Ionophore A23187. Solution conformations of the calcium complex and free acid deduced from proton and carbon-13 nuclear magnetic resonance studies

Abstract

Proton and carbon-13 nuclear magnetic resonance (NMR) spectra have been used to deduce possible biologically relevant conformations of ionophore A23187 as the free acid, and as the calcium complex, in solution. By analysis of coupling constants and dihedral angles obtained from 270-MHz proton NMR, A23187 free acid molecules and individual A23187 anions which comprise the calcium complex are found to differ principally by the rotational state of a carbon-carbon single bond near the benzoxazole ring. Proton Tl relaxation data and measurements of rotational correlation times confirm the dimeric nature of the calcium complex vs. the free acid monomer. Using the deduced pseudocyclic conformation for the A23187 anion, in conjunction with model-building studies, a conformation for the calcium complex is proposed, in which the benzoxazole carboxylate oxygen, the ketopyrrole oxygen, and the benzoxazole (ring) nitrogen atom of each molecule are the major participants in calcium binding. Additional stabilization of the structure is possible through the formation of a postulated hydrogen bond bridge between the pyrrole NH proton and benzoxazole carboxylate oxygen.