Andrea Lepri

Nuclear magnetic resonance study of the oxovanadium (IV)-(glutathione)2 complex

Abstract 13C and 1H NMR relaxation rates were measured for the glutathione-VO2+ 2:1 complex in aqueous solution. The kinetics of the dissociation of the peptide from the coordination sphere were delineated and the structure of the complex was determined. The two carboxyl groups were shown to be the main binding sites.

Conformational features of cell-bound drugs as detected by selective proton relaxation rate investigations

The nonselective and selective longitudinal relaxation rates were measured for procaine protons in the presence of model lipid membranes, biological membranes and whole cells. Unlike the nonselective relaxation rates, the selective rate was shown to be particularly sensitive in detecting binding interactions with macromolecular cell constituents. It was shown that the aromatic moiety of procaine is involved in binding the cell plasma membrane.

Selective 1H-NMR relaxation investigations of membrane-bound drugs in vitro: 1. Colchicine

Binding of colchicine to dipalmitoylphosphatidylcholine bilayer vesicles was detected by measuring the 1H-NMR selective spin-lattice relaxation rates of the low-field protons of colchicine. From the temperature dependence of the selective rates, preferential binding was observed above the temperature of transition. In the same way, binding of colchicine to red blood cells was detected and the equilibrium constant determined. Binding to the lipid matrix of red blood cells accounted only partially for the binding of colchicine to whole cells.

Selective 1H NMR Relaxation Delineation of Receptor Binding Equilibria

The NMR spectrum of a macromolecule is usually a broad envelope which seldom allows one to distinguish any individual resonance line. Moreover it often happens that a low amount of the macromolecule, insufficient to give a strong resonance adsorption, can be isolated. As a consequence, the problem of studying interactions between a small ligand and its macromolecular receptor has then to be approached by observation of any change in the NMR parameters of the ligand caused by the presence of the macromolecule. Exchange between at least two environments, free (f) and bound (b) to the macromolecule must then be considered in developing theoretical equations for the NMR parameters of the ligand; namely either chemical shift or relaxation rates have been shown (1,2) to depend on the rate of chemical exchange. Occasionally it may be possible to detect the resonances of the bound ligand directly, but generally the concentration of the macromolecule, and hence also that of the bound ligand, will be small and this detection will be difficult. If, however, the rate of chemical exchange between free and bound environments is fast in respect of either the difference in chemical shift or the nuclear relaxation rate, then the observed NMR parameters will be a weighted average of those in each environment and thus information on the bound resonance signals can be gained in the bulk.

Ambiguities in the proton NMR studies involving Cu(II) ions as paramagnetic relaxation centres

Abstract The theory of nuclear spin relaxation induced by paramagnetic ions in solution has been given great relevance in delineating structural and kinetic parameters of bioinorganic reactions. Although several criticism and corrections have been worked out, the original form of the Solomon-Bloembergen-Morgan equitions [1, 2] still represents the most suitable starting point for interpreting experimental paramagnetic rates. t001 . Paramagnetic Proton Relaxation Rates of Imidazole H 4 , pH = 7.0; T = 300 K. His ( M ) Cu 2+ (m M ) T −1 1p (sec −1 ) T −1 2p (sec −1 ) (sec −1 ) T − 2p /T − 1p 0.1 0.01 0.02 0.32 16 0.1 0.02 0.03 0.45 15 0.1 0.03 0.06 0.65 11 0.1 0.04 0.14 1.27 7 0.1 0.05 0.18 1.41 10 When dealing with Cu(II) ions as relaxation reagents, the scalar interaction was shown to give significant contributions to NMR line broadening, and T 1 measurements were therefore designed for getting structural information from the dipolar term. In this report we suggest that asymmetric coordination to Cu(II) ions, resulting in large g tensor anisotropies, make the Solomon-Bloembergen equations meaningless, since unreasonable answers are obtained in well defined Cu(II) complexes. The Cu(His) 2 complex was taken as a model complex, since the X-ray structure of crystals obtained from aqueous solution has been reported [3]. The experimental paramagnetic contributions T −1 ip = T −1 i (obs) − T t-1 i (blank) are reported in Table I for H 4 of the imidazole moiety at different [Cu ++ ] tot / [His] tot ratios. In these conditions CuA 2 (His = H 3 A) is almost exclusively present in solution [4] and the T − 2p /T − 1p values are consistent with fast exchange of His molecules from the metal coordination sphere. The correlation time of the dipolar interaction was approximately by measuring that of water protons bound in the complex and the Cu(II)H 4 distance was taken rom crystallorgraphic structure. Calculations based on the Solomon-Bloembergen equations give values of the coordination number ranging between 0.039 and 0.054, which is nonsense. Since the correlation time of the complex is not rapid enough compared with the electron spin anisotropy energy, that is h τ −1 c ⪡|g∥ − g⊥|βB o no theoretical model is available to allow quantitative interpretation of nuclear relaxation. Combined ESR and NMR experiments are therefore suggested to build up a novel theoretical approach.