Characterisation of cation binding and gelation of polyuronates by circular dichroism

In-situ speciation and estimation of iron(II) and iron(III) contents in anisotropic polysaccharide-based hydrogel by 1H low-field nuclear magnetic resonance

This article aims to quantify and differentiate in-situ iron(II) and/or iron(III) in heterogeneous polygalacturonate hydrogels using the 1H-NMR relaxometry technique. This holds significant importance, for example, in addressing iron-deficiency anemia through the oral administration of iron(II) supplements. The NMR dispersion profiles of the gels exhibited markedly distinct relaxation behaviors corresponding to the different iron oxidation states. At 20 MHz, two primary relaxation mechanisms must be considered: relaxation arising from water molecules confined within the polygalacturonate fiber mesh and paramagnetic relaxation due to iron cations. When iron(III) serves as the cross-linking agent, paramagnetic interaction dominates the relaxation, while with iron(II) as the cross-linker, both mechanisms have to be considered. To distinguish labile from structuring iron, we monitored the evolution of iron concentrations within the gels during successive washes using NMR and atomic absorption spectroscopy. Eventually, a gel containing both iron(II) and iron(III) was analyzed, and successful differentiation between the two cations was achieved. NMR relaxometry demonstrates powerful capabilities in terms of in-situ experiments, rapid results, speciation (iron(II)/iron(III)), and quantification (labile/ bridging iron).

Alginate-metal cation interactions: Macromolecular approach

Alginates are a broad family of linear (unbranched) polysaccharides derived from brown seaweeds and some bacteria. Despite having only two monomers, i.e. β-d-mannuronate (M) and its C5 epimer α-l-guluronate (G), their blockwise arrangement in oligomannuronate (..MMM..), oligoguluronate (..GGG..), and polyalternating (..MGMG..) blocks endows it with a rather complex interaction pattern with specific counterions and salts. Classic polyelectrolyte theories well apply to alginate as polyanion in the interaction with monovalent and non-gelling divalent cations. The use of divalent gelling ions, such as Ca2+, Ba2+ or Sr2+, provides thermostable homogeneous or heterogeneous hydrogels where the block composition affects both macroscopic and microscopic properties. The mechanism of alginate gelation is still explained in terms of the original egg-box model, although over the years some novel insights have been proposed. In this review we summarize several decades of research related to structure-functionships in alginates in the presence of non-gelling and gelling cations and present some novel applications in the field of self-assembling nanoparticles and use of radionuclides.

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.

Interpenetrated Biosurfactant-Biopolymer Orthogonal Hydrogels: The Biosurfactant's Phase Controls the Hydrogel's Mechanics

Controlling the viscoelastic properties of hydrogels is a challenge for many applications. Low molecular weight gelators (LMWGs) like bile salts and glycolipids and biopolymers like chitosan and alginate are good candidates for developing fully biobased hybrid hydrogels that combine the advantages of both components. Biopolymers lead to enhanced mechanics, while LMWGs add functionality. In this work, hybrid hydrogels are composed of biopolymers (gelatin, chitosan, and alginate) and microbial glycolipid bioamphiphiles, known as biosurfactants. Besides their biocompatibility and natural origin, bioamphiphiles can present chameleonic behavior, as pH and ions control their phase diagram in water around neutrality under strongly diluted conditions (<5 wt%). The glycolipid used in this work behaves like a surfactant (micellar phase) at high pH or like a phospholipid (vesicle phase) at low pH. Moreover, at neutral-to-alkaline pH in the presence of calcium, it behaves like a gelator (fiber phase). The impact of each of these phases on the elastic properties of biopolymers is explored by means of oscillatory rheology, while the hybrid structure is studied by small angle X-ray scattering. The micellar and vesicular phases reduce the elastic properties of the hydrogels, while the fiber phase has the opposite effect; it enhances the hydrogel's strength by forming an interpenetrated biopolymer-LMWG network.

