Self-assembly of noncyclic bis-D- and L-tripeptides into higher order tubular constructs: design, synthesis, and...

Trans (1R,2R)-diaminocyclohexane was used as a semirigid vicinal diamine to anchor two N-protected tripeptides consisting of L-D-L amino acids as carboxy terminal amides. The bis-tripeptide consisting of L-Ser (OBn)-D-Ser (OBn)-L-Ser (O-p-bromobenzyl) Boc afforded X-ray quality crystals containing benzene and chloroform solvent molecules. Analysis of the solid-state structure revealed a highly H-bonded helical open-ended tubular superstructure. The tripeptide strands intertwine like a pair of self-embracing arms, held together by a gamma-turn and a 14-membered antiparallel H-bonded macroring spanning the first and third amino acid residues within each strand. Whereas the tripeptide from the R,R anchor gave beautiful crystals from benzene and chloroform, the analogous construct from the S,S-anchored diamine gave a gel. Related bis-tripeptides with different amino acids showed extensive intramolecular H-bonding based on NMR titration and dilution experiments.