Conformational Changes and Aggregation of Alginic Acid as Determined By Fluorescence Correlation Spectroscopy

Abstract

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.

Study of the dynamical behavior of sodium alginate/myoglobin aqueous solutions: A dynamic light scattering study

“…6, for the protein/polysaccharide complex, D eff resulted to be significantly affected by the variation of the ionic strength, especially at the lowest NaCl concentrations. Contrarily, according to previous studies [21,22], for the sodium alginate solutions D eff resulted to be independent, within the experimental uncertainties, of the salt concentration in the range 10-50 mM (see the inset of Fig. 6).…”

“…Moreover, in order to relate the differences in the dynamical properties to structural changes occurring in the systems, Fourier Transform Infrared (FTIR) spectroscopy was also employed. The results were compared with previous investigations on aqueous solutions of sodium alginate tested under similar conditions [21][22][23]. Such information is of crucial importance for the formulation and optimization of new biocompatible products.…”

“…The decrease observed under alkaline conditions can be attributed to the depolymerization of the polysaccharide chains [21], whereas as the pH approaches the pKa of the molecule (pKa = 3.2-3.65), the reduction in size of the aggregates is a consequence of the protonation of the alginic acid that determines a decrease of the intramolecular electrostatic repulsion. This was also observed by Avaltroni and colleagues [21], which ascribed the reduction in size by lowering pH to the progressive protonation of the alginic acid with the appearance of hydrophobic segments in alginate chains. The presence of hydrophilic-hydrophobic aggregates in aqueous solutions depending on pH was also observed by Cao and colleagues [52].…”

Lectins and/or xyloglucans/alginate layers as supports for immobilization of dengue virus particles

“…Under alcaline conditions depolymerization of alginic acid by a ␤-alkoxy-elimination mechanism and formation of small more flexible fragments are expected [25,26]. Polysaccharide chains often undergo conformational changes under high pH and high temperature [25][26][27]. Such order-disorder transitions favor the formation of thick adsorbed layers [27].…”

“…Such order-disorder transitions favor the formation of thick adsorbed layers [27]. It is well reported [25,26,[28][29][30] that alginates bind to divalent cations in a chelate type of binding Ca 2+ ions are accommodated in cavities formed by cooperative pairing of contiguous G-residues, the so-called "egg-box" model. Moreover, the balance between the block length distribution of the G residues, polymer concentration, and Ca 2+ might drive coexisting lateral association between G-sequences [28].…”