Rectangular Unit Cell Research Articles

Computer Synthesis and Filtration of Electron Micrographs of Simple Molecular Objects

A flexible computer program for the IBM 360/91 gives us the capability of synthesizing fairly realistic electron micrographs in the form of computerproduced transparencies which have all of the essential features of actual electron-exposed emulsions. We can control the assumed properties of the illuminating electron beam, the contents of the object plane, the electron optical properties of the objective and magnification system, and the properties of the detector system (emulsion). Statistical error (noise) can be realistically introduced. The resulting transparencies give very striking visual evidence concerning the quality of the information obtainable about simple molecular (DNA) samples under various interesting assumptions.As presently operating, we can conveniently place up to three complete periods of the double helical DNA structure (100.8 Å) in the object plane. The object is assumed to repeat indefinitely with a rectangular unit cell, so that a discrete Fourier series can be used. The Fourier coefficients of the image wave (just after sample passage) are computed from the known atomic coordinates.

Untersuchung von oberflächenreaktionen mittels beugung langsamer elektronen (LEED): I. Wechselwirkung von O 2 und N 2O mit (110)-, (111)- und (100)-Kupfer-Oberflächen

The interaction of molecular and (by decomposition of N 2O) atomic oxygen with atomically clean (110)-, (111)-, and (100)-copper surfaces has been studied by low energy electron diffraction (LEED). There is a great difference in the reactivity of the different surfaces. On each of the studied surfaces several structures — dependent of the experimental conditions — could be observed: The (110) plane reacts with very little amounts of O 2 at room temperature to produce a 1 × 2-structure. N 2O causes the same structure, thus indicating the dissociative adsorption of O 2. This structure changes reversibly by oxygen exposures > 10 −4 Torr-min into a new structure with a surface net described by 3 × √(10.25)−62°. On the (111) plane at room temperature only a disordered adsorption takes place, which causes a strong increase in background intensity. At 350 °C a part of the adsorbed oxygen desorbs. At 400 °C a complicated structure is produced with a sevenfold lattice constant, which changes at 450 °C into a 4 × 4- and at last at 500 °C into a 2 × 2-structure. N 2O causes the same stable 2 × 2-structure. The (111) surface can be reduced by H 2 at 450 °C. The interaction of a (100) surface with molecular oxygen produces two types of structures with rectangular unit cells as observed by Lee and Farnsworth 1). Interaction of N 2O at 500 °C causes the appearance of two other structures with the (111) symmetry of the epitaxial Cu 2O. The first has a lattice constant of approximately 1.06 a, and is characterized by groups of diffraction spots which are close together and which are ascribed to multiple scattering. This structure can be reduced by H 2 at 400 °C. The second structure has a lattice constant of a√3 and is more stable than the first one. This structure can be reduced at 450 °C. Larger amounts of oxygen cause the appearance of facets. In no case the formation of the true oxide could be observed.

Structure of Polyglycine

IN a preliminary report on a chemical, X-ray and infra-red investigation of synthetic polypeptides1, one of the forms of polyglycine was tentatively assigned a rectangular unit cell of dimensions : α = 4·36 A. (or twice that), b ≈ 7 A., and c = 3·44 A. I should like now to correct this in the light of more recent considerations.