The reliability of wormlike polysaccharide chain dimensions estimated from electron micrographs

Doubling growth of egg-box structure during Calcium-mediated molecular assembly of alginate

Ca²⁺-mediated molecular assembly of alginate underpins its wide range of applications in foods, pharmaceutics, biomedicines, tissue engineering and environmental treatments. The mode of growth of egg-box structure of alginate in the presence of Ca²⁺ is a long-standing fundamental problem to be concluded. In this work, we investigate the Ca-induced structural evolution of alginate in dilute solution using atomic force microscopy and dilute solution viscometry. It is demonstrated that the structural evolution follows the three critical steps of monocomplexation, dimerization and multimerization, upon binding with Ca²⁺. Interestingly, the alginate single chains grow into dimers and multimers via a doubling mode, i.e., successive emerging of dimer, tetramer, octamer, and hexadecamer. Compared with lower guluronate (G) alginate, higher G alginate exhibits a more pronounced multimerization process occurring at a lower ratio of Ca/G. A mechanistic model depicting the evolution of egg-box structure is proposed. The results would add new knowledge to the current egg-box model regarding the molecular assembly and gelation of an important biopolymer alginate, and provide fundamental basis for molecular engineering of alginate for more advanced applications.

Single-molecule AFM study of hyaluronic acid softening in electrolyte solutions

Investigation of hyaluronic acid (HA) morphology and mechanical properties at a single-molecule level is important for the development of HA based biomaterials. We have developed the atomic force microscopy (AFM) based approach for quantitative characterization of conformation of HA molecules. HA molecules adsorbed on a modified graphitic surface form oriented linear segments. Conformation of HA molecules can be considered as two-dimensional quasi-projection of a three-dimensional conformation locally straightened by a substrate. The persistence length and Young's modulus of biomolecules estimated using wormlike chain model decrease from 15.7 to 9.9 nm, and from ∼21 to ∼13 GPa, respectively, when KCl concentration increases from 0 to 100 mM. The dependence of the persistence length on ionic strength supports the Odijk-Skolnick-Fixman model of polyelectrolyte stiffening in electrolyte solution. The obtained results represent a new insight into the conformation and mechanical characteristics of HA molecules and complement the characterization of this biopolymer by bulk methods.

Effect of Polysaccharide Conformation on Ultrafiltration Separation Performance

The manifold array of saccharide linkages leads to a great variety of polysaccharide architectures, comprising three conformations in aqueous solution: compact sphere, random coil, and rigid rod. This conformational variation limits the suitability of the commonly applied molecular weight cut-off (MWCO) as selection criteria for polysaccharide ultrafiltration membranes, as it is based on globular marker proteins with narrow M_w and hydrodynamic volume relation. Here we show the effect of conformation on ultrafiltration performance using randomly coiled pullulan and rigid rod-like scleroglucan as model polysaccharides for membrane rejection and molecular weight distribution. Ultrafiltration with a 10 kDa polyethersulfone membrane yielded significant different recoveries for pullulan and scleroglucan showing 1% and 71%, respectively. We found deviations greater than 77-fold between nominal MWCO and apparent M_w of pullulan and scleroglucan, while recovering over 90% polysaccharide with unchanged M_w. We anticipate our work as starting point towards an optimized membrane selection for polysaccharide applications.

Chain flexibility and dynamics of alginate solutions in different solvents

Mechanical rheometry, specifically rotational rheometry, squeeze flow, and capillary rheometry, and two microrheology methods, namely multiple-particle tracking (MPT) and diffusing wave spectroscopy (DWS) have been used to get new insight into structural and dynamical properties of alginate dissolved in solvents widely used for bioprinting, namely deionized water, phosphate-buffered saline (PBS), and Dulbecco Modified Eagle Medium (DMEM) cell media. Results demonstrate that alginate rheological properties depend on the solvent quality at concentrations higher than 1 wt.%. In this high concentration regime, in aqueous salt-free and PBS solutions, experimental scaling exponents for the concentration dependence of the specific viscosity ηsp and the plateau modulus G0 agree well with theoretical predictions for neutral polymers in good solvent whereas for the terminal relaxation time TR, the exponent is slightly higher than theoretically predicted, presumably due to the formation of aggregates. For alginate dissolved in DMEM, all exponents for ηsp, G0, and TR agree with predictions for polymers in theta solvents, which might be related to the formation of polyelectrolyte complex as a result of interactions between alginate and amino acids. Chain persistence length lp values, as determined directly from high frequency rheometry for the first time, are independent of alginate concentration and temperature. Lower absolute lp values were found for DMEM solutions compared with the other solvents. Moreover, scaling exponents for ηsp, G0, and TR do not change with temperature, within 20 and 60 °C. These findings suggest no change in the conformation of alginate chains with temperature.

