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Biomedical subjects

S Doniach

Publications and source records attributed to S Doniach.

66 records · Page 4Linked to original sources
PubMed1986

Location of terbium binding sites on acetylcholine receptor-enriched membranes.

Acetylcholine receptor-enriched membranes bind 45 terbium cations per receptor. The Tb(III) X-ray scattering factor changes by as much as 30% over a 50 eV range about the L3 absorption edge. We exploit these changes to modulate the contribution of these ions to the X-ray diffraction pattern of oriented receptor-enriched membranes by varying the incident X-ray energy. Difference Fourier analysis of the meridional diffraction amplitudes at two X-ray energies revealed six localized regions of Tb(III) density across the membrane. Most significant is the finding of 18 Tb(III) ions near the entrance and 11 ions near the exit of the ion channel as well as 4 or 5 Tb(III) ions localized in the channel itself. This evidence strongly suggests the presence of anionic carboxylate side-chains on the channel lining.

Animals↗
PubMed1983

Anomalous x-ray scattering from terbium-labeled parvalbumin in solution.

We have used anomalous small-angle x-ray scattering as a structural probe for solutions of rabbit parvalbumin labeled with terbium. This technique makes use of the large changes in the terbium scattering factor that occur when the x-ray energy is tuned around an L3 absorption edge of this heavy-atom label. These changes in scattering result in changes in the small-angle scattering curve of the labeled protein as a whole, which can then be analyzed to derive structural information concerning the distribution of labels in the protein. Based on a Gaussian model for the protein electron density, the mean distance from the terbiums to the protein center of mass is determined to be 13.2 A and is consistent with crystallographic results. Our results demonstrate the usefulness of terbium as an anomalous scattering label and provide criteria to help establish anomalous scattering as a reliable structural technique for proteins in solution.

Animals↗
PubMed1980

White lines in L-edge x-ray absorption spectra and their implications for anomalous diffraction studies of biological materials.

We have measured high-resolution x-ray absorption spectra of lanthanide (Ln) and heavy transition metal complexes that display prominent narrow absorption peaks near the L2 and L3 absorption edges. The anomalous scattering factors (f' and f"), which are mathematically related to the absorption cross section, have correspondingly sharp changes in their magnitude within 5-10 eV of the absorption edge. Calculations of the magnitude of the change in f' and f" demonstrate that significant changes (on the order of 20 electrons in f') can be expected for these materials. These substantial changes in the anomalous scattering factors have applications to deriving structural information for macromolecules from x-ray diffraction studies. The magnitude of the changes indicate that the anomalous scattering technique is a powerful means of obtaining structural characteristics for macromolecules in single crystals, in solution, and in biological membranes.

Crystallography↗
PubMed1975

X-ray absorption spectroscopy using synchrotron radiation for structural investigation of organometallic molecules of biological interest.

The technique of x-ray absorption spectroscopy using tuneable, very intense x-rays from a high energy electron storage ring has been applied to study of the estended x-ray absorption fine structure for Cu and Ni tetraphenylporphyrin and methemoglobin. Preliminary analysis shows that the spectra may be interpreted as a super-position of modulations arising from the nearest neighbor nitrogen and pyrrole alpha-carbon coordination sheels of the metal atoms. We estimate that with the observed magnitude of noise to modulation amplitude, relative shifts of 0,5% in the metal-nitrogen to metal-carbon bond distances in the prophyrins should be observable using extended x-ray absorption fine structure and that this technique may provide a method of observing these types of structural changes in solution.

Copper↗