Influence of Decavanadate Clusters on the Rheological Properties of Gelatin

The influence of polyoxovanadate clusters ([H2V10O28]4−) on the thermo-reversible gelation of porcine skin gelatin solution (type A, M w ≈ 40 000 g·mol−1, pH = 3.4 ≪ isoelectric point (IEP) ≈ 8) has been investigated as a function of temperature and vanadate concentration by combining rheology and microcalorimetry. This work shows that the rheological properties of the system depend on electrostatic interactions between [H2V10O28]4− and positively charged gelatin chains. In a first stage, we describe the renaturation of the gelatin triple helices in the presence of decavanadate clusters. We reveal that, when gelatin chains are in coil conformation (30 °C < T < 50 °C), the inorganic clusters act as physical cross-linkers that govern the visco-elastic properties of the mixture with an exponential dependence of the (G′, G′′) modulus with the vanadate concentration. Below 30 °C, we show that gelatin triple helix nucleation is slightly favored by the presence of vanadate, but above a helix concentration of 0.012 g·cm−3, G′ is fully governed by the helix concentration. During the melting process, we reveal the non-fully reversible behavior of the vanadate/gelatin rheological properties and the stabilization of gelatin triple helices due to vanadate species until 50°C. This non-reversible character has also been observed in the same experimental conditions with collagen/vanadate solutions. This is the first time that such a stabilization of triple helices has been reported in the case of gelatin hydrogels chemically cross-linked or not. We propose to analyze these results by considering that triple helix aggregates should persist because of decavanadate bridging, that the nucleation of an extended triple helix network may induce a strong modification of the vanadate cross-linker distribution in the system, or both, thus promoting the formation of thermally stable vanadate/gelatin micro-gels in the dangling end of the triple helices.