Brownian dynamics of water confined in AOT reverse micelles: A field-cycling deuteron NMR relaxometry study

Reversed micelles and water in oil micro-emulsions can be used to solubilize biopolymers and genetic materials allowing analyzing their properties in a confined geometry. Nuclear Magnetic Resonance Dispersion (NMRD) provides a powerful and a noninvasive experimental technique to probe the long-term dynamics of these confined systems. However, the first step is to analyze and understand the slow dynamics of water inside these micro-reactors without any guest molecule. This is the aim of this presentation. Experimental results have been obtained for deuteron ²H NMRD of water confined in reverse micelles of bis (2-ethylhexyl) sodium sulfosuccinate (AOT) dispersed in isooctane C₈H₁₈. The water content is expressed as the molar ratio W₀ = [Water]/[AOT]. The radius of the spherical reversed micelles, R_m, increases almost linearly with W₀. In our case, W₀ is chosen in the range 20 ≤ W₀ ≤ 50 (35 ≤ R_m ≤ 80 Å). The frequency dependence for the spin-lattice relaxation rate R₁(ω) exhibits two regimes, for all W₀ values: a plateau at low frequency, proportional to 1/R_m, followed by the beginning of an algebraic decay. These experimental observations are discussed and compared to a numerical simulation of the intermittent Brownian diffusion of a water molecule inside a rotating reverse micelle. The possibility to probe some properties of the confinement, such as the localisation time on the sulfonated palisade and/or the water self-diffusion inside the water pool is emphasised.     

Total synthesis of 3,3-difluorinated 1-deoxynojirimycin analogues

Difluorination of 1-deoxynojirimycin at position C(3) creates a competitive inhibitor 15 of 10 times higher activity against an α-glucosidase than the parent compound. Its screening against a panel of human cell lines showed a low cytotoxicity therefore making this compound an interesting candidate for further clinical investigations.     

Probing microstructure evolution during the hardening of gypsum by proton NMR relaxometry

We report a comprehensive proton NMR relaxation study of the water confined in the evolving porous structure of hardened gypsum prepared with different water-to-plaster ratios (w/p) and increasing additions of crushed gypsum. This study gives some new information on the microstructure, the water distribution, and the hydration kinetics without any drying or perturbing preparation. The bi-exponential transverse magnetization decay reveals the existence of two water populations in slow exchange. However, the different behaviors of these populations during saturation and desaturation experiments show evidence of a fast exchange of each population with the surface. Two modes of organization of the microstructure of this material are identified through an original model of exchange as a function of the water-to-plaster ratio (0.4 < or = w/p < or = 0.6 and 0.7 < or = w/p < or = 1). A clear gap is shown in the exchange rate value above w/p = 0.6 that could be representative of a percolation threshold. Both the method and the theory presented can be applied more widely to other porous media with reactive surface areas.     

Multi-scale approach continuously relating the microstructure and the macroscopic mechanical properties of plaster pastes during their settings

A new multi-scale experimental approach is proposed to continuously relate the microstructure and the macroscopic mechanical properties of plaster pastes during their settings. (1)H NMR relaxometry is used to follow continuously and not destructively, the degree of hydration and the microstructure evolution during the setting and hardening of plaster paste. Transmission of shear and compressional ultrasonic velocities enable the determination of macroscopic mechanical properties of the material during the setting. On the basis of similar behaviors of Young's modulus and NMR-population of confined water as function of the degree of hydration, we conclude that NMR gives a better understanding of the evolution of the microstructure at the origin of a better control of the macroscopic mechanical properties.     

Anomalous surface diffusion of water compared to aprotic liquids in nanopores

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.     

