Silk Fibroin/Montmorillonite Nanocomposites: Effect of pH on the Conformational Transition and Clay Dispersion

Abstract

By adjusting the solution pH value below the isoelectric point (pI) of silk fibroin (SF) protein, the SF was in the cation state and it could interact strongly with unmodified anionic montmorillonite (MMT) surface. In this way, novel SF-MMT nanocomposites with good clay dispersion were successfully obtained, which were confirmed by X-ray diffraction and transmission electron microscopy. Further 1H CRAMPS and 13C CP/MAS NMR experimental results revealed that beta-sheet content of SF was remarkably enhanced for nanocomposite prepared below the pI of SF (SF-MMTA) due to the strong interaction between MMT and SF. In SF-MMTA nanocomposite, clay layers acting as an efficient nucleator could efficiently enhance the beta-sheet crystallization. On the contrary, SF preserved the native random coil conformation in SF-MMTN nanocomposites due to the weak interaction between MMT and SF. A tentative model was suggested and used to explain the mechanism of clay dispersion and conformational transition of silk protein.

Insights into the production and characterization of electrospun fibers from regenerated silk fibroin

“…This lack of one single preferential motif in L is consistent with the flat pattern obtained from our XRD analysis (Fig. 5), and might agree with the random coil conformation proposed in previous studies [69].…”

“…In fact, although the signals attributed by Dang et al to a psheet conformation [69] matched those observed in Silk II by Asakura and Yao [65], the chemical shift identified by the latter author with an ordered repeated pll-turn in Silk I [64,65] was associated by Dang and coworkers with a random-coiled distribution of the fibroin molecules [69]. In the study on the conformational transition from Silk I to Silk II, Zhou and colleagues describe the former state as 'helix dominant' [46].…”

“…Analyses by CP-MAS 13 C NMR have also been conducted on B. morí silk fibroin, as well as related model peptides [46,[64][65][66][67][68][69], in order to characterize the secondary structure of SF in the worm gland before spinning (Silk I) and in the spun solid fiber (Silk II). We used degummed SF fibers (D) and lyophilized regenerated SF powder (L) as a reference for Silk II and Silk I structures, respectively (Fig.…”

“…We wanted, however, to gain insight and quantify the different protein structural motifs coexisting in each sample. To this end, we further analyzed the Ala Cp signal, as it can be used as a diagnostic peak [46,65,66,69]. In fact, this resonance is quite sensitive to the molecular conformation and appears in a clear spectral region not overlapped with other signals.…”

“…Ala Cp peak was deconvoluted into the three identified components of Silk II (p-sheet A, p-sheet B and distorted p-turn) [65] and their ratio estimated by integration ( Fig. 8 and Table 1) [64,65,69]. In order to find the best fit for the Ala Cp signal, the height, width and position of the three component Gaussian peaks were adjusted.…”