The two isostructural compounds, ATh2Se6 (A = K, Rb), adopt the orthorhombic space group Immm. ATh2Se6 has a two-dimensional structure which is related to the ZrSe3-type structure with K+/Rb+ cations stabilized between the layers. These compounds represent the intercalated form of ThSe3 with 0.5 equiv of alkali metal ion. The stacking arrangement of the layers is slightly modified from that of ZrSe3 in order to stabilize the newly introduced alkali metal ions between the layers. Electron diffraction studies reveal a static charge density wave (CDW), due to electron localization, resulting in 4a × 4b superstructure. An atomic pair distribution function analysis and spectroscopy confirmed the presence of diselenide groups in the ZrSe3-type layer (invisible by the single-crystal structure analysis) and support the notion that these Se atoms in the [Th2Se6] layers accept the extra electron from the alkali metal, and this results in breaking one out of four diselenide bonds. The superstructure is due to ordering of the three Se2 2- and two Se2- species along both directions. Optical absorption, Raman spectroscopy, and atomic force microscopy as well as magnetic susceptibility measurements support these conclusions.

The strong specific binding of streptavidin (SA) to biotin is utilized in numerous biotechnological applications. The SA tetramer is also known to exhibit significant stability, even in the presence of sodium dodecylsulfate (SDS). Despite its importance, relatively little is known about the nature of the thermal denaturation pathway for SA. This work uses a homogeneous SA preparation to expand on the data of previous literature reports, leading to the proposal of a model for temperature induced structural changes in SA. Temperature dependent data were obtained by SDS and native polyacrylamide gel electrophoresis (PAGE), differential scanning calorimetry (DSC), and fluorescence and ultraviolet (UV)-visible spectroscopy in the presence and absence of SDS. In addition to the development of this model, it is found that the major thermal transition of SA in 1% SDS is reversible. Finally, although SA exhibits significant precipitation at elevated temperatures in aqueous solution, inclusion of SDS acts to prevent SA aggregation.

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Individual Concanavalin A (ConA) molecules have been imaged at the liquid/solid interface with an atomic force microscope (AFM). Three-dimensional sizing with very high resolution (<5 Å) has been obtained by a novel approach based on height distributions, which avoids the tip convolution effects which normally affect scanning probe microscopy techniques. Each height measurement correlates to a particular molecular orientation on the surface. A large number of such measurements provide a statistical ensemble of orientations. The complete height distribution reflects the three-dimensional size of the protein sample and hence its tertiary and quaternary structure. A surface adsorption and orientation model, based on a minimization of surface adsorption energy, is proposed. This model is in good agreement with the observed height distribution of Con A molecules at the liquid/solid interface. Analysis of Con A and succinylated Con A molecules on mica demonstrates that Con A dimers are the prevalent species at the liquid/solid interface. This is in contrast to the tetrameric organization of Con A normally observed in solution. The new possibilities opened by height distribution analysis on the physical characterization of biomolecules at interfaces are also discussed.

While the binding of biotin by streptavidin does not appear to be cooperative in the traditional sense of altered binding strength, it has been suggested that it may be cooperative in terms of differential structural changes in the protein. In this work we present intrinsic tryptophan fluorescence data as evidence of a cooperative structural change. The technique involves examination of the differences in fluorescence emission corresponding to distinct tryptophan populations accompanying protein-ligand binding. Specifically we note that the 335 nm emission population (i.e. more hydrophobic) saturates prior to the saturation of the 350 nm emission population commonly used in the standard binding activity assay. We also note that the wavelength of maximum emission, total integrated fluorescence emission and full width at half maximum during the titration of ligand into streptavidin also reach saturation before the expected 4:1 stoichiometric end point. This suggests that the binding of the first 3 biotins effect greater structural changes in the protein than the final ligand.