Influence of local molecular motions on the determination of 1H-1H internuclear distances measured by 2D 1H spin-exchange experiments

Analysis of spin-exchange build-up curves obtained by measurement of 2D 1H CRAMPS spectra of alpha-glycine was performed to evaluate the rate of 1H-1H spin-exchange process with respect to the influence of variation in internal molecular motion. Differences in local motions significantly affect spin-exchange constants even in highly rigid organic solids with virtually uniform motion behavior. The polarization transfer between nonequivalent alpha-protons is described by the spin-exchange constant D=0.77 nm(2)ms(-1), while the polarization transfer involving spin exchange between alphaH and NH(3)(+) protons is characterized by D=0.24-0.21 nm(2)ms(-1). This significant decrease corresponds to rotation of hydrogen-bonded amino groups. Neglecting this variation in local spin-exchange constants the resulting calculated 1H-1H distance can be overestimated by up to 100%. Complications following from relayed and back polarization transfer involving the nearest spins within one functional group (e.g., CH(2) and/or NH(3)(+)) and intermolecular spin exchange are discussed. It was shown that 2H quadrupolar splitting determined for selected sites directly correlates with the experimentally observed differences in spin-exchange coefficients. It is also demonstrated that a medium level quantum chemical calculation of molecular dynamics provides relevant data that can be used to estimate differences in molecular motions.

Solid-state 13C and 1H spin diffusion NMR analyses of the microfibril structure for bacterial cellulose

To obtain further information about the cause for the rather large splitting of the C4 resonance line into the downfield (C4D) and upfield (C4U) lines in CP/MAS 13C NMR spectra for native cellulose, 13C and 1H spin diffusion measurements have been conducted by using different types of bacterial cellulose samples. In 13C spin diffusion measurements, the C4D resonance line is selectively inverted by the Dante pi pulse sequence and the 13C spin diffusion is allowed to proceed from the C4D carbons to other carbons including the C4U carbons with use of the 13C4-enriched bacterial cellulose sample. The analysis based on the simple spin diffusion theory for the process experimentally observed reveals that the C4U carbons may be located at distances less than about 1 nm from the C4D carbons. In 1H spin diffusion measurements, poly(vinyl alcohol) (PVA) films in which ribbon assemblies of bacterial cellulose are dispersed are employed and the 1H spin diffusion process is examined from the water-swollen PVA continuous phase to the dispersed ribbon assemblies by the 13C detection through the 1H-13C CP technique. As a result, it is found that the C4D and C4U carbons are almost equally subjected to the 1H spin diffusion from the PVA phase, indicating that the C4U carbons are not localized in some limited area, e.g. in the surfacial region, but are distributed in the whole area in the microfibrils. These experimental results suggest that the C4U carbons may exist as structural defects probably due to conformational irregularity associated with disordered hydrogen bonding of the CH(2)OH groups in the microfibrils.

Rapid method for the determination of trace fluoride and activation of ion-selective electrode

A method for activating ISE is proposed that can allow determination of the fluoride concentration at ng mL(-1) level with good precision and accuracy. Fluoride ISE is activated in 0.5 mol L(-1) HClO4 medium and then fluoride is determined in the same medium. The linear range for the determination of fluoride is between 1.00 x 10(-2)-1.00 x 10(-7) mol L(-1), and the detection limit of the method is 1.0 ng mL(-1). The advantage of this method is that it is free from the use of TISAB solution while being, time-saving and labor-saving. A mechanism study of the activation of FISE in HClO4 medium is explained. The method has been used for the determination of trace fluoride in milk and flour with satisfactory results.

NMR magnetization transfer as a tool for characterization of nanoporous materials

The application of nuclear magnetic resonance magnetization transfer experiments to probe the surface-to-volume ratio and pore morphology of porous materials with characteristic pore sizes of 1-100 nm is described. The method is based on the phenomenon of incomplete freezing of liquids in small pores where a few monolayers adjacent to the pore walls remain liquid. Sufficient difference between the transverse relaxation times in the solid frozen core and liquid surface layer allows the initial preparation and subsequent re-equilibration of a solid-liquid magnetization grating. The method is demonstrated using model nanoporous materials with known characteristics. The ensuing problems of the mechanism of the magnetization transfer through the interface and within the frozen core are discussed and elucidated by pulsed-field-gradient NMR experiments.

Self-diffusion of nonfreezing water in porous carbohydrate polymer systems studied with nuclear magnetic resonance

Water is an integral part of the structure in biological porous materials such as wood and starch. A problem often encountered in the preparation of samples for, e.g., electron microscopy is that removal of water leads to a decreasing distance between supermolecular structural elements and a distortion of the structure. It is, therefore, of interest to find methods to investigate these materials in the native water-swollen state. We present a method to study water-swollen biological porous structures using NMR to determine the amount and self-diffusion of water within the porous objects. The contribution of bulk water to the NMR signal is eliminated by performing experiments below the bulk freezing temperature. Further decrease of the temperature leads to a gradual freezing of water within the porous objects. The contribution of the freezing water fraction to the migration of water through the porous network is, thus, estimated. The results are rationalized in terms of the ultrastructure of the samples studied, namely, wood pulp fibers and potato starch granules.

