The binding of lasalocid A (X537A) to Pr(III) in methanol has been studied by lasalocid fluorescence, circular dichtoism, and 1H and 13C NMR spectroscopy. It is clear that in addition to a mono complex, bis and tris complexes are also formed. Values of the binding constants and spectral properties of the various complexes have been determined by computer fitting of the binding isotherms. The Pr(III) ion binds only at the salicylic "head" of the lasalocid A, in stark contrast with other known structures. The lasalocid A appears to have an "open" conformation in these complexes. The relevance of these results to the structure and conformation of the Ca(II) complex is discussed. The first order rate constant for the dissociation of the tris complex has also been determined.
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