand distances were then used in molecular dynamics (MD) computer simulations of YCC in these three systems to investigate the possible effects of anisotropic ligand motions on the fits of the calculated to the experimental XAFS spectra

Molecular dynamics simulations of a hydrated protein vectorially oriented on polar and nonpolar soft surfaces.

We present a collection of molecular dynamics computer simulation studies on a model protein-membrane system, namely a cytochrome c monolayer attached to an organic self-assembled monolayer (SAM). Modifications of the system are explored, including the polarity of the SAM endgroups, the amount of water present for hydration, and the coordination number of the heme iron atom. Various structural parameters are measured, e.g., the protein radius of gyration and eccentricity, the deviation of the protein backbone from the x-ray crystal structure, the orientation of the protein relative to the SAM surface, and the profile structures of the SAM, protein, and water. The polar SAM appears to interact more strongly with the protein than does the nonpolar SAM. Increased hydration of the system tends to reduce the effects of other parameters. The choice of iron coordination model has a significant effect on the protein structure and the heme orientation. The overall protein structure is largely conserved, except at each end of the sequence and in one loop region. The SAM structure is only perturbed in the region of its direct contact with the protein. Our calculations are in reasonably good agreement with experimental measurements (polarized optical absorption/emission spectroscopy, x-ray interferometry, and neutron interferometry)