Abstract
The transfer of streptothricin antibiotics containing a β-lysine monomer (ST-F), trimer (ST-D), and tetramer (ST-C) at a polarizable nitrobenzene (NB) | 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, ΔOWϕ0S’T, 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+, ST-DL44+ and ST-CL55+ complexes with the ligand in NB. Using the ΔOWϕ0S’T-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 ΔOWϕ0S’T of the ST-F2+ ion was estimated. Interestingly, the determined ΔOWϕ0S’T-value was increasingly negative in the order of ST-F2+(~0.24V)>ST-D4+(0.199V)>ST-C5+(0.194V), indicating that the lysine oligomer affected the phase-transfer characteristics of streptothricin.
Published Version
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