engleski

Mesoscale organization in the gelation of gelatin

We present a comprehensive investigation of the gelation of gelatin water solutions, with the aim of understanding the emergence of a complicated gel network structure from interactions at the residue level and explaining the dyamics of this network. The basic microscopic molecular mechanism responsible for gelation -- the coil-helix transition of the polypeptide chain segments -- has been studied extensively using optical rotation (with comparisons to rheology) [1]. To gain additional insight into this mechanism we performed temperature- and concentration-dependent circular dichroism spectroscopy, which enabled us to follow the helix (un)folding very closely and led to a single 'helix-melting' master curve. This provides a background for mesoscopic organization processes, of special interest due to their importance for transport and gel network dynamics but mostly unexplored. We used PFG-NMR to obtain the complete dynamic structure factor $S(k, t)$ for timescales from 10 to 1000 ms and lengthscales from a few hundred nm up to tens of microns, thus characterising anomalous self-diffusion of the gelatin chains [2] in detail. A complementary transport experiment was performed by measuring conductivity spectra of the gelatin solutions using a novel impedance cell design [3], showing two previously unreported conductivity relaxations ; comparison with NMR results indicates that they are due to chain diffusion processes. The anomalous diffusion scale inferred from these experiments lies in the micron range, and on the same scale TEM micrographs revealed polypeptide bundles forming a network. This suggests that between the single helical segments joining three chains and the macroscopic gel there exist additional levels of organization and that the gel is quite inhomogeneous at the micron scale. We believe that the driving force behind this organization is a short-range hydrophobic interaction between the helical segments. Similar models have been investigated [4] showing spinodal decomposition ; in our case the phase separation will not be complete due to hydrophilic chain segments. To support this view we performed coarse-grained MD simulations allowing for an attractive potential between rigid helices, starting from percolation clusters and relaxing them towards reasonably realistic structures.

gelatin; anomalous diffusion; nuclear magnetic resonance; mesoscale organisation

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