The Effect of Pectin Branching on the Textural and Swelling Properties of Gel Beads Obtained during Continuous External Gelation Process

The aim of the study was to produce gel beads under continuous conditions. Pectins obtained from black and red currants and commercial apple pectin were used as the material. For the production of gel beads, a self-designed device was used. The designed device allows for the production of gel beads in a continuous process, the properties of which are similar to those obtained in the classic, batch process. Thanks to the device, it is possible to obtain a repeatable product while reducing the workload. The produced gel beads were tested for water absorption and textural properties. The water absorption of the obtained gel capsules is strongly influenced by the pectin chain structure. Pectin beads obtained from currant pectins have a less hard structure and are more sensitive to deformation than those from apple pectin. Shorter and more branched chains of currant pectin than apple pectin form gels with a delicate structure, which strongly absorbs water, and unlike apple pectin gel, it disintegrates. The results show that the use of raw material obtained from different sources allows for obtaining products with various properties, using the same method; moreover, the used device is fully scalable and can be used in large scale.

Extraction, characterization and spontaneous gelation mechanism of pectin from Nicandra physaloides (Linn.) Gaertn seeds

Nicandra physaloides (Linn.) Gaertn seeds (NPGS) could be manually scrubbed to obtain water-soluble pectin, which forms gel at room temperature without additives. The extraction, characterization and spontaneous gelation (SG) mechanism of the pectin were studied. The results showed that the pectin was located on the surface of NPGS and easily to be dissolved. Chemically, the pectin was low methoxy pectin with esterification degree of 46.93%, Gal-A content of 65.80%, and average molar weight of 631.15 kDa. The SG occurred at the pectin concentration of 1.5%, it can be destroyed by urea and SDS, however, EDTA cannot. In addition, KCl and NaCl induced the gelation of 1.0% pectin solution and the ions of K, Mg, Ca and Na were detected in the pectin. Hydrogen bonding, electrostatic and hydrophobic interaction contributed to the SG. This study could promote the commercial applications of the pectin in the field of edible colloids and cosmetics.

Alginate Nanoparticles for Drug Delivery and Targeting

Nanotechnology refers to the control, manipulation, study and manufacture of structures and devices at the nanometer size range. The small size, customized surface, improved solubility and multi-functionality of nanoparticles will continue to create new biomedical applications, as nanoparticles allow to dominate stability, solubility and bioavailability, as well controlled release of drugs. The type of a nanoparticle, and its related chemical, physical and morphological properties influence its interaction with living cells, as well as determine the route of clearance and possible toxic effects. This field requires cross-disciplinary research and gives opportunities to design and develop multifunctional devices, which allow the diagnosis and treatment of devastating diseases. Over the past few decades, biodegradable polymers have been studied for the fabrication of drug delivery systems. There was extensive development of biodegradable polymeric nanoparticles for drug delivery and tissue engineering, in view of their applications in controlling the release of drugs, stabilizing labile molecules from degradation and site-specific drug targeting. The primary aim is to reduce dosing frequency and prolong the therapeutic outcomes. For this purpose, inert excipients should be selected, being biopolymers, e.g. sodium alginate, commonly used in controlled drug delivery. Nanoparticles composed of alginate (known as anionic polysaccharide widely distributed in the cell walls of brown algae which, when in contact with water, forms a viscous gum) have emerged as one of the most extensively characterized biomaterials used for drug delivery and targeting a set of administration routes. Their advantages include not only the versatile physicochemical properties, which allow chemical modifications for site-specific targeting but also their biocompatibility and biodegradation profiles, as well as mucoadhesiveness. Furthermore, mechanical strength, gelation, and cell affinity can be modulated by combining alginate nanoparticles with other polymers, surface tailoring using specific targeting moieties and by chemical or physical cross-linking. However, for every physicochemical modification in the macromolecule/ nanoparticles, a new toxicological profile may be obtained. In this paper, the different aspects related to the use of alginate nanoparticles for drug delivery and targeting have been revised, as well as how their toxicological profile will determine the therapeutic outcome of the drug delivery system.

Development of novel gastroretentive salbutamol sulfate-loaded sodium alginate-pectin bubble beads prepared by co-axial needle air-injection method and in vivo clinical evaluation by ultrasound studies

