Biomedical subjects

Y Le Page

Publications and source records attributed to Y Le Page.

4 recordsLinked to original sources

PubMed1992

Accurate d-spacings from zero-order Laue zone patterns.

Experimental d-spacing values are criteria towards the identification of crystallites by electron diffraction. Conclusive identifications often rely on accurate d-spacings. It is shown here that accurate orthogonal components (in mm) for the primitive unit vectors of a zero-level diffraction pattern can be obtained through least-squares processing of (x,y) coordinates for all spots on the film. Valid vectors from the origin spot to any spot in the plane of the film are integer linear combinations of the two selected unit vectors. Accurate lengths and standard deviations for such vectors therefore can be calculated from the least-squares results. Corresponding d-spacings can then be calculated from the vector lengths on the film and the camera constant. In order to obtain d-spacing values that are not only precise, but also accurate, an accurate value of the camera constant should be used. This requires calibration of the experimental setup with reference materials in the same experimental conditions, with careful control of the sample height. For the same quality of measurements, the improvement in the accuracy of the d-spacings obtained with the proposed method is approximately proportional to the square root of the number of measurements taken. Practically, typical improvement in accuracy is about threefold, and accuracies of a fraction of a percent in d-spacings are achievable in this way. The above approach has been programmed as an option in the NRCBED program.

Crystallization↗

PubMed1992

Ab-initio primitive cell parameters from single convergent-beam electron diffraction patterns: a converse route to the identification of microcrystals with electrons.

A new method for the ab initio derivation of Buerger-reduced primitive cell parameters from coordinate measurements of spots on single convergent-beam electron diffraction (CBED) patterns is described, which does not involve trial-and-error. The pattern can be taken along any zone axis, and misorientations of the crystallite by as much as a few degrees are taken into account without loss of accuracy. This derivation of cell parameters by least-squares analysis of the measurements has been automated in a program called NRCBED. Present accuracy is about 1% on lengths and 2 degrees on angles, but could be significantly improved by modelling projector lens aberrations, or by using a microscope without a projector lens. With present technology, it is possible to obtain a CBED pattern and a semi-quantitative energy-dispersive X-ray (EDX) analysis simultaneously from a single microcrystal a few hundred Angströms across. It becomes therefore possible to identify the material of the crystal on a single CBED pattern: a cell parameter database for known compounds is searched with the primitive cell parameters obtained in the above way, and with a mask describing the EDX results qualitatively. Feasibility is demonstrated on a crystallite of CeO2 500 Angströms across. With this new approach, trial-and-error should disappear from the solution of other long-standing problems: interpretation of X-ray powder patterns for new compounds in the presence of impurity lines, or in the case of multiple phases should become straight-forward.

Cerium↗

PubMed1991

Primitive unit cell volumes obtained from unindexed convergent-beam electron diffraction patterns.

Provided that multiple reflection is present, a common occurrence with electron diffraction on microcrystallites, a single unindexed convergent-beam electron diffraction (CBED) pattern taken with the beam axis parallel to any direct lattice row allows the volume of the primitive cell to be calculated. The film measurements required are the diameters of the successive high-order Laue zones (HOLZ), the lengths of any two coprime vectors in the zero-order Laue zone (ZOLZ), and the angle between them. The primitive cell volume is an objective criterion allowing in a simplification in the identification of a phase under study by rapidly eliminating other possible phases. The computer program CELVOL for the calculation of the primitive cell volume from film measurements, or from literature cells, is available from the authors.

Cells↗

PubMed1990

Molecular structure of opiate alkaloids. IV. Structure of two thioniamorphinans.

(I) S(equatorial)-Allyl-3-hydroxy-17-thioniamorphinan perchlorate, C19H25OS+.C1O4-, Mr = 400.91, monoclinic, P2(1)/n, a = 9.4171 (8), b = 10.7425 (10), c = 19.096 (2) A, beta = 95.666 (7) degrees, V = 1922.4 (3) A3, Z = 4, Dx = 1.385 Mg m-3, lambda (Mo K alpha) = 0.70930 A, mu = 0.33 mm-1, F(000) = 847.92, room temperature, final R = 0.052 for 1742 observed reflections. (II) S(axial)-Allyl-3-hydroxy-17-thioniamorphinan perchlorate, C19H25OS+.C1O4-, Mr = 400.91, monoclinic, P2(1), a = 8.4554 (10), b = 11.658 (3), c = 9.5831 (21) A, beta = 95.620 (10) degrees, V = 940.1 (3) A 3, Z = 2, Dx = 1.416 Mg m-3, lambda (Mo K alpha) = 0.70930 A, mu = 0.33 mm-1, F(000) = 423.96, room temperature, final R = 0.054 for 1015 observed reflections. The molecular structures of (I) and (II) are differentiated only by the orientation of the S-allyl substituent: the S-allyl group is equatorial in (I) and axial in (II). It has been shown that the activities as potent and selective blockers of kappa opioid receptors (kappa 2 subtype) are to be attributed to the alpha-thiamorphinan isomer (I). The inactive compound (II) is the beta-isomer. The fused ring systems are almost identical in the two molecules.

Alkaloids↗