Application of the equations‐of‐motion method to the calculation of the indirect nuclear spin–spin coupling tensors

Density functional theory study of calix[4]arene-N-azacrown-5, calix[4]arene-N-phenyl-azacrown-5, and their complexes with alkali-metal cations: Na+, K+, and Rb+

Theoretical studies of 1,3-alternate-25,27-bis(1-methoxyethyl)calix[4]arene-azacrown-5 (L(1)), 1,3-alternate-25,27-bis(1-methoxyethyl)calix[4]arene-N-phenyl-azacrown-5 (L(2)), and the corresponding complexes M(+)/ L of L(1) and L(2) with the alkali-metal cations: Na(+), K(+), and Rb(+) have been performed using density functional theory (DFT) at B3LYP/6-31G* level. The optimized geometric structures obtained from DFT calculations are used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal-ligand and cation-pi interactions are investigated. The results indicate that intermolecular electrostatic interactions are dominant and the electron-donating oxygen offer lone pair electrons to the contacting RY* (1-center Rydberg) or LP* (1-center valence antibond lone pair) orbitals of M(+) (Na(+), K(+), and Rb(+)). What's more, the cation-pi interactions between the metal ion and pi-orbitals of the two rotated benzene rings play a minor role. For all the structures, the most pronounced changes in geometric parameters upon interaction are observed in the calix[4]arene molecule. In addition, an extra pendant phenyl group attached to nitrogen can promote metal complexation by 3D encapsulation greatly. In addition, the enthalpies of complexation reaction and hydrated cation exchange reaction had been studied by the calculated thermodynamic data. The calculated results of hydrated cation exchange reaction are in a good agreement with the experimental data for the complexes.

Evidence against formation of A23187 dimers and oligomers in solution: photo-induced degradation of Ionophore A23187

Ionophore A23187 has been proposed to form Ca(2+)- conducting channels that arise from dimers and oligomers of the compound (e.g., Balasubramanian, S. V., and Easwaran, K. R. K. (1989) Biochem. Biophys. Res. Commun. 158, 891-897). To investigate this possibility, the solution behavior of A23187 in chloroform, n-hexane, ethanol, 80% methanol-water, and palmitoyloleoylphosphatidyl choline (POPC) vesicles was investigated using UV-VIS, circular dichroism (CD), and 1H NMR techniques. The concentration dependence of the UV-VIS and CD spectra obtained in freshly prepared chloroform solutions indicates that neutral A23187 (HA) exists as a monomer for ionophore concentrations in the range of 50-1000 microM. The cause of time- and concentration-dependent spectral alterations which gave rise to the dimer/channel hypothesis was also investigated. For solutions of 50-1000 microM A23187 in chloroform, n-hexane, and ethanol stored in the dark, no spectral changes were observed for periods of 2 months. However, solutions in these solvents did show time-dependent spectral changes when exposed to light. In 80% methanol-water or phospholipid vesicles, similar spectral changes were observed, even when the solutions were protected from light. Application of TLC and MS methods indicate that the time-dependent spectral changes reflect degradation of A23187, not dimer or oligomer formation. The degradative processes proceed with half-lives ranging from approximately 75 to > 400 h, and are influenced by several factors, including solvent, exposure to light, ionophore concentration, pH, and the presence of metal ions, EDTA, dissolved oxygen, and a radical inhibitor. The kinetics of Ca2+ transport into Quin-2-loaded POPC vesicles by authentic A23187 give no evidence of a channel mechanism, even following a previous and lengthy coincubation of the ionophore with the vesicles.