In the present study, the salbutamol sulfate-loaded sodium alginate-pectin (SS-loaded SA-PEC) bubble beads have been optimized and evaluated for drug loading, in vitro drug release, in vivo floating behavior in the stomach, etc. Nine batches (F1-F9) of bubble beads with different SA and PEC contents were prepared by novel co-axial needle air-injection method and related to their percent drug loading efficiency (%DLE) and percent drug release at 4 h (%R_4h) as response factors. The multivariate analysis has shown the effect of SA/PEC ratio, total polymer content, as well as their interaction on %DLE and %R_4h. In the quantitative modeling, the satisfactory adjustment of the linear models (along with interaction terms) with the experimental data for both %DLE and %R_4h has confirmed the findings of the multivariate analysis. The optimized SS-loaded SA-PEC bubble beads based on 2D (contours), 3D, desirability, and overlay plots has exhibited %DLE of 87.35 ± 2.48% (n = 3 and error = 2.94%) and %R_4h of 85.79 ± 2.98% (n = 3 and error = 0.25%). The in vitro drug release studies have shown almost complete (≥85%) SS release from all the batches within 4-6 h in simulated gastric fluid (SGF) pH 1.2. The in vivo clinical findings by ultrasound studies have shown excellent floatation (>6 h) behavior of bubble beads in the upper gastrointestinal tract (GIT) and efficient stomach-specific gastroretention.

Cold and Hot Gelling of Alginate-graft-PNIPAM: a Schizophrenic Behavior Induced by Potassium Salts

Recently, alginates (ALG) characterized by high mannuronic content (M blocks) have been shown to undergo a reversible sol/gel transition during cooling in the presence of potassium salts. Cold gelling takes place at low temperatures, just below 0 °C for a KCl concentration of 0.3 mol/kg, but the aggregation process can be easily shifted to higher temperatures by increasing the salt concentration. In the present paper, we take advantage of this peculiar behavior to design a copolymer with schizophrenic gelling properties. For this purpose, side chains of poly(N-isopropylacrylamide) (PNIPAM), characterized by a Lower Critical Solution Temperature (LCST) in water, were grafted on the alginate backbone. Working in semidilute solutions, we show by coupling DSC and viscoelastic measurements that ALG-g-PNIPAM solutions are able to form gels either by cooling or heating depending on the ionic environment. As the aggregation process of ALG and PNIPAM depends mainly and respectively on the nature of the cations and anions, the choice of the salt is then critical to control the self-assembly behavior and the gel properties. Moreover, as the gelation process of alginates driven by the aggregation of mannuronic sequences is characterized by a large hysteresis of 20-30 °C between gelling and melting, both ALG and ALG-g-PNIPAM polymers offer a large versatility not only in terms of salt (nature and concentration) but also in preparation history as different states (sol or gel) can be obtained at room temperature.

Natural and synthetic polymers in fabric and home care applications

Polymers can be tailored to provide different benefits in Fabric & Home Care formulations depending on the monomers and modifications used, such as avoiding dye transfer inhibition in the wash, modifying the surface of tiles or increasing the viscosity and providing suspension properties to consumer products. Specifically, the rheology modification properties of synthetic and natural polymers are discussed in this chapter. The choice of a polymeric rheology modifier will depend on the formulation ingredients (charges, functional groups), the type and the amount of surfactants, the pH and the desired rheology modification. Natural polymeric rheology modifiers have been traditionally used in the food industry, being xanthan gum one of the most well-known ones. On the contrary, synthetic rheology modifiers are preferably used in paints & coats, textile printing and cleaning products.

Chiroptical characterization of homopolymeric block fractions in alginates

Homopolymannuronic and homopolyguluronic fractions were obtained by partial hydrolysis of the alkaline extracts from the brown seaweeds Ascoseira mirabilis, Desmarestia menziessi, Desmarestia ligulata and Durvillaea sp. collected in southern Chile. Full characterization of the fractions was achieved by FT-IR and NMR spectroscopy. Total hydrolysis with 90% formic acid of the homopolymeric fractions allowed the preparation of mannuronic and guluronic acids. Both monomers and homopolymeric fractions as neutral salts were studied by CD and ORD. Chiroptical spectra were similar in shape and sign to those previously published in the literature, and permitted to assign D configuration to mannuronic acid and L configuration to guluronic acid in alginic acids. Specific optical rotation values at the sodium D light for the homopolymannuronic (∼-100°) and homopolyguluronic (∼-110°) acid fractions were obtained. These high negative values are proposed for the assignment of the absolute configuration of monomers in homopolymeric fractions.

