Self-assembly from low dimension to higher conformation of GGX motif in spider silk protein

Spider silk is intriguing because of its unique structure and high-performance mechanical properties. However, few studies have presented data characterizing the conserved motifs found in the silk protein as an oligopeptide. In this report a designed peptide that mimics the repetitive GGX motif in the silk spidroin was investigated. The peptide undergoes a 'bottom-up' self-assembly which was simply manipulated by metal ions and an ionic self-complementary peptide, RADA16-I. In water, the peptide was observed to form discrete and stiff nanorods (PNrs) via hydrophobic interactions. AFM, TEM and DLS data showed that the addition of MgCl2 or CaCl2 led to the formation of a mass of nanorod bundles (PNrs-b). The addition of EDTA forced the disruption of the bundles, therefore providing evidence that the metal ions and peptide interact. Fourier transform infrared spectroscopy showed the presence of coordination bonds bridging the metal ions and the carbonyl groups of the peptide. Such coordination bonds are considered to facilitate the bundling of the nanorods. Conversely, mixing the GGX peptide with the RADA16-I peptide caused a lengthening of the nanorods (L-PNrs). Here, the length of the nanorods changed from several microns to over ten microns. In addition, AFM was used to follow the elongation process. Such a method provided a clear and direct overview of the kinetics of self-assembly.