Primary, Secondary, Tertiary and Quaternary Structure Levels in Linear Polysaccharides: From Random Coil, to Single Helix to Supramolecular Assembly

Polysaccharides are ubiquitous in nature and represent an essential class of biopolymers with multiple levels of conformation and structural hierarchy. However, a standardized structural nomenclature, as in the case of proteins, is still lacking due to uncertainty on their hierarchical organization. In this work we use carrageenans as model polysaccharides to demonstrate that several structural levels exist and can be unambiguously resolved by statistical analysis on high resolution Atomic Force Microscopy images, supported by spectroscopic, X-ray scattering and rheological techniques. In direct analogy with proteins, we identify primary, secondary, tertiary and quaternary structures. The structure-property relationship induced by monovalent ions for κ-, ι- and the non-gelling control λ-carrageenan is established from the single chain regime to the occurrence of hydrogels at higher concentrations. For κ-carrageenan in the presence of potassium, a disorder-order transition from random coil to single helix is first observed (secondary structure), followed by intrachain supercoiling events (tertiary structure) and macroscopic anisotropic domains which are parts of a network (quaternary structure) with tunable elasticity up to ∼10³ Pa. In contrast, κ-carrageenan in the presence of sodium only produces changes in secondary structure without supercoiling events, prior to formation of gels, highlighting the ion-specificity of the process. Loosely intertwined single helices are observed for ι-carrageenan in the presence of sodium and potassium chloride, providing an elastic mesh with many junction zones, while λ-carrageenan does not undergo any structural change. A generality of the observed behavior may be inferred by extending these observations to a distinct class of polysaccharides, the weak carboxylic polyelectrolyte Gellan gum. These results advance our understanding of ion-specific structural changes of polysaccharides and the physical mechanisms responsible for their gelation.

Nanotemplate-directed DNA segmental thermal motion

Nanotemplate directed DNA segmental thermal motion on molecular nanotemplates on graphite was directly observed and characterized using AFM in a liquid.

Computation of the effective mechanical response of biological networks accounting for large configuration changes

The asymptotic homogenization technique is involved to derive the effective elastic response of biological membranes viewed as repetitive beam networks. Thereby, a systematic methodology is established, allowing the prediction of the overall mechanical properties of biological membranes in the nonlinear regime, reflecting the influence of the geometrical and mechanical micro-parameters of the network structure on the overall response of the equivalent continuum. Biomembranes networks are classified based on nodal connectivity, so that we analyze in this work 3, 4 and 6-connectivity networks, which are representative of most biological networks. The individual filaments of the network are described as undulated beams prone to entropic elasticity, with tensile moduli determined from their persistence length. The effective micropolar continuum evaluated as a continuum substitute of the biological network has a kinematics reflecting the discrete network deformation modes, involving a nodal displacement and a microrotation. The statics involves the classical Cauchy stress and internal moments encapsulated into couple stresses, which develop internal work in duality to microcurvatures reflecting local network undulations. The relative ratio of the characteristic bending length of the effective micropolar continuum to the unit cell size determines the relevant choice of the equivalent medium. In most cases, the Cauchy continuum is sufficient to model biomembranes. The peptidoglycan network may exhibit a re-entrant hexagonal configuration due to thermal or pressure fluctuations, for which micropolar effects become important. The homogenized responses are in good agreement with FE simulations performed over the whole network. The predictive nature of the employed homogenization technique allows the identification of a strain energy density of a hyperelastic model, for the purpose of performing structural calculations of the shape evolutions of biomembranes.

