Tryptocidine C encapsulated nano-silver assemblies with potent antibacterial activity

• Tryptocidine C (TpcC), is a Trp-rich antimicrobial cyclodecapeptide that form nanoparticle templates suitable for silver nanoparticle (AgNP) synthesis. • Trp-clusters in TpcC-NPs facilitate the photo-induced fabrication of AgNPs upon green laser irradiation. • Fluorescence spectroscopy confirmed the role of the Trp-residues in AgNP formation. • Encapsulation of AgNPs in the core of TpcC nanostructures and was observed with high resolution electron microscopy studies. • The size of the TpcC-NPs correlates with those of the observed AgNPs. • These novel Tpc-Ag nanohybrids, with enhanced and potent antibacterial activity towards Gram-positive Streptococcus aureus and Gram-negative Escherichia coli bacteria. Tryptocidine C (TpcC), a Trp-rich antimicrobial cyclodecapeptide, was previously observed to oligomerise and form small nanospheres with narrow polydispersity. This study reports the laser-assisted fabrication and encapsulation of silver nanoparticles (AgNPs) by tryptocidine C nanoparticles (TpcC-NPs). Molecular docking study performed by using YASARA Force Field, demonstrated the formation of TpcC tetramer in which formation of tryptophan pocket can be seen where side chain tryptophan residues participate in intermolecular hydrogen bonding. This tryptophan pocket possibly facilitates to trap silver ions for photo-induced formation of silver nanoparticles by continuous wave green laser (532 nm and 1000 mW). Fluorescence spectroscopy studies confirmed the role of Trp residues in the reaction with silver ions, possibly inside Trp clusters within the TpcC-NPs. Surface plasmon resonance and high-resolution electron microscopy studies confirmed the formation and encapsulation of AgNPs in the core of TpcC-NPs, with AgNPs sizes correlating with TpcC-NP sizes. These novel TpcC-Ag nanohybrids exhibited potent antibacterial activity against both Staphylococcus aureus and Escherichia coli . TpcC alone showed an MIC of 6.25 μM against S. aureus but were inactive against E. coli . Ag(I) alone showed MIC of 100 μM against both bacteria For the TpcC-Ag (1:2) colloid, a 16-fold improvement of MIC against S. aureus was found for Ag/Ag(I) with and 2-fold improvement for TpcC. For the TpcC-Ag (1:3) colloid the E. coli resistance towards TpcC was reversed (MIC of 6.25 μM), combined with a 5-fold lower Ag/Ag(I) MIC. In conclusion, the formation of AgNPs was directed by TpcC oligomerisation, while the antimicrobial activity of TpcC was modulated and improved by hybridising it with AgNPs.