CERCOSPORA-BETICOLA TOXINS .6. PRELIMINARY STUDIES OF PROTONATION AND COMPLEXATION EQUILIBRIA

The biological activity of Cercospora beticola toxins might be enhanced by the complex formation with magnesium. Therefore, protonation and complexation equilibria of beticolins were studied. Beticolins carry three dissociable functions (H3B) two of which dissociate at a physiological pH. In the presence of magnesium, the neutralisation and protonation curves provide evidence for the formation of complexes. At physiological pH, the uncharged complex, Mg2H2B2 is the predominant form. The non-ionised forms of free beticolin-1 and -2 fluoresce in a 50% dioxan-water solution and their emission maxima shift to higher wavelengths in water. The dianion HB2- is non-fluorescent both in water and in less polar media. The formation of the Mg2H2B2 complex which strongly fluoresces in nonpolar media is confirmed by a marked increase in fluorescence at 520 nm and by a shift of the excitation maximum.