Infrared Spectra of Beauvericin-Alkaline Earth Metal Ion Complexes─Ion Preference to Physiological Ions.

Beauvericin (Bv) is a naturally occurring ionophore that selectively transports ions through cell membranes. However, the intrinsic ion selectivity of Bv for alkaline earth metal ions (M) is yet to be established due to inconsistent results from condensed phase experiments. Based on fluorescence quenching rates, Ca appears to be preferred while extraction experiments favor Mg. In this study, we apply cold ion trap─infrared spectroscopy to Bv-M coupled with electrospray ionization mass spectrometry. The mass spectrum shows that Bv favors binding to physiologically active ions Mg and Ca although it can form complexes with all four alkaline earth metal ions. Infrared spectroscopy, as measured by the H tag technique, reveals that Bv binds Mg and Ca ions by six carbonyl oxygens in the center of its cavity. This observation is supported by theoretical calculations. Other alkaline earth metal ions are bound by three carbonyl groups at the amide face. This difference in configuration is consistent with the binding preferences for the alkaline earth metal ions.