RESUMO
Experimental and theoretical data have been revisited to shed light onto the aspects of hydration and chain expansion of pectic acid (galacturonan) upon charging. The prediction of the variation of the number of solvation water molecules between the two limit ionization states from theoretical calculations was confirmed to a very high accuracy by the corresponding number evaluated form dilatometric measurements. The relevance of hydration to the mechanism of bonding of calcium ions by sodium pectate is discussed. Characterization of polymer expansion has been obtained by calculating the values of the characteristic ratio and/or the persistence length on the respective populations and comparing the theoretical predictions with experimental data. The results show that a charged chain in typical conditions of ionic strength is more expanded than its neutral counterpart, whereas the ideal limit (31 and 21) helical conformations in the uncharged and totally charged conditions, respectively, share the same value of the linear advance of the helical repeat, when the ionic strength tends to infinite. Total divergence between theoretical predictions and experimental evidence rules out the possibility that carboxylate charge reduction by protonation and by methyl esterification are equivalent in determining the solution behavior of galacturonan.
Assuntos
Pectinas , Água , Polieletrólitos , Conformação MolecularRESUMO
Chitosan and its highly hydrophilic 1-deoxy-lactit-1-yl derivative (Chitlac) are polysaccharides with increasing biomedical applications. Aimed to unravel their conformational properties we have performed a series of molecular dynamics simulations of Chitosan/Chitlac decamers, exploring different degrees of substitution (DS) of lactitol side chains. At low DS, two conformational regions with different populations are visited, while for DS ≥ 20% the oligomers remain mostly linear and only one main region of the glycosidic angles is sampled. These conformers are (locally) characterized by extended helical "propensities". Helical conformations 32 and 21, by far the most abundant, only develop in the main region. The accessible conformational space is clearly enlarged at high ionic strength, evidencing also a new region accessible to the glycosidic angles, with short and frequent interchange between regions. Simulations of neutral decamers share these features, pointing to a central role of electrostatic repulsion between charged moieties. These interactions seem to determine the conformational behavior of the chitosan backbone, with no evident influence of H-bond interactions. Finally, it is also shown that increasing temperature only slightly enlarges the available conformational space, but certainly without signs of a temperature-induced conformational transition.
Assuntos
Quitosana/química , Lactose/química , Conformação Molecular , Glicosídeos/química , Ligação de Hidrogênio , Simulação de Dinâmica Molecular , Concentração Osmolar , Cloreto de Sódio/química , Álcoois Açúcares/química , Temperatura , Fatores de TempoRESUMO
Recent extensions of counterion condensation theory, originally developed for well-defined linear polyelectrolytes, enable us to analyze the interaction of trace metals with humic acid. In the present model, the heterogeneity of the macromolecule is taken into account as well as the chemical binding of the considered metal ions to the humic material. Experimentally, potentiometric titrations have been performed for humic acid in solution in the presence of different environmentally important (heavy) metals (Ca, Cd, Cu, Ni, and Pb) at various metal concentrations by titrating with potassium hydroxide without additional salt. From proton release data obtained for the initial point in the titration, it was estimated that the interaction of the different metals with the humic acid in terms of binding strength increased in the order Ca < Cd approximately = Ni < Pb approximately = Cu. These results were confirmed by model analysis. Experimentally obtained apparent dissociation constants were in good agreement for the humic acid systems containing Ca, Cd, and Ni at concentrations ranging from 0 up to 0.75 x 10(-3) mol L(-1) and polymer dissociation degree from about 0.1 up to approximately 0.8. Also for the Cu/humic acid and Pb/humic acid systems, the agreement between experimental data and calculated data was satisfactory atthe lowest metal concentrations over the complete titration curve. For elevated levels of Cu and Pb, the agreement between experimental data and theoretical calculations becomes less satisfactory at low degrees of dissociation of the humic acid. This distortion of the potentiometric curves is probably due to changes in the intrinsic pK of the functional groups due to metal binding. This complex process is not included in present polyelectrolytic models.