Ionization and Solubility of Chitosan Solutions Related to Thermosensitive Chitosan/Glycerol-Phosphate Systems

Abstract

Chitosan is a linear cationic biopolymer composed of glucosamine and N-acetyl-glucosamine that is only soluble in acidic aqueous solutions and precipitates when neutralized. However, it was recently discovered that chitosan dissolved in solutions containing glycerol phosphate was soluble at near neutral pH and produced a sol-gel transition when heated. Understanding this unique thermogelling system requires improved characterization of the ionization and solubility behaviors of chitosan, in particular dependencies on temperature, salt, chitosan concentration, and fD, where fD is the fraction of glucosamine monomers (deacetylated monomers) in chitosan. In the current study we performed temperature-controlled titration and dilution experiments on chitosan solutions with fD of 0.72, 0.85, and 0.98 at concentrations ranging from 1.875 to 30 mM of its glucosamine monomer and with 0 to 150 mM added salt. Light transmittance measurements were performed during titration to indicate precipitation. We found the apparent proton dissociation constant of chitosan, pKap, to (1) decrease strongly with increased temperature, (2) increase strongly with increased salt, (3) increase strongly with increased chitosan concentration in low-salt conditions, and (4) decrease weakly with increasing fD. All of the above influences on chitosan pKap were accurately predicted using a mean-field Poisson-Boltzmann (PB) cylindrical cell model where the only adjustable parameter was the temperature-dependent chitosan intrinsic monomeric dissociation constant pK0(T). The resulting chitosan pK0 values at 25 degrees C were in the range from 6.63 to 6.78 for all chitosans and salt contents tested. The temperature dependence of chitosan ionization was found to strongly reduce pK0(T) by 0.023 units per degrees C, for example, resulting in a reduction of chitosan pK0(T) from 7.1 at 5 degrees C to 6.35 at 37 degrees C for fD of 0.72 in 150 mM salt. A similar temperature-dependent reduction of the pKa of the glucosamine monomer was found (-0.027 units per degrees C) while the pKa of glycerol phosphate did not change significantly with temperature. The latter result suggested that chitosan solutions heated in the presence of glycerol phosphate will become partly neutralized by transferring protons to glycerol phosphate and thereby allow attractive interchain forces to form a physically cross-linked gel under the appropriate conditions. Additionally, the degree of ionization of chitosan when it precipitates upon addition of a strong base was measured to be in the range from 0.25 to 0.55 and was found to (1) be insensitive to temperature, (2) increase strongly with increased salt, and (3) increase strongly with fD. The salt effect was accounted for by the PB model, while the influence of fD appeared to be due to acetyl groups impeding attractive chain-to-chain association to increase solubility and require reduced ionization levels to precipitate.

Permeability of Exendin-4-Loaded Chitosan Nanoparticles across MDCK Cell Monolayers and Rat Small Intestine

“…When the pH level increased to 6, a larger particle size of 173 nm and a lower zeta potential of +6.0 mv were observed. The amino group in chitosan is known to have a pK a value of 6.5, 24) which leads to protonation in acidic to neutral solutions with a charge density dependent on pH. A higher pH value reflects a lower hydrogen ion concentration, and therefore fewer protonated amino groups, which may explain the observed decreases in charge density and zeta potential of particles with increasing pH in the current study.…”

What Can We Learn from the Saga of Chitosan Gums in Hyperphosphatemia Therapy?

“…The glucosamine monomer has a pK of 7.8, and therefore, at neutral pH, most of the amine would be in a charged state. However, as a glucosamine polymer (i.e., chitosan), its pK is reduced to about 6.6 (11). As a result, chitosan will be mostly unionized and quite insoluble at the neutral or alkaline pH of saliva (11,12).…”

“…However, as a glucosamine polymer (i.e., chitosan), its pK is reduced to about 6.6 (11). As a result, chitosan will be mostly unionized and quite insoluble at the neutral or alkaline pH of saliva (11,12). In contrast, sevelamer has a much more favorable pK, and therefore, at any given time, 40% of all amines are positively charged, balanced by chloride, or bicarbonate (13).…”

Effect of Intensity of Heating on Qualitative Tests for the Detection of Common Adulterants in Milk

“…Moreover, these reactions are affected by temperature as it plays an important role in solubility and ionization of compounds (Filion et al, 2007).…”