Engineered collagen-PEO nanofibers and fabrics

Type I collagen-PEO fibers and non-woven fiber networks were produced by the electrospinning of a weak acid solution of purified collagen at ambient temperature and pressure. As determined by high-resolution SEM and TEM. fiber morphology was influenced by solution viscosity, conductivity, and flow rate. Uniform fibers with a diameter range of 100-150 nm were produced from a 2-wt% solution of collagen-PEO at a flow rate of 100 microl min(-1). Ultimate tensile strength and elastic modulus of the resulting non-woven fabrics was dependent upon the chosen weight ratio of the collagen-PEO blend. 1H NMR dipolar magnetization transfer analysis suggested that the superior mechanical properties, observed for collagen-PEO blends of weight ratio 1:1, were due to the maximization of intermolecular interactions between the PEO and collagen components. The process outlined herein provides a convenient, non-toxic, non-denaturing approach for the generation collagen-containing nanofibers and non-woven fabrics that have potential application in wound healing, tissue engineering, and as hemostatic agents.

13C CPMAS studies of plant cell wall materials and model systems using proton relaxation-induced spectral editing techniques

The solid state 13C CPMAS NMR spectra of plant cell walls are often complex owing to superposition of resonances from different polysaccharides and the heterogeneity of the cell wall assembly. In this paper, we describe the application of a set of proton relaxation-induced spectral editing (PRISE) experiments which combine 1H relaxation properties (T1, T1rho, T2) with 13C high resolution spectroscopy (CPMAS) to relate the dynamics of the plant cell walls and model systems to their domain structural details. With PRISE it has been found that in plant cell wall materials, cellulose is always associated with the long components of spin-lattice relaxation in both the laboratory and rotating frames whereas non-cellulose polysaccharides (pectin and hemicellulose) are associated with the short ones. For the proton T2 relaxation, cellulose is only associated with the short component (below 20 micros), pectin contributes to both the short component and the long one.

n-Alkyl fluorenyl phases in chromatography. II. Dynamic behavior and high-performance liquid chromatography applications

The dynamic behavior of two n-hexyl fluorenyl phases (fluorene-6A [3a(Tn)(Qm)y], fluorene-6B [3b(Tn)(Qm)y]) and three n-decyl fluorenyl phases (fluorene-10A [4a(Tn)(Qm)y], fluorene-10B [4b(Tn)(Qm)y], and fluorene-10C [4c(M1)(Qm)y]) is investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy using the dipolar filter technique with both 13C and 1H detection. These results are compared with those from other dynamic measurements, like the relaxation times in the rotating frame (T1pH) and the variation of the contact time (T(CH)). Additionally, another type of a fluorenyl phase [5a(Tn)(Qm)y], which has an aromatic moiety connected to the silica gel by amido couplings, was also investigated by the dipolar filter method. The solid-state NMR dynamic measurements indicate an increased mobility of the n-alkyl fluorenyl phases compared to the amido coupled fluorenyl phase. The lower the ligand density of the studied n-alkyl fluorenyl phases, the higher their mobility. The separation behavior of the respective phases in high-performance liquid chromatography was investigated with samples containing polycyclic aromatic hydrocarbons and nitro explosives. Depending on the amount of the chemically bound aromatic moiety and the length of their n-alkyl spacer groups, pi-pi interactions with the solute molecules are involved in the separation process and cause it to proceed at a different rate. Therefore, n-alkyl fluorenyl phases can be classified as mixed-mode phases.

An NMR relaxation study of the state of water in gelatin gels

A combination of 1H and 23Na NMR is used to probe the dynamic state of water in gelatine gels as the water content is lowered from 70% to dryness. A sharp increase in the proton and sodium transverse relaxation rates is observed as the water content falls from 20 to 15% while the proton longitudinal and dipolar cross relaxation rates show a maximum at ca. 15%. We show that these observations can be understood if monolayer coverage occurs at 15% and multilayers of less strongly interacting hydration water are formed between 15 and 20%. Above 20% the water appears to behave as an unperturbed bulk phase.

2D CP/MAS 13C isotropic chemical shift correlation established by 1H spin diffusion

A new 2D solid-state CP/MAS 13C NMR exchange experiment for through-space isotropic chemical shift correlation is proposed and demonstrated. Through-space correlation is established via a second cross polarization from 13C to 1H and subsequent 1H spin diffusion. A third cross polarization results in the final 13C-13C isotropic chemical shift correlation. The 1H spin diffusion time is a variable parameter allowing different mean square magnetization displacements to be probed. Experimental results on mixtures of differently 13C-labeled alanine and polyethylene indicate that this site-selective 2D technique can be used to characterize domain sizes and proximities over a wide range of length scales (1-200 nm) in solids such as polymers or biological materials.

A 129Xe NMR study on an ionomeric polymer blend system

The morphology of an ionomeric polymer blend consisting of an amino-silicone copolymer and zinc neutralized sulfonated polystyrene (ZnSPS) has been studied using proton spin diffusion and small angle X-ray scattering (SAXS). The extent of reaction between the two components in the blend was monitored by 13C CP MAS spectroscopy. All three types of experiment point to domain sizes in the nanometer range. 129Xe NMR was used to study exchange by translational diffusion between domains. A single xenon resonance was detected in temperatures ranging from 25 degrees C to -90 degrees C, and the chemical shift followed a weighted average of the isolated polymer shifts consistent with the small domain sizes. Pulse field gradient 129Xe NMR was used to determine the effective diffusion constants in the amino silicone starting material and the blend. The diffusion constant of xenon in poly(styrene) is known, allowing for comparison of the predictions of effective diffusion constants in the blend based on the values in the constituents of the blend. Simple two-site exchange equations incorrectly predict that diffusion in the blend would be dominated by the constituent with slow diffusion. The blend diffusion constant is close to the value of the amino silicone or the constituent with fast diffusion which is correctly predicted for a rapid exchange solution of the diffusion equation.