Mechanism and dynamics of conformational ordering in xanthan polysaccharide

The thermally induced order-disorder transition of xanthan (extracellular bacterial polysaccharide from Xanthomonas campestris) has been investigated by optical rotation, differential scanning calorimetry, stopped-flow reaction kinetics and low-angle laser light scattering, and the results have been analysed in terms of Zimm -Bragg helix-coil transition theory. The reciprocal of the transition midpoint temperature (Tm) varies linearly with the logarithm of cation (K+) the salt dependence of Tm, is in agreement with Manning polyelectrolyte theory the ordered structure. The associated increase in cation binding, calculated from the salt dependence of tm, is in agreement with the Manning polyelectrolyte theory for one of the candidate structures from X-ray diffraction, a 5(1) single helix stabilized by packing of side-chains along the polymer backbone, but not for the alternative double-helix structure that has also been proposed. At each salt concentration, the two fundamental parameters of the Zimm -Bragg theory, s and sigma, were calculated. The equilibrium constant for growth of the ordered structure (s) is derived directly from calorimetric measurement of transition enthalpy (delta Hcal ), and sigma, which quantifies the relative instability of the helix nucleus, is derived from the ratio of delta Hcal to the apparent transition enthalpy (delta Happ ) obtained by van't Hoff analysis of the optical rotation data. The temperature course of conformational ordering calculated theoretically is in good quantitative agreement with experimental results from both optical rotation and scanning calorimetry. The calculated average length of stable, ordered chain-sequences increases with decreasing temperature, but equals or exceeds the total chain length from light scattering only at temperatures more than approximately equal to 70 K below Tm, suggesting that ordered and disordered regions may co-exist within the same xanthan molecule. Consistent with this interpretation, the observed rate of conformational ordering increases sharply under conditions where the starting solution for dynamic measurements is partially ordered, suggesting that ordered sequences within each chain may act as helix nuclei for adjacent disordered regions, so that helix growth, rather than the slower nucleation process, becomes rate limiting.     

On the Robustness of the Geometrical Model for Cell Wall Deposition

All plant cells are provided with the necessary rigidity to withstand the turgor by an exterior cell wall. This wall is composed of long crystalline cellulose microfibrils embedded in a matrix of other polysaccharides. The cellulose microfibrils are deposited by mobile membrane bound protein complexes in remarkably ordered lamellar textures. The mechanism by which these ordered textures arise, however, is still under debate. The geometrical model for cell wall deposition proposed by Emons and Mulder (Proc. Natl. Acad. Sci. 95, 7215–7219, 1998) provides a detailed approach to the case of cell wall deposition in non-growing cells, where there is no evidence for the direct influence of other cellular components such as microtubules. The model successfully reproduces even the so-called helicoidal wall; the most intricate texture observed. However, a number of simplifying assumptions were made in the original calculations. The present work addresses the issue of the robustness of the model to relaxation of these assumptions, by considering whether the helicoidal solutions survive when three aspects of the model are varied. These are: (i) the shape of the insertion domain, (ii) the distribution of lifetimes of individual CSCs, and (iii) fluctuations and overcrowding. Although details of the solutions do change, we find that in all cases the overall character of the helicoidal solutions is preserved.     

Effect of sublethal concentrationsof mercury, cadmium, and copper salts on the lysozyme content in fryof the lena river sturgeon acipenser baeri

The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon exposed to the presence of sublethal concentrations of Hg2⁺, Cd²⁺, and Cu²⁺ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and structural-functional organization of the studied organs.