Novel imaging technologies for characterization of microbial extracellular polysaccharides

Understanding of biology is underpinned by the ability to observe structures at various length scales. This is so in a historical context and is also valid today. Evolution of novel insight often emerges from technological advancement. Recent developments in imaging technologies that is relevant for characterization of extraceullar microbiological polysaccharides are summarized. Emphasis is on scanning probe and optical based techniques since these tools offers imaging capabilities under aqueous conditions more closely resembling the physiological state than other ultramicroscopy imaging techniques. Following the demonstration of the scanning probe microscopy principle, novel operation modes to increase data capture speed toward video rate, exploitation of several cantilever frequencies, and advancement of utilization of specimen mechanical properties as contrast, also including their mode of operation in liquid, have been developed on this platform. Combined with steps in advancing light microscopy with resolution beyond the far field diffraction limit, non-linear methods, and combinations of the various imaging modalities, the potential ultramicroscopy toolbox available for characterization of exopolysaccharides (EPS) are richer than ever. Examples of application of such ultramicroscopy strategies range from imaging of isolated microbial polysaccharides, structures being observed when they are involved in polyelectrolyte complexes, aspects of their enzymatic degradation, and cell surface localization of secreted polysaccharides. These, and other examples, illustrate that the advancement in imaging technologies relevant for EPS characterization supports characterization of structural aspects.

Statistical analysis of molecular nanotemplate driven DNA adsorption on graphite

In this work, we have studied the conformation of DNA molecules aligned on the nanotemplates of octadecylamine, stearyl alcohol, and stearic acid on highly oriented pyrolytic graphite (HOPG). For this purpose, fluctuations of contours of adsorbed biopolymers obtained from atomic force microscopy (AFM) images were analyzed using the wormlike chain model. Moreover, the conformations of adsorbed biopolymer molecules were characterized by the analysis of the scaling exponent ν, which relates the mean squared end-to-end distance and contour length of the polymer. During adsorption on octadecylamine and stearyl alcohol nanotemplates, DNA forms straight segments, which order along crystallographic axes of graphite. In this case, the conformation of DNA molecules can be described using two different length scales. On a large length scale (at contour lengths l > 200-400 nm), aligned DNA molecules have either 2D compact globule or partially relaxed 2D conformation, whereas on a short length scale (at l ≤ 200-400 nm) their conformation is close to that of rigid rods. The latter type of conformation can be also assigned to DNA adsorbed on a stearic acid nanotemplate. The different conformation of DNA molecules observed on the studied monolayers is connected with the different DNA-nanotemplate interactions associated with the nature of the functional group of the alkane derivative in the nanotemplate (amine, alcohol, or acid). The persistence length of λ-DNA adsorbed on octadecylamine nanotemplates is 31 ± 2 nm indicating the loss of DNA rigidity in comparison with its native state. Similar values of the persistence length (34 ± 2 nm) obtained for 24-times shorter DNA molecules adsorbed on an octadecylamine nanotemplate demonstrate that this rigidity change does not depend on biopolymer length. Possible reasons for the reduction of DNA persistence length are discussed in view of the internal DNA structure and DNA-surface interaction.

Equivalent mechanical properties of biological membranes from lattice homogenization

The goal of this manuscript is to set up a novel methodology for the calculation of the effective mechanical properties of biological membranes viewed as repetitive networks of elastic filaments, based on the discrete asymptotic homogenization method. We will show that for some lattice configurations, flexional effects due to internal structure mechanisms at the unit cell scale lead to additional flexional effects at the continuum scale, accounted for by an internal length associated to a micropolar behavior. Thereby, a systematic methodology is established, allowing the prediction of the overall mechanical properties of biological membranes for a given network topology, as closed form expressions of the geometrical and mechanical micro-parameters. The peptidoglycan and the erythrocyte have been analyzed using this methodology, and their effective moduli are calculated and recorded versus the geometrical and mechanical lattice parameters. A classification of lattices with respect to the choice of the equivalent continuum model is proposed: The Cauchy continuum and a micropolar continuum are adopted as two possible effective medium, for a given beam model. The relative ratio of the characteristic length of the micropolar continuum to the unit cell size determines the relevant choice of the equivalent medium. In most cases, the Cauchy continuum is sufficient to model membranes in most of their configurations. The peptidoglycan network may exhibit a re-entrant hexagonal lattice, for which micropolar effects become important. This is attested by the characteristic length becoming larger than the beam length for such configurations. The homogenized moduli give accurate results for both membranes, as revealed by comparison with experimental measurements or simulation results from the literature at the network scale. A first insight into the nonlinear mechanical behavior of the hexagonal and triangular networks is lastly investigated using a perturbative method.

