Abstract:

Bolaamphiphiles ("bolas") containing two secondary amide groups at the ends of an oligomethylene chain and two amino acid type headgroups were synthesized. The structures of crystals, of noncovalent fibers, and of surface monolayers on gold strongly depended on odd-even effects. In the crystal structures of alanine-alanine dipeptides with C₁₁- and C₁₂-,-amino acid linkers, helical (even number of methylene groups in the chain) or sheetlike (odd) arrangements of the headgroups were found. Bolas containing two different amino acid end groups, namely, D- or L-alanine and L-lysine, connected by the same C₁₁- and C₁₂ linkers did not crystallize. Only the even-numbered bolas gave fibers. L- and D- configurations of alanine headgroups affected the curvature of the fibers. Diamido bolas with terminal SH-groups self-assembled on gold. Only those with even-numbered chains gave rigid monolayers. Simple stereochemical arguments concerning the fitting of amide hydrogen bond chains on both ends of the bolas are given to explain the observed differences in crystals, fibers, and monolayers.