Formation and structures of tyrocidine B oligomers in solution in the presence of water

Small-angle x-ray scattering and quasi-elastic light scattering measurements were conducted to analyze correlations between the structure and short-time dynamics of tyrocidine B·hydrochloride (TrcB) in ethanol, acetonitrile, and (R,S)-2-methylbutanol dispersions in the presence of 20.0% (v/v) water at 278 K and 298 K. The three TrcB dispersions exhibited peak position shifts that varied with the volume fraction. The experimental data were fitted to a model that considered the effective interaction potential, short-range attraction, and long-range repulsion. This model of repulsively interacting single TrcB particles is incompatible with the presence of equilibrium aggregate phases. The self-diffusion coefficient at the short-time limit (Dsh) decreased more as the TrcB concentration increased than one would expect for a corresponding hard-sphere or charged particle at the same volume fraction. At low volume fractions, the system consisted of monomers, dimers, and trimers. At high TrcB volume fractions, the main particles were larger aggregates. The collective diffusion coefficient, Dc, was constant when Q > Qc, where Qc is the position of the interference peak, which implies that there were no inter-monomer TrcB oligomer dynamics. This is because Dsh/D0 decayed much more quickly than the TrcB monomer as a function of the volume fraction. In vitro experiments revealed that antimicrobial activities were preserved at all volume fractions notwithstanding the presence of various oligomers.