Higher order structure of short immunostimulatory oligonucleotides studied by atomic force microscopy

Immunostimulatory CpG-DNA activates the innate immune system by binding to Toll-like receptor 9. Structurally different CpG-containing oligonucleotides trigger a different type of immune response while activating the same receptor. We therefore investigated the higher order structure of two different classes of immunostimulatory CpG-DNA. Class A, which contains a partly self-complementary sequence and poly-G ends, forms duplexes and nanoparticles in salt solution, while class B, which does not contain these features and is purely linear, does not form a duplex or nanoparticles. Results obtained here by high-resolution atomic force microscopy of classes A and B CpG-DNA, reflect these differences in secondary structure. Detailed structural analysis of the atomic force microscopy topographs is presented for two different sample preparation methods.

Individual bottle brush molecules in dense 2D layers restoring high degree of extension after collapse-decollapse cycle: directly measured scaling exponent

We prepared dense films of adsorbed brush-like macromolecules on mica substrate by transfer of compressed Langmuir monolayers from water subphase. The main macromolecular contours in the dense films were clearly resolved by SFM. The films were subjected to successive treatments by ethanol and water vapours. In accordance with previous results for isolated macromolecules, the films underwent collapse and subsequent decollapse morphological transformations in the changing vapour environment. Statistical analysis of the macromolecular dimensions in the films allowed us to measure the values of the scaling exponent nu determining the correlation between mean lateral and linear dimensions of the macromolecules. The analysis showed that the macromolecular conformations in the film as transferred were similar to the previously described conformations of the same macromolecules deposited directly on mica as isolated chains at much lower surface densities. The determined nu was close to the 0.75 value corresponding to the 2D SAW statistics. We assumed that the molecules retained the high degree of extension during the compression step due to suppressed reorganisation of the side chains. Differently from previous observations for isolated macromolecules, the restored conformations in the dense films after collapse-decollapse cycle were more extended with the nu of about 0.73 value. A theoretical explanation of the high degree of re-extension is proposed.

Explicit-solvent molecular dynamics simulations of the polysaccharide schizophyllan in water

Schizophyllan is a beta(1-->3)-D-glucan polysaccharide with beta(1-->6)-branched lateral glucose residues that presents a very stiff triple-helical structure under most experimental conditions. Despite the remarkable stability of this structure (which persists up to 120 degrees C in aqueous solution), schizophyllan undergoes a major change of state around 7 degrees C in water that has been hypothesized to result from an order-disorder transition in the lateral residues. This hypothesis is only supported by indirect experimental evidence and detailed knowledge (at the atomic level) concerning hydrogen-bonding networks, interactions with the solvent molecules, orientational freedom of the lateral residues, and orientational correlations among them is still lacking. In this study explicit-solvent molecular dynamics simulations of a schizophyllan fragment (complemented by simulations of its tetrasaccharide monomer) are performed at three different temperatures (273 K, 350 K, and 450 K) and with two different types of boundary conditions (finite nonperiodic or infinite periodic fragment) as an attempt to provide detailed structural and dynamical information about the triple-helical conformation in solution and the mechanism of the low-temperature transition. These simulations suggest that three important driving forces for the high stability of the triple helix are i), the limited conformational work involved in its formation; ii), the formation of a dense hydrogen-bonding network at its center; and iii), the formation of interchain hydrogen bonds between main-chain and lateral glucose residues. However, these simulations evidence a moderate and continuous variation of the simulated observables upon increasing the temperature, rather than a sharp transition between the two lowest temperatures (that could be associated with the state transition). Although water-mediated hydrogen-bonded association of neighboring lateral residues is observed, this interaction is not strong enough to promote the formation of an ordered state (correlated motions of the lateral residues), even at the lowest temperature considered.

Conformational changes and aggregation of alginic acid as determined by fluorescence correlation spectroscopy

Insight into the conformations and aggregation of alginic acid was gained by measuring its diffusion coefficient at very dilute concentrations using fluorescence correlation spectroscopy. Both the pH and ionic strength (I) had an important influence on the diffusion coefficient of the polysaccharide. For pH, three effects were isolated: (i) below pH 4, the charge density decreased causing increased aggregation; (ii) between pH 4 and 8, a molecular expansion was observed with increasing pH, whereas (iii) above pH 8 some dissociation of the polymer was observed. Increasing I from 0.001 to 0.1 M resulted in a ca. 20% increase in the diffusion coefficient. By coupling these measurements to molar mass determinations obtained by size exclusion chromatography and monomer size estimations determined from ab initio calculations, it was possible to determine the radii of gyration via de Gennes renormalization theory. From diffusion coefficients and radii of gyration obtained as a function of ionic strength, persistence lengths (total, electrostatic, and intrinsic) were calculated from the Benoit-Doty relationship.

