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

Beauvericin(Bv) is a naturally occurring ionophore thatselectivelytransports ions through cell membranes. However, the intrinsic ionselectivity of Bv for alkaline earth metal ions (M2+) isyet to be established due to inconsistent results from condensed phaseexperiments. Based on fluorescence quenching rates, Ca2+ appears to be preferred while extraction experiments favor Mg2+. In this study, we apply cold ion trap infrared spectroscopyto Bv-M2+ coupled with electrospray ionization mass spectrometry.The mass spectrum shows that Bv favors binding to physiologicallyactive ions Mg2+ and Ca2+ although it can formcomplexes with all four alkaline earth metal ions. Infrared spectroscopy,as measured by the H-2 tag technique, reveals that Bv bindsMg(2+) and Ca2+ ions by six carbonyl oxygens inthe center of its cavity. This observation is supported by theoreticalcalculations. Other alkaline earth metal ions are bound by three carbonylgroups at the amide face. This difference in configuration is consistentwith the binding preferences for the alkaline earth metal ions.