Several paramagnetic electron-rich Fe(III) mononuclear arylacetylide complexes of formula [(η2-dppe)(η5-C5Me5)Fe(C⋮C−Ar)]+ in which Ar represents a functional aryl group were studied by means
of multinuclear NMR. All signals detected for the various nuclei were assigned. Hyperfine coupling
constants for selected nuclei of the arylacetylide ligand were derived from 1H or 19F NMR contact shifts.
These NMR data are diagnostic of a metal-centered unpaired electron partly residing in a π molecular
orbital on the arylacetylide ligand, in line with DFT computations. We show here that the 1H NMR
paramagnetic shifts of the ortho (H1) and meta (H2) arylacetylide protons convey decisive information
on the charge distribution in the aryl ring. Estimates of the relaxation rates of the unpaired electron were
also derived from half-widths of the 1H NMR signals. Finally, line-broadening studies of Fe(II)/Fe(III)
mixtures allowed extracting the self-exchange rates for several redox couples among these complexes.
The self-exchange rates appear slightly substituent dependent and are apparently larger for compounds
with electron-withdrawing substituents on the aryl ring. Reorganization energies of ca. 4000 cm-1 could
be derived for these outer-sphere electron-transfer processes.
We have investigated the spin distribution and determined the magnetic exchange coupling J(ab) (defined according to the following Hamiltonian: H(spin) = -2J(ab)S(a).S(b)) for three arylethynyl-bridged organoiron(III) diradicals containing [(eta(2)-dppe)(eta(5)-C(5)Me(5))Fe(III)](+) fragments. Considering the distance separating the Fe(III) centers (>or=11 A), remarkably large intramolecular magnetic interactions between unpaired spins were found for two of them. Thus, an antiferromagnetic coupling (J(ab)) of ca. -190 cm(-1) was experimentally determined for the binuclear Fe(III) species featuring a 1,4-diethynylbenzene bridge 1[PF(6)](2), while a ferromagnetic interaction of over +150 cm(-1) was evidenced for its 1,3-substitued analogue 2[PF(6)](2). We also show that a much weaker interaction (0 > J(ab) >or= -1 cm(-1)) takes place in the 4,4'-biphenyl analogue of 1[PF(6)](2) (3[PF(6)](2)), evidencing that insertion of an additional 1,4-phenylene unit in the bridge severely disrupts the magnetic communication in these diradicals. With the help of NMR and density functional theory, the magnetic properties of these compounds were rationalized and compared to those of the corresponding mononuclear Fe(III) relatives 4[PF(6)] and 5[PF(6)]. Finally, it is shown that, for all of these dinuclear Fe(III) complexes, the structural changes between singlet and triplet spin isomers remain very small regarding the carbon-rich bridge. Thus, even for a strongly coupled diradical such as 1[PF(6)](2), a dominant diradicaloid character dominates the valence-bond description of the singlet state unpaired electrons.
Grapevine stem extracts are viticulture byproducts rich in stilbenes that are increasingly studied for their potential biological activities. This study aimed to investigate some biological activities of a grape byproduct with high stilbenoid content and to point out the molecules responsible of these beneficial activities. As a consequence, the extract was subjected to a bioguided fractionation and separation by centrifugal partition chromatography. The obtained fractions were characterized by liquid chromatography coupled to mass spectrometry and nuclear magnetic resonance. Fractions were purified further by column chromatography and resulted in the purification of the main constituents. Thirteen stilbenes have been quantified. The most abundant compounds were ε-viniferin, resveratrol, and, in lesser amounts, isohopeaphenol and ampelopsin A. The extract, fractions, and major stilbenes were tested for their antioxidant activity by oxygen radical absorbance capacity and their cyprotective effects against β-amyloid on rat pheochromocytoma cells. Among them, fraction 5 showed significant antioxidant activity and fraction 2 had a significant cytoprotective effect against β-amyloid-induced toxicity. Two putative inhibitors of β-amyloid toxicity have been identified: ampelopsin A and piceatannol.
Detailed characterization of protein, peptide or drug interactions with natural membrane is still a challenge. This review focuses on the use of nuclear magnetic resonance (NMR) for the analysis of interaction of molecules with small unilamellar vesicles (SUV). These phospholipid vesicles are often used as model membranes for fluorescence or circular dichroism experiments. The various NMR approaches for studying molecule-lipid association are reviewed. After a brief survey of the SUV characterization, the use of heteronuclear NMR (phosphorous, carbon, fluorine) is described. Applications of proton NMR through transferred nuclear Overhauser effect to perform structural determination of peptide are presented. Special care is finally given to the influence of the kinetic of the interactions for the proton NMR of bound molecules in SUV, which can constitute a good model for the study of dynamical processes at the membrane surface.
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