Natural Rheological Modifiers for Personal Care

The development of personal care formulations incorporates the deliberation of the prerequisite product rheology and the right rheology modifier to deliver these effects. While the effectiveness of the finished product will hinge on the selection and level of active ingredients and excipients, the aesthetics and even the penetration of these actives into the stratum corneum will be affected by the product rheology. Rheology modifiers are frequently denoted as thickeners and, whilst increasing the apparent viscosity should enhance the perception of quality in a formulation, this is only one aspect of rheological control. The product itself can be Newtonian or pseudoplastic, thixotropic, exist as a ringing gel or a stringy flowable liquid. This changes the way that the product appears in the bottle, the ease with which aliquots are poured or scooped from the packaging, the process of rubbing it into the skin or along the hair shaft, and upon using, the rinsing and removal of the product. It will also be vital to select the correct rheological characteristics to guarantee the stability of the finished formulation. By changing the structure of polymeric rheology modifiers, the impact on the sensorial properties and performance characteristics can be investigated. Careful selection of monomers, structure and other co-ingredients will result in products optimised for use in skin care formulations.

Ca(ii) and Ce(iii) homogeneous alginate hydrogels from the parent alginic acid precursor: a structural study

Alginate hydrogels are suitable for the encapsulation of biomolecules and microorganisms for the building of bioactive materials.

Structural behaviour differences in low methoxy pectin solutions in the presence of divalent cations (Ca(2+) and Zn(2+)): a process driven by the binding mechanism of the cation with the galacturonate unit

In this paper, we compare the interactions between low methoxy pectin (LMP) and either Ca(2+) or Zn(2+) in semi-dilute solutions. Intrinsic viscosity and turbidity measurements reveal that pectin-calcium solutions are more viscous, but yet less turbid, than pectin-zinc ones. To get a molecular understanding of the origin of this rather unexpected behavior, we further performed isothermal titration calorimetry, small angle neutron scattering experiments, as well as molecular dynamics simulations. Our results suggest that calcium cations induce the formation of a more homogeneous network of pectin than zinc cations do. The molecular dynamics simulations indicate that this difference could originate from the way the two cations bind to the galacturonate unit (Gal), the main component of LMP: zinc interacts with both carboxylate and hydroxyl groups of Gal, in a similar way to that described in the so-called egg-box model, whereas calcium only interacts with carboxylate groups. This different binding behavior seems to arise from the stronger interaction of water molecules with zinc than with calcium. Accordingly, galacturonate chains are more loosely associated with each other in the presence of Ca(2+) than with Zn(2+). This may improve their ability to form a gel, not only by dimerization, but also by the formation of point-like cross-links. Overall, our results show that zinc binds less easily to pectin than calcium does.

Formation and rupture of Ca(2+) induced pectin biopolymer gels

When calcium salts are added to an aqueous solution of polysaccharide pectin, ionic cross-links form between pectin chains, giving rise to a gel network in dilute solution. In this work, dynamic light scattering (DLS) is employed to study the microscopic dynamics of the fractal aggregates (flocs) that constitute the gels, while rheological measurements are carried out to study the process of gel rupture. As the calcium salt concentration is increased, DLS experiments reveal that the polydispersity of the flocs increase simultaneously with the characteristic relaxation times of the gel network. Above a critical salt concentration, the flocs become interlinked to form a reaction-limited fractal gel network. Rheological studies demonstrate that the limits of the linear rheological response and the critical stresses required to rupture these networks both decrease with the increase in salt concentration. These features indicate that the ion-mediated pectin gels studied here lie in a 'strong link' regime that is characterised by inter-floc links that are stronger than intra-floc links. A scaling analysis of the experimental data presented here demonstrates that the elasticities of the individual fractal flocs exhibit power-law dependences on the added salt concentration. We conclude that when both pectin and salt concentrations are increased, the number of fractal flocs of pectin increases simultaneously with the density of crosslinks, giving rise to very large values of the bulk elastic modulus.

Structural regime identification in ionotropic alginate gels: influence of the cation nature and alginate structure

The morphologies of several ionotropic alginate hydrogels and aerogels were investigated by SAXS according to the nature of the divalent metal cation (Mn(2+), Co(2+), Zn(2+), Cu(2+)) and the guluronic fraction of the alginate. All alginate hydrogel and aerogel samples show isotropic small-angle X-ray scattering. Gelation results from cooperative associations of cations and chain segments and yields different nanostructures, that is, nanofibrillar morphology or multiple junction morphology, according to cation type and eventually mannuronic/guluronic ratio. Therefore, Mn and Cu gels present the same morphology whatever the guluronic ratio, whereas Co and Zn gels yield different nanostructures. In the size range investigated by SAXS (~10-200 Å), the structure of aerogels obtained by CO(2) supercritical drying is found to be inherited from the morphology of the parent hydrogel whatever the initial structural regime.

