Ion-transfer voltammetry of streptothricin antibiotics with differently sized lysine oligomers at a nitrobenzene | water interface

• Streptothricins gave reversible voltammograms due to multivalent ion transfers.
• The formal potential became more negative with increasing the lysine oligomer size.
• Dibenzo-18-crown-6 facilitated their ion transfer.

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 +(~ 0.24 V) > ST-D4 +(0.199 V) > ST-C5 +(0.194 V), indicating that the lysine oligomer affected the phase-transfer characteristics of streptothricin.