Distribution of Cr and Pb in artificial sea water and their sorption in marine sediments: an example from experimental mesocosms.

The uptake of two heavy metals (chromium and lead) in sediments in experimental mesocosms under exposure to different metal concentrations was evaluated by monitoring their concentrations over time both in seawater and in sediment. Two separate experiments under laboratory-controlled conditions were carried out for the two metals. Sediments were collected from a protected natural area characterized by low anthropic influence and were placed in mesocosms that were housed in aquaria each with seawater at a different metal concentration. At pre-established time intervals, seawater and sediment samples were collected from each mesocosm for chemical analyses. Quantification of chromium and lead concentration in seawater and sediment samples was carried out by atomic absorption spectrometer with graphite furnace. Low doses of chromium and lead (<1 mg L-1) do not entail an uptake in sediments and waters. At doses ≥1 mg L-1, evolution of concentrations over time shows significant differences between these two metals: (i) chromium absorption from seawater is twice faster than lead; (ii) lead accumulates in considerable amount in sediments. The different behaviour of the two investigated heavy metals could be ascribed to different interactions existing between metal ions and different components of sediment.

Calcium-binding properties and molecular organization of bradykinin A solution 1H-NMR study.

The NMR features of bradykinin were investigated in dimethylsulfoxide containing 1% water. The temperature dependence of chemical shifts and ROESY maps were monitored for the major species where all X-Pro bonds are trans. The occurrence of a head-to-tail ionic interaction and intramolecular hydrogen bonds stabilizing a pseudo cyclic arrangement was inferred, a beta turn at the C-terminus being the main feature of the secondary structure. Calcium was shown to bind to the peptide with a dissociation constant Kd = 2.8 + 0.2 mm. 2Pro and 3Pro carbonyls, as well as the 9Arg carboxyl, were assigned as the metal-binding sites. A molecular model of the 1 : 1 metal-complex was obtained. In light of conformational changes experienced by the peptide upon interaction with calcium, a role for the metal was hypothesized in the process of conformational selection from the free to the receptor-bound state of bradykinin.

Delineation of conformational and structural features of the amikacin-Cu(II) complex in water solution by 13C-NMR spectroscopy.

The copper (II) complex of amikacin in water solution at pH 5.5 was investigated by 13C-NMR. The temperature dependence of spin-lattice relaxation rates was measured and fast exchange conditions were shown to apply. The motional correlation time of the complex was approximated by the pseudo-isotropic rotational correlation time of free amikacin in water solution (tau c = 0.17 ns at 300 K). Formation of a pseudo-tetrahedral 1:1 complex was demonstrated by relaxation rates analysis and also by UV-Vis spectrophotometry. Two amino nitrogens of amikacin, together with the amide nitrogen and the hydroxyl in the hydroxyl-aminopropyl carbonyl side chain, were assigned as the copper-binding sites and a model of the complex was built by using copper-carbon distances obtained by NMR analysis as input parameters.

Conformational features of charged dibucaine in solution as investigated by 13C- and 1H-NMR.

Conformational features of charged dibucaine in [2H6]DMSO were elucidated by measuring 13C and 1H spin-lattice relaxation rates and 1H-(1H) and 13C-(1H) nuclear Overhauser effects. The reorientational correlation time of the aromatic moiety was evaluated at 0.16 ns at room temperature and side chains were observed to display segmental motion. Relevant distances were calculated by isolating dipolar interaction terms of 1H-1H or 13C-1H pairs. The 'preferred' conformation in solution was shown to present several analogies, but also some differences, with the structures obtained by solid state experiments, energy calculations and LIS data.

Interaction of Daunomycin with Dipalmitoylphosphatidylcholine Model Membranes. A 1H NMR Study

1H NMR parameters were obtained for daunomycin in water solution in the free state as well as in the presence of dipalmitoylphosphatidylcholine model membranes. Spin-lattice relaxation rates were measured under nonselective, single-selective, and double-selective irradiation modes, and 2D NOESY spectra were obtained at several values of the mixing time. Proton-proton distances were calculated and the motional correlation time was evaluated in both the free and bound states. NMR parameters were used to show that ring A and the glucosamine moiety of daunomycin strongly interact with the external surface of the bilayer, while the rest of the molecule penetrates the membrane without crossing it. The structures of both free and bound daunomycin were obtained and compared by using molecular modeling.

1H-NMR and 13C-NMR investigation of complexes of Mn2+ with ocytocin analogues in (2H6)dimethylsulfoxide.

