Lyotropic aggregate of tripeptide derivatives within organic solvents: study on dynamic property of molecular assembling

Tripeptide derivatives possessing two hydrophobic chains aggregated in organic solvents, which were accompanied by changes in the Fourier transform infrared (FT-IR) spectra. A marked shift from a nonbonding to a bonding amide band was used for the diagnostics of the aggregation in a light and transparent nonaqueous solution in which it is difficult to determine aggregate formation based on macroscopic, rheologic and thermodynamic observations. The diagnostics was supported by the band shift in a CH2 stretching mode. Using the spectral changes, we demonstrated that the tripeptide derivatives are divided into two types. One was observed for amphiphilic tripeptide derivatives such as N-[N-11-trimethylammonioundecanoyl)-di-l-diphenylalanyl]-O,O′-didodecyl-l-glutamate bromide. The other was observed for hydrophobic tripeptide derivatives lacking a hydrophilic ammonium head, which were the intermediates of the amphiphiles. The tripeptide-containing amphiphiles formed a gel at about 10−3 M, whereas the later hydrophobic tripeptide derivatives generally only formed a viscoelastic solution with increase in the concentration. However, aggregate formation of the molecules was completed prior to the formation of a gel. The tripeptide-containing amphiphiles formed a stable aggregate at a very low concentration (<10−5 M). On the other hand, the hydrophobic tripeptide derivatives stepwise aggregated at a higher concentration. In this case, the monomeric species and the aggregate coexisted at equilibrium over the wide concentration range (10−2–10−3 M). We pointed out that the concentration-induced assemblage formed an artifact during preparation of the air-dried sample, which was provided for the structural analysis such as AFM observation.