NMR Studies of Ca2+ Complexes of Annonaceous Acetogenins

13C NMR longitudinal relaxation times (T 1) have been determined for both annonacin (1) and squamocin (2) in the absence and presence of Ca2+ ions. These data are used to assess structural changes that accompany complexations. Even though acetogenins of the Annonaceae present no ionophoric effects in biological studies with living cells, we assume that they have a role in the bioavailability of the cations in the cell membranes, due to both their lipophilic and polar natures. These results also show differences in the stoichiometry of the complexes of a mono-tetrahydrofuranic acetogenin and a bis-tetrahydrofuranic acetogenin with Ca2+ ions.