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Biomedical subjects

D F Parsons

Publications and source records attributed to D F Parsons.

At least 73 records · Page 4Linked to original sources
PubMed1972

Reply to "myelin membrane structure as revealed by x-ray diffraction" by David Harker.

The interpretation of the heavy metal-labeled data can either be accomplished with the analysis of the observed intensity differences (Akers and Parsons) or with the analysis of the observed structure amplitude changes (Harker). Both methods of analysis give essentially the same results: that two possible electron density distributions are valid, within experimental error, depending on whether there are one or two metal-labeling sites within the membrane. At present, the correct choice must rest on either the introduction of additional physical and chemical data on the position of the proteins and lipids or on an independent phasing technique such as the Hosemann-Bagchi Q-function.

Animals↗
PubMed1970

X-ray diffraction of myelin membrane. I. Optimal conditions for obtaining unmodified small angle diffraction data from frog sciatic nerve.

The X-ray diffraction pattern of myelin of frog sciatic nerve has been investigated, using a Kratky small angle slit camera to obtain the electron density distribution across the membrane. All major reflections observed were related to a fundamental repeat distance of 171 +/- 2.8 A. There was no further increase in the number of reflections on varying the experimental conditions (varying pH, applying tension, immersion in various isotonic buffer solutions, etc.) or by varying the camera slit arrangement. The degree of disorder within the myelin sheath was examined by comparing the crystallite size to the half-width of the diffraction peak at half-height. The limiting of the diffraction spectra to five major reflections was determined not to be caused by disorder. It is concluded that the observed X-ray diffraction pattern is a consequence of the particular electron density distribution of the membrane. Therefore, the membrane cannot contain sharply distinct step-function regions of electron density, but approaches a modified cosine distribution.

Animals↗
PubMed1970

X-ray diffraction of myelin membrane. II. Determination of the phase angles of the frog sciatic nerve by heavy atom labeling and calculation of the electron density distribution of the membrane.

The phase signs of the five main X-ray reflections from normal frog sciatic nerve have been determined as all positive using a technique of labeling with very small amounts of heavy metal. The changes in intensity of the individual reflections were studied as a function of uptake of metal label by the membrane. The possible localization of the metal label was decided from computer-analogue studies and from Patterson calculations. These phases are different from those determined by previous workers using techniques of trial of the best set of phases, or a step model, to give the best fit of the combined intensity data of normal and swollen myelin membranes. The electron density map has been calculated using eight reflections and their experimentally determined phases. The map shows an inner low electron density region which is different from that shown by earlier calculations. The center of the low electron density region shows a small region of increased electron density. However, without fixing absolute electron density levels in the map, it is not yet possible to allocate regions of low electron density to pure lipid or lipoprotein. The map shows the two sides of the membranes to be different in molecular structure without significant water spaces between the membranes.

Animals↗
PubMed1969

Neutron diffraction of cell membranes (myelin).

Small-angle neutron diffraction (wavelength 4.05 angstroms) of human and rabbit sciatic nerve has been carried out by means of the Brookhaven high flux beam reactor with an automated slit camera. Most of the free water of the nerves was substituted in order to minimize incoherent scatter of hydrogen atoms. The differences in amplitude and phase shifts between neutrons and x-rays resulted in a neutron diffraction pattern that was completely different from the x-ray pattern. The neutron pattern consisted of a single peak of about 89-angstrom spacing in the region examined (up to 6-angstrom spacing). The strong third, fourth, and fifth order reflections (about 60, 45, and 36 angstroms) seen in the x-ray pattern were suppressed. The neutron data indicated a strong scattering from one portion of the membrane.

Animals↗