Highly selective metallogel from 4-biphenylcarboxy capped diphenylalanine and FeCl3

The gelation property of the iron(iii) complex of 4-biphenylcarboxy capped diphenylalanine (l-Phe-l-Phe, the hydrophobic core motif of Alzheimer's beta-amyloid polypeptide) has been reported. Gelation was observed selectively for FeCl3 and 4-biphenylcarboxy capped diphenylalanine, whereas Cd, Cu, Zn, Cr, Ni, Hg, Mg, Mn, Pb, and Pd complexes were not able to form a metallogel. However, Boc and acetyl capped diphenylalanine have failed to form a metallogel. The storage modulus of the metallogel was approximately an order of magnitude larger than the loss modulus, which indicates the elastic nature of the gel and the physical crosslink. The metallogel exhibits an entangled fiber network which is stabilized by weak interactions such as coordination, hydrogen bonding and pi-pi interactions. The solid-state study shows that the acetyl protected diphenylalanine adopts an extended conformation and forms a supramolecular helix-like structure through multiple intermolecular hydrogen bonds. However, the 4-biphenylcarboxy protected diphenylalanine shows a supramolecular sheet-like structure through intermolecular hydrogen bonds and pi-pi stacking interactions between the biphenyl moieties.