Small angle x-ray scattering study of local structure and collapse transition of (1,3)-β-D-glucan-chitosan gels

Scleroglucan is a (1,3)-beta-D-glucan polysaccharide produced by the fungus Sclerotium. Dissolved in water at room temperature it adopts a linear, rigid, triple helical structure. Gelation of scleroglucan can be obtained by Schiff-base formation between partly periodate oxidized scleroglucan and the primary amine groups of chitosan. The scleraldehyde-chitosan gels exhibit a collapse transition when exposed to volume fractions of isopropanol, Wp, larger than 65%. The aim of the present study is to provide structural information concerning the local polymer distribution and the collapse transition in (1,3)-beta-D-glucan-chitosan gels. Small angle x-ray scattering was used to investigate solutions and gels of scleroglucan in water, as well as in an aqueous mixture containing 65% isopropanol. The results reveal that in aqueous solution, the polysaccharide scleroglucan chains have an approximately cylindrical cross section of external diameter close to 17 A. The gels display the same local structure, but form clusters on a longer distance scale. For the collapsed gels in the water-isopropanol mixture, partial phase separation occurs in which ordered domains of approximate size of 110 A develop. This study indicates that local ordering in liquid-crystalline-type domains is a possible molecular mechanism contributing to the collapse of gels composed of semiflexible polymers. The triple helical structure of the molecule appears not to be conserved in the majority phase in this solvent, but it is conserved in the liquid crystalline domains.

Morphologies and conformation transition of lentinan in aqueous NaOH solution

Molecular morphologies and conformation transition of lentinan, a beta-(1-->3)-D-glucan from Lentinus edodes, were studied in aqueous NaOH solution by atomic force microscopy (AFM), viscometry, multiangle laser light scattering, and optical rotation measurements. The results revealed that lentinan exists as triple-helical chains and as single random-coil chains at NaOH concentration lower than 0.05M and higher than 0.08M, respectively. Moreover, the dramatic changes in weight-average molecular weight Mw, radius of gyration [s2](1/2), intrinsic viscosity [eta], as well as specific optical rotation at 589 nm [alpha]589 occurred in a narrow range of NaOH concentration between 0.05 and 0.08M NaOH, indicating that the helix-coil conformation transition of lentinan was carried out more easily than that of native schizophyllan and scleroglucan, and was irreversible. For the first time, we confirmed that the denatured lentinan molecule, which was dissolved in 0.15M NaOH to be disrupted into single coil chains, could be renatured as triple helical chain by dialyzing against abundant water in the regenerated cellulose tube at ambient temperature (15 degrees C). In view of the AFM image, lentinan in aqueous solution exhibited the linear, circular, and branched species of triple helix compared with native linear schizophyllan or scleroglucan.

Hyaluronan conformations on surfaces: effect of surface charge and hydrophobicity

Extended, relaxed, condensed, and interacting forms of the polysaccharide hyaluronan have been observed by atomic force microscopy (AFM). The types of images obtained depend on the properties of the surfaces used. We have investigated several different surface conditions for HA imaging, including unmodified mica, mica chemically modified with two different kinds of amino-terminated silanes (3-aminopropyltriethoxysilane and N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride), and highly oriented pyrolytic graphite. We found the degree of HA molecular extension or condensation to be variable, and the number of bound chains per unit area was low, for all of the mica-based surfaces. HA was more easily imaged on graphite, a hydrophobic surface. Chains were frequently observed in high degrees of extension, maintained by favorable interaction with the surface after molecular combing. This observation suggests that the HA macromolecule interacts with graphite through hydrophobic patches along its surface. AFM studies of HA behavior on differing surfaces under well-controlled environmental conditions provides useful insight into the variety of conformations and interactions likely to be found under differing physiological conditions.

Extended, relaxed, and condensed conformations of hyaluronan observed by atomic force microscopy

The conformation of the polysaccharide hyaluronan (HA) has been investigated by tapping mode atomic force microscopy in air. HA deposited on a prehydrated mica surface favored an extended conformation, attributed to molecular combing and inhibition of subsequent chain recoil by adhesion to the structured water layer covering the surface. HA deposited on freshly cleaved mica served as a defect in a partially structured water layer, and favored relaxed, weakly helical, coiled conformations. Intramolecularly condensed forms of HA were also observed, ranging from pearl necklace forms to thick rods. The condensation is attributed to weak adhesion to the mica surface, counterion-mediated attractive electrostatic interactions between polyelectrolytes, and hydration effects. Intermolecular association of both extended and condensed forms of HA was observed to result in the formation of networks and twisted fibers, in which the chain direction is not necessarily parallel to the fiber direction. Whereas the relaxed coil and partially condensed conformations of HA are relevant to the native structure of liquid connective tissues, fully condensed rods may be more relevant for HA tethered to a cell surface or intracellular HA, and fibrous forms may be relevant for HA subjected to shear flow in tight intercellular spaces or in protein-HA complexes.

