| Abstract: | The changes in conformation undergone by α-gelatin molecules on quenching aqueous solutions to below the temperature at which they can gel have been monitored by nuclear magnetic resonance and dielectric relaxation techniques. The relative rates of these conformational transitions are compared with changes in rheological properties. The nmr spectral intensity changes for 0.2 and 0.5% w/v α-gelatin solutions correspond to a unimolecular process with k ～ 10?2 min?1 at 15°C; this process occurs independently of whether or not the solution is concentrated enough to form a gel. The process involves a slow intramolecular nucleation step, followed by a rapid conformational change of the whole molecule from random coil to a rigid stage. Comparison with other data suggests that the transition gives rise to a triple collagen-like helix. In dilute solution (but above the critical concentration for gel formation, e.g., 0.5% w/v), the gelatin process follows the formation of the rigid molecular species. It probably involves the formation of junction zones consisting of three polypeptide chains in a collagen-like triple-helical conformation. These junctions may form, at low concentrations, from a reorganization of previously formed, intramolecular, triple helices. Solutions below a concentration of about 0.4% w/v α-gelatin cannot gel by this mechanism, and only form viscous liquids. |