Application of the Nernst-Planck approach to lead ion exchange in Ca-loaded Pelvetia canaliculata

Ca-loaded Pelvetia canaliculata biomass was used to remove Pb(2+) in aqueous solution from batch and continuous systems. The physicochemical characterization of algae Pelvetia particles by potentiometric titration and FTIR analysis has shown a gel structure with two major binding groups - carboxylic (2.8 mmol g(-1)) and hydroxyl (0.8 mmol g(-1)), with an affinity constant distribution for hydrogen ions well described by a Quasi-Gaussian distribution. Equilibrium adsorption (pH 3 and 5) and desorption (eluents: HNO(3) and CaCl(2)) experiments were performed, showing that the biosorption mechanism was attributed to ion exchange among calcium, lead and hydrogen ions with stoichiometry 1:1 (Ca:Pb) and 1:2 (Ca:H and Pb:H). The uptake capacity of lead ions decreased with pH, suggesting that there is a competition between H(+) and Pb(2+) for the same binding sites. A mass action law for the ternary mixture was able to predict the equilibrium data, with the selectivity constants alpha(Ca)(H)=9+/-1 and alpha(Ca)(Pb)=44+/-5, revealing a higher affinity of the biomass towards lead ions. Adsorption (initial solution pH 4.5 and 2.5) and desorption (0.3M HNO(3)) kinetics were performed in batch and continuous systems. A mass transfer model using the Nernst-Planck approximation for the ionic flux of each counter-ion was used for the prediction of the ions profiles in batch systems and packed bed columns. The intraparticle effective diffusion constants were determined as 3.73x10(-7)cm(2)s(-1) for H(+), 7.56x10(-8)cm(2)s(-1) for Pb(2+) and 6.37x10(-8)cm(2)s(-1) for Ca(2+).

Development of novel spray coated soft elastic gelatin capsule sustained release formulations of nifedipine

Nifedipine release from coated commercially available immediate release soft elastic gelatin capsules was investigated. Capsules were spray coated using two different polymeric combinations, ethylcellulose and hydroxypropylmethylcellulose or pectin, at different coating loads. In vitro drug release studies were conducted in three different dissolution media: with gastric pretreatment, without gastric pretreatment, and in water to investigate the pH effect on nifedipine release. Convolution of in vitro dissolution data for selected formulations and commercially available sustained release nifedipine formulations showed that the tested formulations provided release profiles of nifedipine that are very promising in terms of desirable sustained release formulations.

Multiple Steps and Critical Behaviors of the Binding of Calcium to Alginate

Previous research on the binding and gelation of calcium/alginate in aqueous solution were mostly conducted in the (semi-)concentrated regime, and it did not provide details of the binding process and the formation of egg-box junctions. In the present investigation, the binding of calcium to alginate, of low and high molecular weight and different guluronate/mannuronate ratios, was investigated in dilute solutions using isothermal titration calorimetry (ITC), Ca2+-selective potentiometry, and viscometry techniques. The results reveal three distinct and successive steps in the binding of calcium to alginate with increased concentration of Ca ions. They were assigned to (i) interaction of Ca2+ with a single guluronate unit forming monocomplexes; (ii) propagation and formation of egg-box dimers via pairing of these monocomplexes; and (iii) lateral association of the egg-box dimers, generating multimers. The third step has different association modes depending on the molecular weight of alginate. The boundaries between these steps are reasonably critical, and they closely correlate with the Ca/guluronate stoichiometry expected for egg-box dimers and multimers with 2/1 helical chains. The formation of egg-box dimers and their subsequent association are thermodynamically equivalent processes and can be fitted by a model of independent binding sites. The binding of Ca to alginates of different guluronate contents is controlled by a balance between enthalpy and entropy.

