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Wild-type bacteriophage T4 lysozyme contains a hydrophobic cavity with binding properties that have been extensively studied by X-ray crystallography and NMR. In the present study, the monitoring of ¹H chemical shift variations under xenon pressure enables the determination of the noble gas binding constant (K = 60.2 M1-). Although the interaction site is highly localized, dipolar cross-relaxation effects between laser-polarized xenon and nearby protons (SPINOE) are rather poor. This is explained by the high value of the xenon-proton dipolar correlation time (0.8 ns), much longer than the previously reported values for xenon in medium-size proteins. This indicates that xenon is highly localized within the protein cavity, as confirmed by the large chemical shift difference between free and bound xenon. The exploitation of the xenon line width variation vs xenon pressure and protein concentration allows the extraction of the exchange correlation time between free and bound xenon. Comparison to the exchange experienced by protein protons indicates that the exchange between the open and closed conformations of T4 lysozyme is not required for xenon binding. [ABSTRACT FROM AUTHOR]

The hydrophobic cavity of Lipid Transfer Protein 1 from Nicotiana tabacum is investigated in detail by NMR using xenon as a spy. The analysis of the 129Xe chemical shifts and self-relaxation times gives evidence of protein-xenon interaction. Thermodynamics of the binding is characterized through the study of aliphatic ¹H and 13C chemical shift variation as a function of xenon pressure. The binding constant is evaluated to 75.5 ± 1.0 M-1 at 293 K. The location of xenon inside the cavity is deduced from SPINOE experiments. The noble gas appears to occupy four sites, and xenon self-relaxation experiments indicate that it quickly jumps between different sites. The chemical shifts of amide protons and nitrogens also depend on the xenon concentration, either specifically or nonspecifically for atoms at the external surface of the protein. Yet, contrary to aliphatic atoms, they do not correspond to short-range interactions as confirmed by magnetization transfer experiments between laser-polarized xenon and protons in H2O. These 15N chemical shift variations, used in combination with 15N transverse self-retaxation rates to determine the lower limit of the binding rate, consequently reveal subtle changes in the structure of the protein upon binding. [ABSTRACT FROM AUTHOR]

Cryptophanes bearing OCH2COOH groups in place of the methoxy groups represent a new class of xenon-carrier molecules soluble in water at biological pH. By using ¹H and 129Xe NMR (thermally- and laser-polarized dissolved gas), the structural and dynamical behaviors of these host molecules as well as their interaction with xenon are studied. They are shown to exist in aqueous solution under different conformations in very slow exchange. A saddle form present for one of these conformations could explain the ¹H NMR spectra. Whereas the cryptophanes in such a conformation are unable to complex xenon, unprecedented high binding constants are found for cryptophanes in the other canonical crown-crown conformation. These host molecules could therefore be valuable candidates for biosensing using 129Xe MRI. [ABSTRACT FROM AUTHOR]

Stable complexes of pentavalent uranyl [UO2(salan-tBu2)(py)K)n(3), [UO2(salan-tBu2)(py)K(18C6)] (4), and [UO2(salophen-tBu2)(thf)]K(thf)2)n (8) have been synthesized from the reaction of the complex ([UO2py5][Kl2py2])n (1) with the bulky amine-phenolate ligand potassium salt K2(salan-tBu2) or the Schiff base ligand potassium salt K2(salophen-tBu2) in pyridine. They were characterized by NMR, Rl, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [UVIO2(salan-tBu2)(py)][Cp*2Co], 6, were also isolated from the reduction of [UIVO2(salan-tBu2)(py)], 5, with Cp*2Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO2py5]l.py (2). The measured redox potentials E1/2 (UVI/UV) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give μeff of 2.20 and 2.23 μB at 300 K respectively, which is compared with a μeff of 2.6(1) μB (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (gx = 1.98; gy = 1.25; gz = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the πu. The presence of a back-bonding interaction is likely to play. a role in the observed solution stability of the [UO2(salan-tBu2)(py)K] and [UO2(salophen-tBu2)(py)K) complexes with respect to disproportionation and hydrolysis. [ABSTRACT FROM AUTHOR]