Conformation of tetranactin in solution

The conformation of tetranactin, an ionophore, in chloroform was investigated by infrared and Raman spectra and by proton and 13C magnetic resonances. The infrared spectra show that the structure of its K+ complex in the solution is quite similar to that in crystals. The proton spin–spin coupling constants are explained well by assuming that the crystalline structure is retained in solution. The spin–lattice relaxation times of the 13C nuclei of the K+ complex indicate that its framework is rigid. The correlation time of the overall reorientation of the molecule was calculated to be 9 X 10−11 sec. On the other hand, the conformation of the complexed form in chloroform differs from that in crystals. Despite the geometrical nonequivalence of the four subunits in the crystalline state, the nuclear magnetic resonance spectra show their magnetic equivalence in the solution. The proton spin–spin coupling constants have values that are averaged by rapid internal rotation. The spin–lattice relaxation times of the 13C nuclei in its framework are unexplained by the overall reorientation of the molecule, and reveal the existence of internal motion in the framework. The rate of the local motion of the framework is between 102–1010 sec−1. By comparison of the infrared spectra, it can be said that the mean conformation of the fluctuated framework of the uncomplexed tetranactin in the solution is similar to that of nonactin in the crystalline form, which has an S4 symmetry axis through the center of the macrocyclic ring.