Metal-complexed nanofiber formation in water from dicarboxylic valylvaline bolaamphiphiles

Nanofiber formation of dipeptide-based bolaamphiphiles, bis (N-alpha-amide--valyl--valine) 1,n-alkane dicarboxylate (n=6, 8, 10, and 12) in water was analyzed by TEM, SEM, IR, and XRD. The bolaamphiphiles proved to be coordinated to divalent transition-metal cations, such as Co2+, Ni2+, Cu2+, and Zn2+, giving precipitates, colloidal dispersions (loose hydrogels), and hydrogels upon self-assembly at 23 or 70 degrees C. Longer oligomethylene chains and strong interaction between the metal cations and the carboxylate anions are responsible for the hydrogel formation. Energy-filtering transmission electron microscopy (EF-TEM) and field-emission scanning electron microscopy (EF-SEM) images revealed that the colloidal dispersions and the hydrogels consist of a large number of nanofibers with widths of 15-20 nm and lengths of several micrometers. FT-IR and powder XRD measurement supported the existence of a beta-sheet structure-based nanofibers complexing with metal cations.