Dynamical heterogeneity in the gelation process of a polymer solution with a lower critical solution temperature.

The gelation of a hydrophobically modified hyaluronic acid aqueous solution which shows a lower critical solution temperature of about 25 °C was investigated by multi-particle tracking microrheology. The linear viscoelasticity of the gelling system is converted from the microrheological data. The critical gelling temperature Tgel = 36.3 °C was determined from the loss tangent by the Winter-Chambon criterion. The critical exponent n = 0.62 was determined from the shift factors of the time-cure superposition. The length scales of the dynamic heterogeneity of the gelling system were analyzed using a proposed framework where single-particle and multi-particle non-Gaussian parameters were compared. The length scale of the dynamic heterogeneous regions monotonically decreases during the gelation process, consistent with the nucleation-and-growth mechanism of phase separation. Distributions of local viscosity in the gelling system were extracted from the observed distributions of particle displacement as a time-dependent fingerprint of the dynamic heterogeneity of the gelling system. The results and analyzing methods proposed in the present work can be applied to other microrheological studies.

Cation-amino acid interactions: Implications for protein destabilization.

The mechanism for protein stabilization or destabilization has long been an open quest. In the present study, we have studied the interactions between amino acids and guanidinium (Gdm+)/ammonium (NH4+) ions by using low field nuclear magnetic resonance (LF-NMR), where Gdm+ and NH4+ are denaturant and stabilizer for proteins, respectively. It shows that Gdm+ favors to bind to the thiol group or the hydroxyl group on the side chain but weakly interacts with the α-carboxyl group. In contrast, NH4+ prefers to bind to the α-carboxyl group but slightly interacts with the thiol group or the hydroxyl group on the side chain of amino acids. 1HNMR reveals the hydrogen bonding between NH4+ and the α-carboxyl group, which is not involved in the interactions between Gdm+ and cysteine. Our study demonstrates that the strong interactions between the denaturant and the sulfur atom or the disulfide bond promote the direct binding of the denaturant toward proteins, leading to the destabilization.