Vibrational Study on Structure and Bioactivity of Protein Fibers Grafted with Phosphorylated Methacrylates

Abstract

In the last decades, silk fibroin and wool keratin have been considered functional materials for biomedical applications. In this study, fabrics containing silk fibers from Bombyx mori and Tussah silk fibers from Antheraea pernyi, as well as wool keratin fabrics, were grafted with phosmer CL and phosmer M (commercial names, i.e., methacrylate monomers containing phosphate groups in the molecular side chain) with different weight gains. Both phosmers were recently proposed as flame retarding agents, and their chemical composition suggested a possible application in bone tissue engineering. IR and Raman spectroscopy were used to disclose the possible structural changes induced by grafting and identify the most reactive amino acids towards the phosmers. The same techniques were used to investigate the nucleation of a calcium phosphate phase on the surface of the samples (i.e., bioactivity) after ageing in simulated body fluid (SBF). The phosmers were found to polymerize onto the biopolymers efficiently, and tyrosine and serine underwent phosphorylation (monitored through the strengthening of the Raman band at 1600 cm−1 and the weakening of the Raman band at 1400 cm−1, respectively). In grafted wool keratin, cysteic acid and other oxidation products of disulphide bridges were detected together with sulphated residues. Only slight conformational changes were observed upon grafting, generally towards an enrichment in ordered domains, suggesting that the amorphous regions were more prone to react (and, sometimes, degrade). All samples were shown to be bioactive, with a weight gain of up to 8%. The most bioactive samples contained the highest phosmers amounts, i.e., the highest amounts of phosphate nucleating sites. The sulphate/sulphonate groups present in grafted wool samples appeared to increase bioactivity, as shown by the five-fold increase of the IR phosphate band at 1040 cm−1.

Vibrational Study on the Structure, Bioactivity, and Silver Adsorption of Silk Fibroin Fibers Grafted with Methacrylonitrile

“…In agreement with a previous study [ 30 ], the A 1060 /A Amide I absorbance ratio was calculated to evaluate the nucleation of a calcium phosphate phase; in fact, near 1000 cm −1 , the phosphate antisymmetric stretching has been reported to fall [ 48 ]. The values of this spectroscopic marker are reported in Figure 16 .…”

“…In particular, the position and the FWHM of the Amide I band did not change. The same result was observed after 7 days of immersion in SBF of silk fibroin grafted with phosphorylated methacrylates [ 30 ]. The trend of the I 1266 /I 1229 intensity ratio ( Figure S19, Supplementary Material ) suggested the occurrence of little structural changes after 7 days of immersion.…”

“…Raman spectroscopy could not detect any nucleation of an inorganic phase even on the samples having positive weight gains, since the technique is more sensitive to the bulk of the material [ 30 ]. However, it proved suitable to determine whether the samples’ weight variation was associated with a mass loss due to the grafting agent.…”

“…In fact, silver-loaded silk membranes have already been described as anti-microbial wound dressing [ 26 , 27 , 28 , 29 ]. Moreover, the bioactivity of the grafting fibers (i.e., their calcium phosphate nucleation ability) was evaluated in vitro by ageing studies in simulated body fluid (SBF), mimicking the composition of human plasma [ 5 , 30 ].…”