The transfer of streptothricin antibiotics containing a beta-lysine monomer (ST-F), trimer (ST-D), and tetramer (ST-C) at a polarizable nitrobenzene (NB) broken vertical bar water (W) interface was studied by cyclic voltammetry. Under acidic conditions, the streptothricins existed in fully protonated forms, as ST-F2+, ST-D4+ and ST-C5+ cationic species. Within the potential range tested, ST-D4+ and ST-C5+ ions gave reversible voltammograms due to their transfer across the NB W interface. The formal potentials, Delta(w)(o)phi(S) (T),(0) (,) of ST-D4+ and ST-C5+ ions were determined. In the presence of a neutral ligand (L), dibenzo-18-crown-6, in NB, ST-F2+, ST-D4+ and ST-C5+ ions gave reversible voltammograms due to their transfer assisted by the formation of ST-FL22+, sr-DL44+ and ST-CL55+ complexes with the ligand in NB. Using the Delta(w)(o)phi(S) (T) (0)-values, association constants between the NH3+-groups of ST-D4+ and ST-C5+ ions and the crown ether in NB were determined. The association constants were in agreement with each other. By assuming that the association constant of ST-F2+ was equal to those of the ST-D4+ and ST-C5+ ions, the Delta(w)(o)phi(S) (0)(T) (,) of the ST-F2+ ion was estimated. Interestingly, the determined Delta(w)(o)phi(S) (0)(T) (,)-value was increasingly negative in the order of ST-F2+ (similar to 0.24 V) > ST-D4+ (0.199 V) > ST-C5+ (0.194 V), indicating that the lysine oligomer affected the phase-transfer characteristics of streptothricin. (C) 2015 Elsevier B.V. All rights reserved.