Formation of peptide nanospheres and nanofibrils by metal coordination

Two amphipathic polypeptides were coordinated to the cis positions of a square planar Pt(II) complex in order to provide the metal center with two noncovalent oligomerization domains. This resulted in the formation of new metal-peptide nanoassemblies which are shown to exist as nanometer-sized spheres and fibrils. Construction of these assemblies was based on the 30-residue polypeptide AQ-Pal14 which was designed for its ability to self-assemble into the common protein oligomerization motif of a noncovalent coiled-coil, and modified to contain a metal-binding 4-pyridylalanine residue at its surface. When AQ-Pal14 was reacted with Pt(en)(NO3)(2), a new metal-peptide complex was formed in which two AQ-Pal14 peptides were coordinated to a single metal center as determined by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and electrospray ionization mass spectrometry (ESI-MS). When the reaction mixture was analyzed under nondenaturing conditions by high performance size exclusion chromatography (HPSEC), it was found that all species present eluted at the column void volume, indicating the formation of very large metal-peptide assemblies. This was verified by multiangle light scattering (MALS) which showed that the metal-peptide assemblies have a weight-averaged molecular mass and z-average root-mean-square radius of M-W = (7 +/- 4) x 10(6) g/mol and R-Z = 18 +/- 4 nm, respectively. The presence of such nanometer scale assemblies was confirmed by transmission electron microscopy and atomic force microscopy which showed the existence of both spherical and fibrillar nanostructures.