Formation of Polysaccharide Gel Layers in the Presence of Ca2+ and K+ Ions: Measurements and Mechanisms

Understanding the adsorption properties of polysaccharides in terms of substrate affinity, kinetics, and layer structure is of paramount importance in numerous industrial and natural systems. The structural growth of the layers of two model polysaccharides--sodium alginate and polygalacturonic acid (PGA)--was characterized by quartz crystal microbalance with dissipation and atomic force microscopy. Monitoring the variations in frequency and dissipation energy provides insights into both the average adsorbed mass and the viscoelastic properties of the adsorbed layer of polyelectrolytes along with the associated ions and water molecules. Both polysaccharides had similar adsorption patterns with increasing ionic strengths and showed significant complexation of calcium ions. In the presence of calcium, the alginate gel layer exhibited significant swelling with an increasing concentration of monovalent salt that the PGA gel layer did not manifest. Basing our discussion on the "egg-box model", we interpreted these different swelling behaviors as resulting from differences in the complexation modes of the two polysaccharides. The dimerization of the polymers by cross-linking and the weaker dimer-dimer associations play major roles in the sensitivity of the polysaccharide gel matrix to high salt concentration environments.

FTIR Spectroscopy of NH3 on Acidic and Ionotropic Alginate Aerogels

The acidity of alginate aerogel films has been investigated by infrared spectroscopy of adsorbed NH(3). Supercritical drying of the alginate provided samples with a surface area of several hundred square meters per gram, in which the probe molecule could reach all acidic sites. Free carboxylic groups were studied on acid-gelled alginates and were found to behave as effective Brønsted acid sites. Ionotropic alginate gels formed by alkaline earth cations presented only the Lewis acidity of the cations. Ionotropic gels formed by transition metal cations presented both Lewis and Brønsted sites, because of the presence of a fraction of free carboxylic groups. The incomplete salification was correlated to the pH of the gelling solutions.

Characterisation of Ca– and Al–pectate gels by thermal analysis and FT-IR spectroscopy

Thermal analysis (TG-DTA) and FT-IR spectroscopy have been performed on calcium-pectate membranes to investigate their structure and the consequent variation caused by aluminium sorption. Calcium-polygalacturonate (Ca-PG) membranes, model systems of the soil-root interface, were exposed to aluminium solutions at different concentrations (25-800 microM). Three different pHs (3.50, 4.00 and 4.50) were chosen to study the influence of different aluminium species, such as [Al(H2O)6]3+, [Al(OH)(H2O)5]2+ and [Al(OH)2(H2O)(4)]+, on the structure of the Ca-PG membrane. The DTA profiles and FT-IR spectra showed how aluminium sorption induces structural modifications leading to a reorganisation of the chain aggregates and a weakening of the structure. Higher pH, that is, 4.00 and 4.50, and thus hydrolytic aluminium species and related higher calcium content maintain a more regular structure than at pH 3.50. At pH 3.50, both the effect of [Al(H2O)6]3+ and a major calcium release had a greater impact and thus induced a greater weakening of the structure.

Cross-linking of dried paracetamol alginate granules

This paper reports on the cross-linking of dried (<5% moisture) paracetamol alginate granules with calcium chloride solutions. The effect of calcium concentration, temperature of the treatment solution, stirring speed and time used during cross-linking of granules on water uptake by the granules during cross-linking and physical properties of the cross-linked and dried granules were studied. A full factorial study of these factors each at two levels was used (CaCl2.2H2O: 20, 100 mg/ml; temperature: 25, 45 degrees C; stirrer speed: 25, 240 rpm; time: 1.5 and 5.5 min) to treat dried stock granules (size: 0.8-1.0 mm) containing the model drug paracetamol and sodium alginate powder (1:1) which were prepared using conventional aqueous granulation under low shear. In addition to SEM and photomicrography, the physical properties studied were water uptake during cross-linking, yield, aggregation behaviour, moisture content, drug content, early stage drug release [over 10 s (R10) and the next 50 s (R50)] and calcium and sodium content of the unwashed cross-linked granules. Dry granules were successfully cross-linked. The treatment factors significantly affected most of the response variables. The variables most affected were water uptake (78-254%), drug entrapment (58-86%), early release (R10: 1.2-6.4% and R50: 3.0-12.2%), granule aggregation (0-70%), calcium (6.02-12.4%) and sodium content (1.2-6.44%). SEM photographs suggest that low calcium treated granules were less porous in nature compared to high calcium treated granules. Low shear drug alginate granules can be cross-linked in dried state. The properties of the cross-linked granules can be modified by altering the treatment process.