Characterisation of bacterial polysaccharides: steps towards single-molecular studies

Techniques used in studies of polysaccharides, including chemical composition, linkage pattern, and higher order structures are in constant development. They provide information necessary for understanding of the polysaccharide properties and functions. Here, recent advancements in studies of the polysaccharides at the single-molecule level are highlighted. Over the last few years, single-molecule techniques such as force spectroscopy have improved in sensitivity and can today be used to detect forces in the pN range. In addition, these techniques can be used to investigate properties of single molecules close to physiological conditions. The challenges in the interpretation of the observations are aided by control experiments using well-characterised polysaccharides and by data provided by complementary methods. This field is expected to have increasing impact on the further advancement of the molecular understanding of the role of polysaccharides in various biological processes such as recognition and cell adhesion.

Rheology of concentrated isotropic and anisotropic xanthan solutions: 3. Temperature dependence

The oscillatory rheology of one rodlike and one semiflexible xanthan sample has been investigated as a function of temperature in the range of xanthan concentrations where the polymer forms a lyotropic liquid crystalline phase in aqueous NaCl solutions. Readily observed changes in the rheological observables at temperatures corresponding to phase boundaries permit construction of the biphasic chimney region of the temperature-composition phase diagram. The chimney region leans toward larger values of the polymer concentration with increasing temperature, presumably as a consequence of a reduction in the effective axial ratio of the helical polymer with increasing temperature. The results permit construction of plots of the rheological observables as a function of polymer concentration at temperatures T in the range 20 <or= T <or= 90 degrees C. Characteristic features of these curves observed at room temperature are preserved at higher temperatures, provided the xanthan double helix remains intact. The temperature dependence of the viscosity of isotropic xanthan solutions can be described with the Arrhenius law. For anisotropic solutions the viscosity increases with T at the higher end of the experimental temperature range, presumably because higher temperatures reduce the order parameter of the liquid crystalline phase with a concomitant increase in viscosity. At low NaCl concentration, and low polymer concentration, the xanthan helix order-disorder transition occurs at temperatures T(m) below 90 degrees C. At temperatures above T(m) the rheological observables reveal the onset of network formation involving xanthan chains released from the ordered helical structure. When these systems are cooled back below T(m), extensive network formation develops with large increases in viscosity and in the storage and loss moduli.

Macromolecular triplex zipping observed in derivatives of fungal (1 --> 3)-beta-D-glucan by electron and atomic force microscopy

Scleroglucan, a comb-like branched (1 --> 3)-beta-D-glucan, dissolves in water as a stiff, triple-helical structure with the single glucose branches extending from the surface. The aim of this study is to investigate structural changes in the triple-helical structure associated with selective chemical modification of the side chains. Electron and atomic force microscopy, respectively, were used to investigate the macromolecular structures of aldehyde and carboxylated derivatives of scleroglucan-namely, scleraldehyde and sclerox-with different degrees of substitution. Scleraldehyde was observed to have structures resembling the triplex of the unmodified scleroglucan for all degrees of substitution up to 1.0. Additionally, an increasing tendency to aggregate for the higher degrees of substitution was observed. Fully carboxylated scleroglucan, sclerox(1.0), prepared from solutions at ionic strengths below 1.0M, revealed dispersed, flexible, coil-like structures. This indicates an electrostatic-driven strand separation of the scleroglucan triple-helical structure occurring concomitant with an increasing fraction of the side chains bearing carboxylate groups. Annealed sclerox(1.0) samples in aqueous 1.0 and 1.5M NaCl exhibited partly, or completely, reassociated triplex ensembles, with species ranging from apparently fully zipped linear and circular topologies, partly zipped structures with triplex strand separation occurring at the ends, to dispersed single-strands with random coil-like appearance. This study shows that periodate oxidation of the scleroglucan side chains is not a sufficient modification of the side chains to induce dissociation of the triple-helical structure, whereas further oxidation of the side chains to carboxylic groups dissociates the triple-helical structure when the degree of substitution is above 0.6.

