Structural study of the solutions of acidic polysaccharides. II. Study of some thermodynamic properties of the dilute pectin solutions with different degrees of esterification

Light-scattering and surface tension measurements have been used to study the structure and thermodynamic properties of aqueous-salt solutions of pectins, with the degree of esterification (DE) varying from 5 to 100%. Pectin macromolecules were found in the form of aggregates with $\text{M}{}_{\mathrm{w}}\text{= 5 × 10}{}^5\text{to}\text{1·1 × 10}{}^6\text{D}$. The second virial coefficient of pectin was negative in these poor solvents and independent of the DE. Pectins exhibited surface activity at the air-water interface, the magnitude of this activity also being independent of the DE.     

Structural study of the solutions of acidic polysaccharides. I. Study of the solutions of acidic polysaccharides by measuring the activity coefficients of counterions

The activity coefficient of sodium ions in the presence of acidic polysaccharides (alginates, pectins and κ-carrageenan) has been measured potentiometrically. The activity determined in limit-dilute solutions normally exceeded the values calculated from the Manning theory. The difference (ΔγNa+) was regarded as a measure of the chain flexibility. The change in ΔγNa+ when a polysaccharide is transferred from water to 8 m urea solution gave information about the contribution of hydrogen bonds to the stabilisation of the polysaccharide conformation in aqueous solutions. The function n is independent of polymer concentration at high concentration. where γNa+exp is the experimentally determined value of the activity coefficient of the counterions and γNa+NaCl is the activity coefficient of sodium ions in a NaCl solution of the same equivalent concentration as the polymer solution.This suggests that the polysaccharide solution has a microheterogeneous structure. The results of the viscosity measurements suggest that there is agreement between equilibrium thermodynamic and rheological evaluation criteria for the structure of polymer solutions.     

Sorption of anionic polysaccharides by cellulose

The sorption of anionic polysaccharides pectin, alginate, and xanthan with cellulose were investigated in presence of calcium. Calcium sorption to cellulose was limited by the carboxyl group content in fibers. Atomic Absorption Spectroscopy (AAS) analysis was used to measure the calcium in cellulose fibers and chemical oxygen demand (COD) analysis reveals that the divalent ions calcium can bind the polysaccharide onto cellulose fibers. The amount of calcium and polysaccharide bound in Ca²⁺/polysaccharide modified cellulose fibers was 5.8-12.5 mM Ca²⁺/kg fibers and 1500-2400 mg polysaccharide/kg fibers, respectively. Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR) analysis confirmed the presence of polysaccharide on calcium containing cellulose fibers. The results of alizarin dyeing experiments at the end of polysaccharide sorption further confirmed the presence of calcium in Ca²⁺/polysaccharide modified cellulose fibers. The basic phenomenon of interaction of soluble ionic polysaccharide and cellulosic fibers in presence of divalent cations such as calcium is a key to understand biological functions and technological applications.     

Structural studies on extracellular acidic polysaccharides secreted by three non-nodulating rhizobia

The structures of extracellular, acidic polysaccharides from three non-nodulating rhizobia, Rhizobium trifolii AHU 1134, Rhizobium phaseoli AHU 1133, and Rhizobium lupini KLU were studied by a method involving successive fragmentation with specific two β-d-glycanases of Flavobacterium M64. These three polysaccharides are composed of repeating units of the octassacharide shown. Half of the terminal d-galactose residues are substituted by pyruvic acid acetal groups.

     

Pre-nucleation crystallization studies on aminoacyl-tRNA synthetases by dynamic light-scattering.