New Insights into the Mechanism of Gelation of Alginate and Pectin: Charge Annihilation and Reversal Mechanism

Studies have been undertaken on the binding of Mn2+ ions to two alginate samples of different mannuronate:guluronate ratios (M:G), a sample of low-ester amidated pectin and poly(acrylic acid) (PAA). The binding of Ca2+ ions has also been included for the latter for comparison. The binding curves showed an initial steep rise at low additions of Mn2+ or Ca2+ indicating that all of the ions were bound to the polymer chains with none remaining in solution. At higher additions, the binding curves showed a plateau region and the maximum amount bound, theta, was found to be 0.2, 0.2, 0.25, and 0.33 mol M(2+)/mol COO- for high M:G alginate, low M:G alginate, pectin, and PAA, respectively. The binding curves for Mn2+ and Ca2+ with PAA were superimposable. In all cases, theta was less than the stoichiometric equivalent and also less than predicted by Manning counterion condensation theory. The linear charge density, xi, for the polymers is 1.49, 1.55, 1.62, and 2.85, and it was found that at maximum binding the effective linear charge density, xi(effective), decreased to a value close to 1 in each case and not 0.5 as predicted from Manning's two-variable theory. The mobility of the PAA chains has been followed by electron spin resonance spectroscopy using nitroxide spin labels covalently attached to the polymer, and the gelation of the pectin and alginate samples has been monitored using small deformation oscillatory experiments. For PAA at maximum binding, it was noted that there was a loss of chain mobility and precipitation. For pectin and alginate, gelation occurred and the stoichiometric ratio for maximum binding corresponded to the stoichiometric ratio for the maximum in G'. Precipitation and gelation are attributed to the formation of polymer-metal complexes involving one or two carboxylate groups resulting in charge reversal or charge annihilation.

The complexation of metal cations by d-galacturonic acid: a spectroscopic study

Solid complexes of D-galacturonic acid (GalA) with cobalt(II), copper(II), nickel(II) and oxovanadium(IV) (1-4) were prepared and characterised. The metal-to-ligand molar ratio was 1:2 for complexes 1-3 and 1:1 for complex 4. The alpha- and beta-anomers of GalA were detected in all the complexes in solid state and in solutions. An addition of small amounts of the paramagnetic complexes to the D2O solution of pure ligand led to NMR line broadening of some 1H and 13C nuclei. This broadening was sensitive to the anomeric state of GalA in the case of complexes 1 and 4. NMR and vibrational spectroscopic data indicate the formation of carboxylate complexes of all the cations, while noncarboxylic oxygens are also involved into the metal bonding in some cases. VCD spectra of complexes 1-4 in D2O and Me2SO-d6 solutions confirm that GalA carboxylic group may participate in the formation of optically active species around the metal cation. Possible ways of GalA coordination by metal cations of this study were proposed and discussed.

A review of the biochemistry of heavy metal biosorption by brown algae

The passive removal of toxic heavy metals such as Cd(2+), Cu(2+), Zn(2+), Pb(2+), Cr(3+), and Hg(2+) by inexpensive biomaterials, termed biosorption, requires that the substrate displays high metal uptake and selectivity, as well as suitable mechanical properties for applied remediation scenarios. In recent years, many low-cost sorbents have been investigated, but the brown algae have since proven to be the most effective and promising substrates. It is their basic biochemical constitution that is responsible for this enhanced performance among biomaterials. More specifically, it is the properties of cell wall constituents, such as alginate and fucoidan, which are chiefly responsible for heavy metal chelation. In this comprehensive review, the emphasis is on outlining the biochemical properties of the brown algae that set them apart from other algal biosorbents. A detailed description of the macromolecular conformation of the alginate biopolymer is offered in order to explain the heavy metal selectivity displayed by the brown algae. The role of cellular structure, storage polysaccharides, cell wall and extracellular polysaccharides is evaluated in terms of their potential for metal sequestration. Binding mechanisms are discussed, including the key functional groups involved and the ion-exchange process. Quantification of metal-biomass interactions is fundamental to the evaluation of potential implementation strategies, hence sorption isotherms, ion-exchange constants, as well as models used to characterize algal biosorption are reviewed. The sorption behavior (i.e., capacity, affinity) of brown algae with various heavy metals is summarized and their relative performance is evaluated.