Reversible and irreversible biopolymer gels — Structure and mechanical properties

Abstract

The majority of synthetic polymer gels are formed by the covalent cross‐linking of linear or branched macro‐molecules using multi‐functional cross‐linking agents. Such gels are networks, or true macromolecules with (nominally) infinite molecular weight and consequently they swell rather than dissolve if immersed in a good solvent. However, there are also a whole class of materials called physical gels where non‐covalent crosslinks occur. These show similarities to covalent networks, but because the cross‐links are not permanent, they will show creep behaviour at very long times. Some of these form on cooling a heated solution, such as gelatin gels, while others form only on heating. Also, some gels are thermoreversible, while others are not. Physical gels can be formed from synthetic or biopolymers. In the latter case non‐covalent cross‐links often comprise more specific and complex mechanisms involving, rather than point‐like cross‐links, “junction zones” of known, ordered secondary structure such as multiple helices. Typically there is a specific, and often intricate, hierarchy of arrangements, which are more familiar to molecular biologists than to polymer physical chemists. In this paper we introduce viscoelastic techniques for characteristing physical gels, and then relate the properties to the underlying structure at the macromolecular and junction zone level. The parallels between synthetic and biopolymer gels will also be illustrated. Finally we describe recent work on the heat‐set gelation of globular proteins, and an attempt to relate the important parameter of the gelation time, tc, to both polymer concentration and temperature.

Kinetic and mechanistic considerations in the gelation of genipin-crosslinked gelatin

“…Although the G -G crossover is often used to determine gel time [9], experimental noise below the gel point precluded its use. Ross-Murphy [10] has reported the Winter-Chambon method [9] to be problematic for many gelling systems (including heat-set protein gels) where obtaining good data below the gel point is difficult. The method described provided reproducible gel times for our replicate samples.…”

“…2. The plot is essentially the linearized form of an integrated rate law expression, and assumes that t gel occurs at a critical conversion of gelatin [10]. The slope is E a /R, where E a is the activation energy and R is the gas constant (8.3145 J K −1 mol −1 ).…”

Multi-technique approach on the effect of surfactant concentrations on the thermal unfolding of rabbit serum albumin: Formation and solubilization of the protein aggregates

“…The appearance of turbidity on increasing the temperature diminishes on further increase in concentration of CTAB and on reaching 2 mM CTAB the turbidity in the solution disappears completely. This may probably due to the formation of protein aggregates at low surfactant concentration [22] which dissolve at higher surfactant concentrations (more than the critical micelle concentration, cmc, of the CTAB [37]) as surfactants are very important agents to solubilize the protein aggregates (generally above cmc) and to suppress the aggregate formation in presence of other denaturing stimulants [38]. Therefore, it can be concluded that a high concentration of surfactant is capable to solubilize the protein aggregates as well as to hinder the protein aggregation against thermal denaturation.…”

“…However, no fractal structure of the protein-surfactant complex was observed at this temperature, though the size of protein increases drastically (Table 1). It is believed that protein unfolding at higher temperatures takes place due to exposure of hydrophobic patches of protein to water [21,22] and a consequent aggregation of the protein macromolecules (the build-up of the cross-section in the low-Q region is a sign of such aggregation) to hinder the unfavorable interactions between water and the hydrophobic patches following the gelation of protein as, at this temperature, turbidity of the protein solution increases noticeably, and SANS data at this gelation temperature show a linear region of scattering cross-section on log-log scale for values of Q < 0.07 Å −1 . It was observed that addition of small amount of surfactant causes the denaturation of protein, furthermore, a collective effect of temperature and surfactant (in low concentration region) promotes the protein aggregation (vide supra).…”

Effect of pH and Protein Concentration on Rheological and Structural Behavior of Temperature-induced Bovine Serum Albumin Gels

“…Electron microscopy has directly revealed the structural arrangements of aggregates and shown that on the basis of the strength of the electrostatic repulsion amongst the protein macromolecule, fibril or clumpy aggregates can be formed. [26][27][28] Rheology, on the other hand, gives the flow and elastic properties of the gel, from which has the existence of a critical concentration has been shown only above which a perfect gel is formed. The solid-like property of the gel increases with the protein concentration.…”