Dynamic light-scattering (DLS) studies on solutions of proteins approaching their precipitation point were made with asparaginyl- (NRSEC), leucyl- (LRSEC) and valyl- (VRSEC) tRNA synthetases from Escherichia coli. The three aminoacyl-tRNA synthetases have not been crystallized previously. As a control system, we used E. coli polypeptide elongation factor Tu (EF-Tu). Apart from the different proteins used here, the methods we employed differed from previous studies in that (1) instead of making a series of measurements on individual samples at various concentrations, the protein solutions were titrated with the precipitants, and (2) the results of the light-scattering measurements were analysed by a new maximum entropy procedure that calculates a particle size distribution in a highly reproducible way. The particle size distributions of protein solutions titrated with precipitants showed two major peaks in most cases. For both peaks, relative areas and mean diffusion coefficients were determined. The diffusion constants were corrected for the viscosity of the solutions. From comparing the results on the proteins known to crystallize (EF-Tu) with the amorphously precipitating systems (LRSEC, NRSEC) we find two necessary, but not sufficient, conditions for the formation of crystals: the diffusion coefficient of the monomer peak stays constant until very close to the precipitation point; the percentage of large aggregates stays small (less than 10% of the scattered light intensity) during the titration. For VRSEC, both ammonium sulphate and sodium citrate showed a low percentage of large aggregates and a constant diffusion coefficient of the main (protein monomer) peak below the precipitation point. This indicates that both would be possible precipitants for the crystallization of this enzyme. Crystallization trials using both these salts were carried out, and although no condition could as yet be found for obtaining crystals with ammonium sulphate solutions, crystals of the enzyme have been obtained with sodium citrate.     

Quasielastic light-scattering study of polyadenylic acid solutions. II

Quasielastic light scattering is used to study saline solutions of polyadenylic acid with varying polymer concentrations and molecular masses. These experiments clearly show the existence of two relaxation times. For dilute solutions, when the chains are mutually independent, the fast mode is due to the free diffusion of the polymer chains. For concentrations above the overlap concentration C*, the fast mode is due to the propagation of collective excitations of the pseudolattice of polymer chains. The slow modes are observed when the polymer concentration is in the vicinity of the overlap concentration C*. A series of experiments shows that both their relaxation time and amplitude depend only on the polymer concentration and not on the polymer molecular mass. This result rules out any previous explanation based on individual chain motion. Furthermore, since the amplitudes depend on the time elapsed from the preparation of the solution, the slow modes are due to the diffusion of concentration inhomogeneities in the pseudolattice.     

Structural studies of polysaccharides from aloe vera

The mucilaginous jelly from the leaves of Aloe vera is composed of at least four different partially acetylated glucomannans that differ in their glucose-to-mannose ratios and acetyl contents. Methylation and periodate-oxidation studies indicate that the glucomannans are linear polymers containing (1→4)-glycosidic linkages.     

Structural studies of arabinogalactan and pectin from Silene vulgaris (M.) G. callus

Arabinogalactan and pectin (named silenan) were isolated from Silene vulgaris (M.) G. callus. Fractionation by ion-exchange chromatography on DEAE-cellulose and digestion with pectinase demonstrated that silenan from S. vulgaris callus (80% of D-galacturonic acid) and silenan from the aerial part of the campion S. vulgaris are similar: both pectins contain a high quantity of homogalacturonan segments. The NMR spectral data and mass spectrometry of the purified polysaccharide and its fragment obtained by Smith degradation confirmed that the core of the arabinogalactan consisted of the different segments of β-1,3-D-galactopyranan. Some of the β-galactopyranose residues of the backbone are branched at O-6. The side chains of the arabinogalactan were shown to contain residues of terminal and 3-O-substituted β-galactopyranose, terminal α-arabinofuranose and α-rhamnopyranose, and 2-O-substituted α-rhamnopyranose. The α-rhamnopyranose residues in the sugar chain appeared to be 2-O-glycosylated by the β-1,4-D-galactopyranosyl uronic acid residues. Published in Russian in Biokhimiya, 2006, Vol. 71, No. 6, pp. 798–807.     

Quasi-elastic light-scattering studies of aqueous biliary lipid systems. Cholesterol solubilization and precipitation in model bile solutions