Several ocytocin analogues were synthesised by substitution of the Pro residue with sarcosine or N-methylalanine, the glutamine residue with threonine and one of the cysteines with 2-mercaptopropionic acid. All the derivatives were investigated by NMR in dimethylsulfoxide solutions and evidence was obtained for similar preferred conformations in the solution free state. All peptides were shown to form complexes with Mn2+ in solution by the strong paramagnetic effects experienced by several proton resonances. Two structures could be determined, one formed by peptides containing threonine and the other by the remaining peptides. The two structures were delineated by molecular modelling using the Mn(2+)-proton distances obtained by NMR as restraints.

Conformational features of erythrocyte-bound chloroquine from double-selective 1H NMR relaxation.

The interaction between the antimalarial drug chloroquine and red blood cells was investigated by measuring 1H NMR spin-lattice relaxation rates upon double-selective excitation of dipolarly connected spin pairs within the ligand molecule. Geometric and dynamic features were inferred, for the free as well as for the bound ligand, by the cross-relaxation terms between protons at fixed and time-dependent distances, respectively. A change in orientation of the side chain in respect of the aromatic moiety was shown to result from the binding interaction.

NMR investigations of Mn(II)-leucine-enkephalin complexes in [2H6]-DMSO solution.

Structural and kinetic features of the Mn(II)-Leu-enkephalin binding equilibria were delineated by measuring 13C and 1H NMR spin-lattice relaxation rates. The temperature dependence of such rates showed that some carbons were experiencing slow exchange regimes such that kinetic parameters at room temperature could be calculated (k(off) = 1400 sec-1, delta H* = 12.0 kcal/mol, delta S* = -9.9 e.u.). The paramagnetic rates of fast exchanging carbons were interpreted by the Solomon-Bloembergen-Morgan theory to provide structural parameters. The terminal carboxyl and amino groups were shown to be the binding sites. The motional correlation time (tau c = 0.6 nsec at 298 K) was calculated by measuring selective and double-selective 1H spin-lattice relaxation rates for the free peptide. The number of coordinated ligands was evaluated by considering the distance of the Leu CO in the complex at 2.54 A, as shown by molecular models. Finally, carbon-Mn(II) distances were calculated and the molecular model of the 1:1 complex was built.

Nuclear magnetic resonance spectrometric investigations of calcium antagonist drugs. III: Conformational and dynamic features of diltiazem in solution.

Conformational features of diltiazem in D2O have been delineated by measuring 13C and 1H NMR parameters. The data obtained by interpretation of spin-lattice relaxation rates and 13C-[1H] nuclear Overhauser effects were used to build a model of the most probable arrangement in solution. The molecular motion of the two aromatic rings and of the thiazepinic ring is anisotropic, and the assumption that the molecule is an ellipsoid undergoing Brownian motion leads to calculation of the diffusion constants for the rotation around the longer axis and the shorter axis. The dimethylaminoethyl chain seems folded back, and it displays the typical features of segmental motion; in the same way, the methoxy- and the acetyloxymethyls behave like free rotors.

Nuclear magnetic resonance investigations of calcium antagonist drugs. II: Conformational and dynamic features of verapamil in [2H6]DMSO.

Conformational features of verapamil in [2H6]DMSO have been delineated by measuring 13C and 1H NMR parameters. Spin-lattice relaxation rates were interpreted within the frame of an extended alkane chain with segmental motions hampered by the aromatic substituents at both ends. The effective correlation times were all evaluated in the range 0.16-0.22 ns at 293 K. The aromatic rings were found to lie somewhat out-of-plane with the alkane chain. The NMR data were used to construct a Dreiding model of the most probable spatial arrangement and a molecular modeling system was utilized to represent the "preferred" conformation of verapamil in solution.

Conformational dynamics of cytochalasin B in solution as detected by 13C and 1H NMR relaxation rates.

13C and 1H NMR spin-lattice relaxation rates have been measured for cytochalasin B in [2H]6DMSO. Motional features have been interpreted in terms of nearly isotropic reorientation of the whole molecule with few additional internal motions. The 'isotropic' reorientational correlation time was evaluated at 0.21 ns at room temperature. By using selective and double-selective excitation techniques, relevant cross-relaxation terms have been obtained wherefrom proton-proton distances have been calculated. A Dreiding model of the 'preferred' conformation in solution has been built, yielding evidence of a strong similarity between solution and solid state structures of cytochalasin B.