Tunable High-Molecular-Weight Silk Fibroin Polypeptide Materials: Fabrication and Self-Assembly Mechanism

Silk fibroin is a multisegment natural protein composed of a heavy (H) chain, a light (L) chain and a P25 glycoprotein chain. Herein, we developed a dialysis separation technique under reducing conditions to break the disulfide bond between the H-chain and L-chain and remove the low-molecular-weight portions of the protein. Thus, a high-molecular-weight silk fibroin polypeptide (HSF) material was obtained. SDS-PAGE electrophoresis showed that the molecular weight of HSF was over 80 kDa, similar to the size of the silk fibroin H-chain. Amino acid analysis result demonstrated that the amino acid composition of HSF was almost identical to that of H-chain composition. Importantly, the HSF material obtained has a high surface activity, which can reduce the surface tension of water to below 20 mN/m; at high temperature and high concentration, it can also form a unique nanofibrous network with a lamellar crystalline structure. HSF can further form a rod-shaped structure in a strong polar environment and become a star-shaped fibrous network in a weak polar environment. When the pH value of HSF solution was adjusted from 6 to 8, a structural transition from a folded crank sheet-like structure with micellar beads to a ring-like fibrous structure was observed. During the conversion of HSF from colloidal particles to nanofibers, its molecular conformation also transformed from random coils to β-sheets. These tunable properties indicate that HSF materials have a wide range of applications in biomedical and green chemistry fields.

Characterization and performance of short cationic antimicrobial peptide isomers

Cationic antimicrobial peptides (CAMPs) represent an ancient defense mechanism against invading bacteria, with peptides such as the cathelicidins being essential elements of vertebrate innate immunity. CAMPs are typically associated with broad-spectrum antimicrobial potency and limited bacterial resistance. The cathelicidin identified from the elapid snake Naja atra (NA-CATH) contains a semi-conserved repeated 11-residue motif (ATRA motif) with a sequence pattern consistent with formation of an amphipathic helical conformation. Short peptide amides (ATRA-1, -1A, -1P, and -2) generated based on the pair of ATRA motifs in NA-CATH exhibited varied antimicrobial potencies. The small size of the ATRA peptides, coupled with their varied antimicrobial performances, make them interesting models to study the impact various physico-chemical properties have on antimicrobial performance in helical CAMPs. Accordingly, the D- and L-enantiomers of the peptide ATRA-1A, which in earlier studies had shown both good antimicrobial performance and strong helical character, were investigated in order to assess the impact peptide stereochemistry has on antimicrobial performance and interaction with chiral membranes. The ATRA-1A isomers exhibit varied potencies against four bacterial strains, and their conformational properties in the presence of mixed zwitterionic/anionic liposomes are influenced by anionic lipid content. These studies reveal subtle differences in the properties of the peptide isomers. Differences are also seen in the abilities of the ATRA-1A isomers to induce liposome fusion/aggregation, bilayer rearrangement and lysing through turbidity studies and fluorescence microscopy. The similarities and differences in the properties of the ATRA-1A isomers could aid in efforts to develop D-peptide-based therapeutics using high-performing L-peptides as templates.