The cytokine stimulating activity of (1-->3)-beta-D-glucans is dependent on the triple helix conformation

The immunomodulating properties of comb-like branched (1-->3)-beta-D-glucans scleroglucan, schizophyllan and lentinan depend on branching pattern, molecular weight and higher-order structure. The effect of weight average molecular weight Mw and higher order structure of scleroglucan, on stimulation of human monocytes cultured in vitro to secrete tumor necrosis factor-alpha (TNF-alpha) was investigated. The higher order structures of the scleroglucan samples were determined by electron microscopy. The data showed that the samples with a linear wormlike, triple helical structure with Mw less than 50 x 10(4) g/mol or larger than 110 x 10(4) g/mol stimulated the monocytes more efficiently than samples with Mw in the range (67-110) x 10(4) g/mol. The denaturation of the linear triple helices by NaOH (> 0.25 M), followed by neutralization yielded blends of linear and macrocyclic topologies with concomitant irreversible reduction of the cytokine inducing activity compared with the untreated scleroglucans. The dose-dependent ability to activate monocytes to cytokine production was not restored following annealing of the denatured-renatured samples, despite the fact that electron micrographs revealed similar structures of these annealed samples to the starting material. Pre-incubation of monocytes with antibodies against cluster of differentiation antigens CD14 or CD11b reduced the scleroglucan potency to stimulate TNF-alpha secretion mainly for mAb against CD14 in the presence of serum.

Tapping mode Atomic Force Microscopy of scleroglucan networks

Tapping mode Atomic Force Microscopy (TmAFM) has been used to study the fungal polysaccharide scleroglucan deposited from aqueous solution and dimethyl sulfoxide (DMSO) onto a mica surface. The solutions from which the microscope samples were produced were prepared by first dissolving the solid scleroglucan in 0.1M NaOH, then neutralizing the solution with HCl, followed by dilution to the required concentration in either water or DMSO. It was found that from the aqueous solution described above, scleroglucan forms networks. Based on a comparison of the denatured-renatured and aqueous solution samples, network formation is due to the imperfect registration between the chains forming the triple helices. The relatively large stiffness of the scleroglucan triple helix is also assumed to contribute to the formation of the extended networks. The triple helix diameter was measured to be 0.92 +/- 0.27 nm, which is in the same range as that obtained by other researchers using similar techniques. Denatured scleroglucan, deposited from DMSO onto mica, forms a web-like layer on top of which there are sphere-like structures. These morphologies are believed to be due to triple helix denaturation yielding highly flexible single chains in DMSO, which results in coiling and web-like dense packing of scleroglucan upon deposition onto mica. Most interestingly after additional of water to the samples deposited from DMSO, some of the chains can be renatured into short, stiff rod-like structures which are similar to the structures observed by others researchers. The imaging data for aqueous solution deposition can be analyzed by plotting maximum end-to-end distance versus the perimeter of the networks deposited onto mica. This yields a Flory-like exponent of 0.67, which is almost similar in value to that obtained by other researchers for linear structures of scleroglucan but less than that expected for a polymer chain following a self-avoiding walk (upsilon = 0.75) model on a two-dimensional surface. The fractal dimension that can be used to characterize the networks was determined graphically to be 1.22 +/- 0.06.

Imaging of carrageenan macrocycles and amylose using noncontact atomic force microscopy

Samples of kappa-carrageenan, iota-carrageenan, and synthetic amylose have been examined by atomic force microscopy (AFM). All samples were spray deposited from aqueous solutions onto freshly cleaved mica, air dried, and imaged in air using noncontact atomic force microscopy (NCAFM). Images of single stranded amylose and carrageenan are presented. At relatively low polymer concentrations in the presence of NaCl iota-carrageenan formed circles that appear to be predominantly head-to-tail associated unimeric duplex (double stranded) structures. At higher iota-carrageenan concentrations the polymer forms circles and aggregates that appear to involve dimeric duplex structure. Direct comparison of synthetic amylose molecular weights determined from NCAFM images with results from solution measurements showed that NCAFM provides an excellent way to measure amylose molecular weight and molecular weight distribution. It is shown that synthetic amylose is single stranded in aqueous solution and that the chain length distribution is broader than the Poisson distribution anticipated from polymerization theory.

