A Molecular Orbital Study on Tetranactin-NH_4+ Complex

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The inclusion complex of NH_4+ with tetranactin increases the fluxes of ions across thin lipid membranes. When NH_4+ is inserted into the ionophore of tetranactin, the conformation changes greatly ; this structural change is called an "induced fit." When the conformation change of tetranactin is induced by the approach of NH_4+, the four ether oxygens of the tetrahydrofuran rings and the four carbonyl oxygens of the ester groups function to include NH_4+ in the molecule. The ether oxygens rotate inwards to hold NH_4+ inside tetranactin, and the carbonyl oxygens rotate inwards to prevent NH_4+ from escaping. The four ether oxygens form linear hydrogen bonds with NH_4+. Although the four carbonyl oxygens approach NH_4+ at distances similar to those of the four ether oxygens, the carbonyl oxygens do not form linear hydrogen bonds with NH_4+. In order to clarify the induced fit mechanism of the ionophore, in this paper, the inclusion complex of NH_4+ with tetranactin was studied with regard to the arrangement of NH_4+ in the tetranactin molecule from a quantum-chemical point of view. Since tetranactin is too large for double zeta ab initio SCF calculations, functional groups which have large contributions to the insertion of NH_4+ were selected as a model for the tetranactin ; the model consisted of four HCHO molecules and four H_2O molecules. The model was in good accord with the actual result, and thus appears to be appropriate. Tetranactin is composed of four homonactinic acid moieties, and the interaction energy between each of them and NH_4+ is significant for the arrangement of NH_4+. The interaction energy between NH_4+ and the four ether groups was -73.2 kcal/mol, and the interaction energy between NH_4+ and the four carbonyl groups was -39.4 kcal/mol. The interaction energy between NH_4+ and the ether groups was thus about 1.9 times larger than that between NH_4+ and the ester groups. The interaction energy between NH_4+ and the eight ether and ester groups was -109.0 kcal/mol. From the results of geometry optimizations, the interaction energy between NH_4+ and CH_3OCH_3 was larger than that between NH_4+ and HCOOH by 5.8 kcal/mol due to the electrostatic interaction energy. Therefore, even though the four carbonyl groups may move to accommodate NH_4+ in tetranactin, the four ether groups of tetranactin will interact more strongly with NH_4+ than the four carbonyl groups, and NH_4+ will orient to the ether groups. Moreover, when perturbation of the ionophore by the rotation of NH_4+ is neglected, the rotational inhibition energy of NH_4+ in tetranactin was calculated to be 9.8 kcal/mol ; thus, NH_4+ is expected to orient to the four ether oxygens.
社団法人日本薬学会の論文
1980-03-25

A Molecular Orbital Study on Tetranactin-NH_4+ Complex

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