We have employed quasi-elastic light-scattering methods to characterize micellar aggregates and microprecipitates formed in aqueous solutions containing sodium taurocholate (TC), egg lecithin (L), and cholesterol (Ch). Particle size and polydispersity were studied as functions of Ch mole fraction (XCh = 0-15%), L/TC molar ratio (0-1.6), temperature (5-85 degrees C), and total lipid concentration (3 and 10 g/dL in 0.15 M NaCl). For XCh values below the established solubilization limits (XChmax) [Carey, M. C., & Small, D. M. (1978) J. Clin. Invest. 61, 998], added Ch has little influence on the size of simple TC micelles (type 1 systems), on the coexistence of simple and mixed TC-L micelles (type 2 systems), or on the growth of "mixed disc" TC-L micelles (type 3 systems). For supersaturated systems (XCh/XChmax greater than 1), 10 g/dL type 1 systems (L/TC = 0) exist as metastable micellar solutions even at XCh/XChmax = 5.3. Metastability is decreased in type 2 systems (0 less than L/TC less than 0.6), and "labile" microprecipitation occurs when XCh/XChmax exceeds approximately 1.6. In 10 g/dL mixtures, the microprecipitates initially range in size from 500 to 20000 A and later coalesce to form a buoyant macroscopic precipitate phase. In 3 g/dL mixtures, the microprecipitates are smaller (200-400 A) and remain as a stable, noncoalesced microdispersion. Transmission electron microscopy of the microprecipitates formed at both concentrations indicates a globular noncrystalline structure, and lipid analysis reveals the presence of cholesterol and lecithin in a molar ratio (Ch/L) of approximately 2/1, suggesting that the microprecipitates represent a metastable cholesterol-rich liquid-crystalline phase. In supersaturated type 3 systems (0.6 less than L/TC less than 2.0), the precipitate phase is a lecithin-rich liquid-crystalline phase which likewise coalesces in a 10 g/dL system but forms stable vesicle (liposomal) structures (600-800 A radius) in 3 g/dL systems. In conjunction with these experimental data, we present a quantitative thermodynamic analysis of Ch solubilization in model bile systems from which rigorous deductions of the free energy and enthalpy change for solubilization of cholesterol monohydrate in type 1 and type 2 systems are obtained. In addition, we employ homogeneous nucleation theory to analyze the origin of the metastable/labile limit in supersaturated systems and to deduce the interfacial tension between microprecipitates and solution. On the basis of these experimental data and theoretical analyses, we offer new hypotheses on the structure and physiology of bile and the pathogenesis of Ch gallstones. In particular, it is suggested that the "stable" microprecipitates observed in 3 g/dL type 2 systems may provide a secondary vehicle (in addition to micelles) for cholesterol transport in supersaturated hepatic bile.     

Quasi-elastic light-scattering studies of single skeletal muscle fibers.

Measurements were made of the intensity autocorrelation function, g(2)[tau], of light scattered from intact frog muscle fibers. During the tension plateau of an isometric tenanus, scattered field statistics were approximately Gaussian and intensity fluctuations were quasi-stationary. The half time, tau 1/2, for the decay of g(2)[tau] was typically 70 ms at a scattering angle of 30 degrees. The decay rate, 1/tau 1/2, of g(2)[tau] varied roughly linearly with the projection of the scattering vector on the fiber axis. 1/tau 1/2 was greater during the tension creep phase of tetani of highly stretched fibers, but was roughly independent of sarcomere length during the tension plateau. g(2)[tau] measured during rest or on diffraction pattern maxima during isometric contraction were flat with low amplitudes. These results are consistent with a model of a 200-mu m segment of an isometrically contracting fiber in which scattering material possesses relative axial velocities of 1-2 mu m/s accompanied by relative axial displacements greater than 0.1 mu m. The slow (1-2 mu m/s) motion of one portion of the fiber relative to another observed under the microscope (500X) during isometric contraction is consistent with the light-scattering results. Structural fluctuations on the scale of the myofibrillar sarcomere which may arise from asynchronous cycling of cross-bridges must involve relative axial velocities less than 3 mu m/s or relative axial displacements less than 0.05 mu m.     

Static and dynamic light-scattering studies of pectic polysaccharides from the middle lamellae and primary cell walls of cider apples

Mild, selective, and sequential methods have been used to extract pectic polysaccharides from cider apple pomace. The extracts have been characterised by static and dynamic light-scattering. CDTA (cyclohexane-trans-1,2-diaminetetra-acetate)-soluble pectic polysaccharides from the middle lamellae were characterised as having a broad-molecular-weight distribution (M_w = 4.2 × 10⁶)_of non-free-draining stiff coils. CDTA-insoluble and Na₂CO₃-soluble pectic polysaccharides (M_w = 1.1 × 10⁶) from the primary cell walls showed properties consistent with branched, cross-linked, microgel structures.