G. Valensin

1H-NMR studies on the self-association of chloroquine in aqueous solution

The concentration dependences of 1H-NMR chemical shifts and spin-lattice relaxation rates were measured for chloroquine in aqueous solution. The weak self-association constant was evaluated according to a dimerization equilibrium with the formation of self-stacked adducts (Kd = 4.52 +/- 0.68 l mol-1). The motional correlation times were evaluated for the monomer and the dimer by measuring intramolecular dipolar cross-relaxation rates of aromatic vicinal protons (tau cm = 0.06 ns and tau cd = 0.26 ns). The geometry of the stacked dimer was elucidated by measuring intermolecular dipolar cross-relaxation rates and interpreted in terms of partial superposition of quinoline moieties.

13C and 31P NMR studies of phosphomycin, a phosphonate antibiotic

Abstract 13C and 31P NMR parameters were obtained for phosphomycin in water solution. The pH dependence of chemical shifts was used to calculate the pK of the second deprotonation process (pK2 = 6.4). The pH dependence of 13C[sbnd]31P and 13P[sbnd]31H coupling constant was pointing to a change in dihedral angles and hence in the geometry of phosphomycin during deprotonation. The pH dependences of 31P spin-lattice relaxation rates and 13P[sbnd]{1H} NOEs were interpreted in terms of relative weigth of the 31P-1H dipolar interaction in determining the relaxation mechanism.

H-1 Delineation of Conformational Parameters of Constituents of Silybum Marianum in Solution

Abstract Silybin, silydianin and silychristin, the three major constituents of the crude extract of Silybum marianum, were investigated by means of 1H-NMR in [2H6]-acetone. Non selective, single-selective and doubleselective 1H-NMR spin-lattice relaxation rates were measured and interpreted in terms of dipolarly interacting proton pairs. The motional correlation time was found in the range 0.50–0.60 ns for all compounds.

1H and 13C n.m.r. Conformational Analysis of Adrenergic Drugs. Part 2. N-Isopropyl-N- {3-[4(4-Methoxybenzoylamino)Phenoxy]-2-Hydroxypropyl} Ammonium Chloride, a New β1-Blocking Agent

Abstract Dynamic and structural features of N-Isopropyl-N- {3-[4(4-Methoxybenzoylamino)Phenoxy]-2-Hydroxypropyl} Ammonium Chloride in [2H6]DMSO were investigated by measuring 13C and 1H spin-lattice relaxation rates and 13C- {1H} and 1H- {1H} n.O.e. Correlation times for main and internal reorientational motions were interpreted in terms of internal rotation around the two planal axes. Selective and double-selective 1H spin-lattice relaxation rates were measured, wherefrom relevant proton-proton intramolecular distances were calculated. It was shown that the β1− blocking agent assumes a preferred conformation where extensive intramolecular H-bonding prevents segmental motion along the quaternary ammonium sidechain.

1H and 13C NMR conformational analysis of adrenergic drugs. III: Dichloroisoproterenol, a nonselective β-blocking agent

Abstract 13C and 1H NMR parameters were measured for dichloroisoproterenol in solution. Spin-lattice relaxation rates, nuclear Overhauser effects and J couplings were determined and compared to those obtained from isoproterenol. The t rotamer was shown to occur with a much higher probability than the two g rotamers. Dynamics in solution were interpreted in terms of a nearly isotropic motion of an extended molecular backbone. Some interesting differences were given evidence between the ‘preferred’ conformations in solution of dichloroisoproterenol and isoproterenol.

13C- and 1H-NMR relaxation investigation of a polyciclic nitrogen compound: 3,3-dimethyl-1,5-diphenyl-9-hydroxy-bispidinium iodide

Abstract The rigid polycyclic nitrogen compound was considered as a test for the reliability of internuclear distances calculated by 1H-NMR spin-lattice relaxation rates. The ‘isotropic’ motional correlation time was calculated from 13C relaxation rates (τC = 0.11 ns at 298 K). Dipolar cross-relaxation rates were calculated by measuring non-, mono- and double-selective proton spin-lattice relaxation rates. All the experimental relaxation rates were thoroughly accounted for by dipolar pairwise interactions. Only at high temperatures a certain contribution from the spin rotational mechanism was apparent.