Modulation of lysozyme charge influences interaction with phospholipid vesicles

Lysozyme is a globular protein which is known to bind to negatively charged phospholipid vesicles. In order to study the relationship between charge state of the protein and its interaction with negatively charged phospholipid membranes chemical modifications of the proteins were carried out. Succinylation and carbodiimide modification was used to shift the isoelectric point of lysozyme to lower and higher pH values, respectively. The binding of the modified lysozyme to phospholipid vesicles prepared from phosphatidic acid (PA) was determined using microelectrophoresis and ultracentrifugation. At acidic pH of the solution all lysozyme species reduced the surface charges of PA vesicles. Succinylated lysozyme (succ lysozyme) reduced the electrophoretic mobility (EPM) to nearly zero, whereas native lysozyme and carboxylated lysozyme (carbo lysozyme) changed the surface charge to positive values. At neutral pH, the reduction of surface charges was less for carbo lysozyme and unmodified lysozyme. Succ lysozyme did not change the EPM. Unmodified and carbo lysozyme decreased the magnitude of EPM, but the whole complex was still negatively charged. The bound fraction of all modified lysozyme to PA vesicles at high lysozyme/PA ratios was nearly constant at acidic pH. At low lysozyme/PA ratios the extent of bound lysozyme is changed in the order carbo>unmodified>succ lysozyme. Increasing the pH, the extent of bound lysozyme to PA large unilamellar vesicles (LUV) is reduced, at pH 9.0 only 35% of carbo lysozyme, 23% of unmodified lysozyme is bound, whereas succ lysozyme does not bind at pH 7.4 and 9.0. At low pH, addition of all lysozyme species resulted in a massive aggregation of PA liposomes, at neutral pH aggregation occurs at much higher lysozyme/PA ratios. Lysozyme binding to PA vesicles is accompanied by the penetration of lysozyme into the phospholipid membrane as measured by monolayer techniques. The penetration of lysozyme into the monolayer was modulated by pH and ionic strengths. The interaction of lysozyme with negatively charged vesicles leads to a decrease of the phospholipid vesicle surface hydration as measured by the shift of the maximum of the fluorescence signal of a headgroup labeled phospholipid. The binding of bis-ANS as an additional indicator for the change of surface hydrophobicity is increased at low pH after addition of lysozyme to the vesicles. More hydrophobic patches of the lysozyme-PA complex are exposed at low pH. At low pH the binding process of lysozyme to PA vesicles is followed by an extensive intermixing of phospholipids between the aggregated vesicles, accompanied by a massive leakage of the vesicle aqueous content. The extent of lysozyme interaction with PA LUV at neutral and acidic pH is in the order carbo lysozyme>lysozyme>succ lysozyme.     

Probing the surface area of a cement-based material by nuclear magnetic relaxation dispersion

We show how nuclear magnetic spin-lattice relaxation dispersion of 1H water can provide a direct reliable value of the specific surface area of a cement-based material. The remarkable features of the relaxation dispersion support an interpretation in terms of coupled solid-liquid relaxation at pore interfaces, surface diffusion, and nuclear paramagnetic relaxation. The measurement is sufficiently fast to be applied continuously during the progressive hydration and setting of the material. This method is relevant to other chemically reactive porous media in chemical engineering and oil recovery.     

Water-proton-spin-lattice-relaxation dispersion of paramagnetic protein solutions

The paramagnetic contributions to water-proton-spin-lattice relaxation rate constants in protein systems spin-labeled with nitroxide radicals were re-examined. As noted by others, the strength of the dipolar coupling between water protons and the protein-bound nitroxide radical often appears to be larger than physically reasonable when the relaxation is assumed to be controlled by 3-dimensional diffusive processes in the vicinity of the spin label. We examine the effects of the surface in biasing the diffusive exploration of the radical region and derive a relaxation model that incorporates 2-dimensional dynamics at the interfacial layer. However, we find that the local 2-dimensional dynamics changes the shape of the relaxation dispersion profile but does not necessarily reproduce the low-field relaxation efficiency found by experiment. We examine the contributions of long-range dipolar couplings between the paramagnetic center and protein-bound-water molecules and find that the contributions from these several long range couplings may be competitive with translational contributions because the correlation time for global rotation of the protein is approximately 1000 times longer than that for the diffusive motion of water at the interfacial region. As a result the electron-proton dipolar coupling to rare protein-bound-water-molecule protons may be significant for protein systems that accommodate long-lived-water molecules. Although the estimate of local diffusion coefficients is not seriously compromised because it derives from the Larmor frequency dependence, these several contributions confound efforts to fit relaxation data quantitatively with unique models.     

The magnetic field and temperature dependences of proton spin-lattice relaxation in proteins

The nuclear magnetic relaxation dispersion profiles of lyophilized globular proteins were measured in the frequency range of 10 kHz-30 MHz at temperatures from 156 to 302 K. The existent theory of proton relaxation in immobilized protein systems was critically tested and expended to include contributions of rapid motions of protein side-chain groups. The new theory takes into account the strong coupling between the side-chain protons and the protein backbone, when correlation function cannot be written as a product of the contributions. The measurements showed that while the relaxation rate constant of the protein backbone protons is a linear function of the absolute temperature the side-chain groups exhibit an exponential temperature dependence corresponding to an activated process. Measurements carried out on simple homopolypeptides, polyglycine and polyalanine, provide strong support of the proposed new theory.     

4,5‐Dihydro‐1,2,3‐oxadiazole: A Very Elusive Key Intermediate in Various Important Chemical Transformations

4,5‐Dihydro‐1,2,3‐oxadiazoles are postulated to be key intermediates in the industrial synthesis of ketones from alkenes, in the alkylation of DNA in vivo, and in the decomposition of N‐nitrosoureas; they are also a subject of great interest for theoretical chemists. In the presented report, the formation of 4,5‐dihydro‐1,2,3‐oxadiazole and the subsequent decay into secondary products have been studied by NMR monitoring analysis. The elusive properties evading characterization have now been confirmed by ¹H, ¹³C, and ¹⁵N NMR spectroscopy, and relevant 2D experiments at very low temperatures. Our experiments with suitably substituted N‐nitrosoureas using thallium(I) alkoxides as bases under apolar conditions answer important questions on the existence and the secondary products of 4,5‐dihydro‐1,2,3‐oxadiazole.