RESUMEN
The thermodynamic properties of the dissociation of natural polyuronic acids in dilute aqueous solution at 25 degrees C are reported. The investigation concerns polygalacturonic acid and alginic acids from different sources, including highly homogeneous mannuronic and gulronic fractions and involved potentiometry, isothermal calorimetry, dilatometry and circular dichroism measurements. The dissociation of polyuronic acids was studied as a function of degree of dissociation, concentration of the polymer and ionic strength. Abnormal behavior upon dissociation was observed only in polyguluronic and polygalacturonic acids and attributed to a pH-induced conformational transition and/or to an interchain disaggregation. Direct molecular weight evidence supports the proposed intramolecular transition of polygalacturonate chain.
Asunto(s)
Alginatos/química , Pectinas/química , Calorimetría , Dicroismo Circular , Ácido Glucurónico , Ácidos Hexurónicos , Concentración de Iones de Hidrógeno , Conformación Molecular , Peso Molecular , Potenciometría , Protones , TermodinámicaRESUMEN
The flow behaviour and the viscoelastic properties of N-(carboxymethyl) chitosan aqueous systems in the sol and gel domains have been investigated by means of dynamic, steady and transient shear techniques. For polymer concentrations Cp up to 1%, a typical response of moderately concentrated polymer solutions was observed under continuous and oscillatory shear conditions. No time-dependent properties were detected during transient shear experiments. On the other hand, for all the samples with Cp greater than 1%, the rheological properties were more similar to those of a weak gel system. The continuous shear flow behaviour was of the plastic type and the viscoelastic quantities G' and G" were parallel to each other and slightly dependent on the frequency of oscillation omega. Stress overshoots were observed during transient shear experiments, and the kinetics of the structural breakdown and build-up processes were found to be dependent both on the polymer concentration and the applied shear rate.
Asunto(s)
Quitina/análogos & derivados , Quitosano , Animales , Quitina/química , Crustáceos , Geles , Concentración de Iones de Hidrógeno , Conformación Molecular , Reología , Soluciones , Temperatura , Factores de Tiempo , ViscosidadRESUMEN
The influence of human serum albumin on the decomposition rates of some arylmonomethyltriazenes in buffered aqueous solution was investigated. From the experimental data, a model for the triazene-albumin interaction was derived, and the thermodynamic parameters were systematically calculated by two independent methods. The results show marked dependence of the energetics of binding on the substituent in the triazene aromatic ring. For most of the triazenes studied, the binding with albumin was mainly enthalpy driven. Measurements also were performed using bovine and murine serum albumins.