Bovine Serum Albumin Hydrogel Formation: pH Dependence and Rheological Analyses.

In vitro evaluation of the physical properties of biopolymer-based hydrogels can help in understanding certain phenomena, such as liquid-liquid phase separation. The formation of bovine serum albumin (BSA) hydrogels was investigated in the pH range of 1.0 to 4.0. Hydrogels were formed in the pH range of 3.0 to 4.0, whereas viscous solutions were formed in the pH range of 1.5 to 2.5. Unexpectedly, formation of BSA hydrogel was observed in extremely acidic condition (pH 1.0). The circular dichroism spectra of BSA solutions were recorded at pH values of 1.0, 2.0, 3.0, and 7.0, and α-helix contents were determined from the ellipticity data at 222 nm. The α-helix content decreased with a decrease in pH, and this decrease was associated with the partial denaturation (F-isoform) and the denaturation (E-isoform) of BSA. However, the α-helix contents at pH 1.0 and 3.0 were similar. BSA hydrogels at pH 1.0 and 3.5 showed similar dynamic viscoelastic properties, further supporting the stereo structural change of BSA from the denatured E-isoform to the partially denatured F-isoform at pH 1.0. The study also focused on measuring viscoelasticity, a fundamental physical property of hydrogels, using traditional rheometer and with minimal sample volume. A highly reproducible procedure for measuring the viscoelastic properties of hydrogels was established using sample volumes of 200 and 350 µL.

Formation of pH-Responsive Supramolecular Hydrogels in Basic Buffers: Self-assembly of Amphiphilic Tris-Urea.

An amphiphilic tris-urea compound (1) containing hydrophilic resorcinol units was designed and synthesized. Compound 1 formed supramolecular hydrogels in basic buffers, such as glycine-NaOH, phosphate-NaOH, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-NaOH, and borate-NaOH. The optimum pH range of the buffer solution for gelation was 10-11 and insoluble suspensions or solutions were formed when the pH was outside this range. When the borate-NaOH buffer was used, supramolecular hydrogels were formed over a wide pH range (7.5-11.0). The thermal stabilities and viscoelastic properties of the supramolecular hydrogels were determined from the gel-to-sol phase transition temperatures and rheological properties, respectively. The supramolecular hydrogel formed from compound 1 and the borate-NaOH buffer exhibited a pH-responsive reversible gel-to-sol phase transition property. Gel-to-sol phase transition could be achieved by adding NaOH and regelation of the sol was realized by adding an appropriate amount of boric acid. Increasing the amount of the acid resulted in a gel-to-sol phase transition.

Effect of Alkyl Chain Length of N-Alkyl-N'-(2-benzylphenyl)ureas on Gelation.

Urea derivatives that were substituted with a 2-benzylphenyl group and an alkyl group functioned as low molecular weight gelators for various organic solvents and ionic liquids. Urea derivatives with long alkyl chains were effective for the gelation of polar solvents. However, they were not suitable for the gelation of non-polar solvents, whereas urea derivatives with short alkyl chains were effective. Ionic liquids were similar to polar solvents in that urea derivatives with long alkyl chains were the most effective gelators. The physical properties of the formed supramolecular gels were analyzed by dynamic viscoelasticity measurements using a rheometer.

Amphoteric Homotropic Allosteric Association between a Hexakis-Urea Receptor and Dihydrogen Phosphate.

Conformationally flexible hexakis-urea 1 was synthesized efficiently by condensing hexakis(aminomethyl)benzene with 4-nitrophenyl-(3,5-di-tert-butylphenyl)carbamate. The hexakis-urea 1 is unexpectedly soluble in organic solvents of low polarity due to intramolecular hydrogen bonding. The hexakis-urea 1 recognizes chloride, bromide, and acetate in a 1:2 host-guest ratio and in a positive allosteric manner in CDCl3 . The ability of 1 to recognize dihydrogen phosphate is a unique outcome, and the structure of the associated complex, which contains four dihydrogen phosphate ions, was clarified by single-crystal X-ray structural analysis. However, in solution, a complex with three dihydrogen phosphate ions was identified. The dihydrogen phosphate association in CDCl3 proceeds in an amphoteric allosteric manner; in a positive allosteric manner (K1 K3 ) in the subsequent step.