Conformational and configurational features of acidic polysaccharides and their interactions with calcium ions: a molecular modeling investigation

Modeling simulations have been performed on the four regular glycuronans: alpha-D-(1--->4) polygalacturonic, alpha-L-(1--->4) polyguluronic, beta-D-(1--->4) polymannuronic, and beta-D-(1--->4) polyglucuronic acids. The goal of this study was to characterize the similarities and differences in conformational and configurational behavior as well as in calcium binding in order to progress in the understanding of the physicochemical properties of the parent polysaccharides of industrial interest, namely pectin, alginate and glucuronan. This required the evaluation of the accessible conformational space for the disaccharide subunits of the four homopolymers, using the flexible residue protocol of the MM3 molecular mechanics procedure. The results were used to access the configurational statistics of representative polysaccharide chains, as well as for the determination of the regular polysaccharide helices and their conformational transitions. The surfaces of all regular helices likely to occur for each polyuronide were explored for cation binding using the GRID procedure. Both alpha-D-(1--->4) polygalacturonate and alpha-L-(1--->4) polyguluronate chains exhibit a high specificity for calcium binding, and have well-defined chelation sites. In contrast, beta-D-(1--->4) polymannuronate and beta-D-(1--->4) polyglucuronate chains do not display any stereospecificity for calcium binding. The results gathered from molecular modeling lead to a clear understanding of the different structural features that are displayed by the four ionic polymers.

Static curvature and flexibility measurements of DNA with microscopy. A simple renormalization method, its assessment by experiment and simulation

We present the derivation of equations based on statistical polymer chain analysis and a method to quantify the average angle value of intrinsic bends and the local flexibility at a given locus on DNA fragments imaged by electron microscopy. DNA fragments of n base-pairs are considered as stiff chains of n jointed unit rigid rods. If the DNA fragments are composed of two branches A0Am and A0Bn, with, respectively, m and n base-pairs, where the standard deviations of the angle formed by two consecutive base-pairs are uniform over each branch, respectively, sigmathetaA and sigmathetaB, we show that the standard deviation of the angle AmA0Bn is: [formula: see text] where sigmatheta0 is the standard deviation of the angle at locus A0. This equation is established for small angular deviations by analysis of DNA at different scales and the validity of the methodology is controlled with the computation of the reduced chi2 statistical test. The length of the DNA fragments must be of the order of, or below, the persistence length, as determined by sets of statistics from computer simulations of DNA fragments. This is verified experimentally by a detailed analysis of the digitized contours of homogeneous linear 139 base-pair DNA fragments observed by electron microscopy. The images are compared to the reconstruction of DNA fragments from the measurements. The value found, sigma0=4.6 degrees/bp, is consistent with the well-accepted value for DNA in a plane. We discuss the relationship between the standard deviation of the measured angles and the flexibility at the base-pair level. This method is useful to quantify directly from microscopy techniques, such as electron or scanning force microscopy, the true bending angle, either intrinsic or induced by a ligand, and its associated flexibility at a given locus in any small DNA fragment.

Imaging of individual biopolymers and supramolecular assemblies using noncontact atomic force microscopy

A variety of biopolymers is imaged using noncontact atomic force microscopy. Samples are prepared by aerosol spray deposition of aqueous solutions on freshly cleaved mica followed by air drying. The distributions of contour lengths and chain or fibril thicknesses normal to the mica substrate can be measured for individual polymer molecules or molecular assemblies. In many cases it is possible to conclude that the structures imaged and quantitatively analyzed are representative of those present in solution and not artifacts of the deposition/dessication process. Imaging of linear and cyclic triple helices of the polysaccharide scleroglucan is demonstrated. Measurements of the triple helix thickness normal to the mica surface are analyzed, and successful measurements of the molecular weight distribution and mean molar mass are described. It is demonstrated that the extent of chain association in the polysaccharide xanthan can be modulated by the addition of low molecular weight salts. The contour length and chain thickness distributions in a xanthan fraction are presented. Increases in the extent of chain association with increasing polymer concentration are documented for the gelling polysaccharide gellan, and the formation of stiff fibrillar gellan aggregates in the presence of added low molecular salt is demonstrated. Images are presented of the polysaccharide kappa-carrageenan in its disordered, and presumably single-stranded, state. Biopolymers other than polysaccharides can be imaged by the same technique; this is demonstrated with the fibrous protein collagen. In general it is shown that aerosol spray deposition of biopolymer samples can be used in conjunction with noncontact atomic force microscopy to provide a fast, reliable, and reproducible method for assessing the size and shape distributions of individual biological macromolecules and macromolecular assemblies in solution with a minimum of time and effort devoted to sample preparation.