Site-resolved (2)H relaxation experiments in solid materials by global line-shape analysis of MAS NMR spectra.

We investigate a way one can achieve good spectral resolution in (2)H MAS NMR experiments. The goal is to be able to distinguish between and study sites in various deuterated materials with small chemical shift dispersion. We show that the (2)H MAS NMR spectra recorded during a spin-relaxation experiment are amenable to spectral decomposition because of the different evolution of spectral components during the relaxation delay. We verify that the results are robust by global least-square fitting of the spectral series both under the assumption of specific line shapes and without such assumptions (COmponent-REsolved spectroscopy, CORE). In addition, we investigate the reliability of the developed protocol by analyzing spectra simulated with different combinations of spectral parameters. The performance is demonstrated in a model material of deuterated poly(methacrylic acid) that contains two (2)H spin populations with similar chemical shifts but different quadrupole splittings. In (2)H-exchanged cellulose containing two (2)H spin populations with very similar chemical shifts and quadrupole splittings, the method provides new site-selective information about the molecular dynamics.

Nuclear magnetic resonance and small-angle neutron scattering studies of anionic surfactants with macrocounterions: tetramethylammonium dodecyl sulfate.

Micellar solutions of tetramethylammonium dodecyl sulfate have been studied to determine the degree of counterion binding. Tetramethylammonium chloride was added over a wide range of surfactant concentrations such that the total concentration of tetramethylammonium ions in solution remained constant. Small angle neutron scattering experiments showed a constancy in aggregation number across this series, consistent with the constant C(aq) concept of Bales et al. (J. Phys. Chem. B 2001, 105, 6798). Pulsed-field gradient and electrophoretic NMR experiments were used to determine the degree of counterion dissociation, alpha, which was found to be 0.33. This value is in contrast to the value from conductivity measurements (alpha = 0.2), but supports the concept of an aggregation number based definition of alpha.

Macroscopic background gradient and radiation damping effects on high-field PGSE NMR diffusion measurements.

The effects of macroscopic background gradients due to susceptibility differences at the sample interfaces and of radiation damping on pulsed-gradient spin-echo (PGSE) experiments are examined. Both phenomena can lead to the seemingly strange effect of the echo signal growing as the gradient strength increases at low applied gradient strengths. For a freely diffusing species, background gradients manifest themselves as slight concave or convex inflections in the linearized PGSE attenuation curve, depending on the polarity of the applied gradient. The various means of overcoming macroscopic background gradient problems, including bipolar gradients, and their efficacy are examined experimentally and discussed. The effects of radiation damping can also result in the attenuation curve being nonlinear but, different from the effect of background gradients, the nonlinearity does not change with the polarity of the applied gradient. The vulnerability of the stimulated echo-based PGSE sequence and variations of Hahn-based PGSE sequences is investigated. Both background gradients and radiation damping have serious implications for accurate diffusion measurement determination.

Component separation in NMR imaging and multidimensional spectroscopy through global least-squares analysis, based on prior knowledge.

Use of prior knowledge with regard to the number of components in an image or NMR data set makes possible a full analysis and separation of correlated sets of such data. It is demonstrated that a diffusional NMR microscopy image set can readily be separated into its components, with the extra benefit of a global least-squares fit over the whole image of the respective diffusional rates. As outlined, the computational approach (CORE processing) is also applicable to various multidimensional NMR data sets and is suggested as a potentially powerful tool in functional MRI.

NMR "diffusion-diffraction" of water revealing alignment of erythrocytes in a magnetic field and their dimensions and membrane transport characteristics.

"Diffusion-diffraction" experiments on water, yielding "q-space" plots, were conducted on suspensions of oxygenated (diamagnetic) human erythrocytes. (i) These suspensions displayed diffusion-diffraction of water; (ii) the shape of the q-space plots depended on the direction along which the diffusion was measured, thus implying alignment of the cells in the magnetic field of the NMR spectrometer; (iii) the diffusion anisotropy was altered in a predictable way by converting the hemoglobin to a paramagnetic form; (iv) the shapes of the q-space plots were altered in a predictable way by inhibiting water transport; (v) the pseudo-first order rate constant characterizing the covalent inhibition of water transport, by p-chloromercuribenzenesulfonate (p-CMBS), was measured; and (vi) the cell diameter and intercellular spacing were measured from the positions of the interference minima and maxima in the q-space plots. The relevance of these findings to NMR-based histological characterization of tissues, and the implications, for magnetic resonance imaging (MRI), of erythrocyte alignment in the small vessels of the brain in particular, are noted.

Localized interaction of the polyamine methylspermidine with double-helical DNA as monitored by 1H NMR self-diffusion measurements.

The 1H NMR pulsed field gradient self-diffusion method has been used to measure the diffusion coefficient of the polyamine analogue methylspermidine (completely N-methylated spermidine) in DNA solution, as a function of the concentration ratio of methylspermidine to DNA phosphate. Three different DNA's have been investigated: d(GC)4 (8 base pairs), core length calf thymus DNA (approximately 120 base pairs), and sonicated high molecular weight calf thymus DNA (average 7500 base pairs). For a constant ratio of methylspermidine to DNA phosphate, the diffusion coefficient decreases with increasing DNA length. Moreover, at low concentration ratios the diffusion coefficient of methylspermidine approaches a limiting value that is close to that of the DNA molecule. The experimental data are well reproduced by a two-state diffusion model. In this model the diffusion coefficient of the polyamine is a population-weighted average of polyamine associated with DNA (with a diffusion coefficient given by that of the DNA molecule) and polyamine free in solution.

1H, 7Li and 133Cs multicomponent self-diffusion NMR study on ion binding of Li+ and Cs+ to nucleotides and aggregation of nucleotides in aqueous solution.

The Fourier transform NMR pulsed-gradient spin-echo self-diffusion technique was used for studies of nucleotides (AMP, CMP, GMP and UMP) with Li+ or Cs+ added, in 2H2O. 1H-, 7Li- and 133Cs-NMR-based self-diffusion data on the constituents provide a picture of both the degree of ion binding to nucleotides and the self-association of nucleotides in aqueous solution. Self-diffusion coefficients were investigated in a concentration range up to 0.3 molal nucleotide in 2H2O, while keeping the metal ion concentration of Li+ or Cs+ at twice the nucleotide concentration throughout the investigations. The self-association studies reveal that the aggregation constants of the Li salts differ only slightly from the corresponding constants for the disodium salts of the mononucleotides. Within a two-site bound-free model for the counterions and a cooperative indefinite aggregation model for the nucleotides one finds that the degree of ion binding for all these nucleotide systems remains approximately constant, in spite of increasing aggregate concentration. This corresponds to the well-known polyelectrolyte ion condensation behaviour, indicating that large aggregates are formed, supporting previous findings by the present authors on the aggregation behaviour of nucleotides. An observed large effect on the 17O relaxation of water in nucleotide systems can only be reconciled with the presence of relatively large aggregates in solution.

A multicomponent self-diffusion NMR study of aggregation of nucleotides, nucleosides, nucleic acid bases and some derivatives in aqueous solution with divalent metal ions added.

The self-aggregation of the mononucleotides AMP, CMP, and UMP with Mg2+ added (nucleotide concentration = Mg2+ concentration) up to 0.4 molal or to their solubility limit in 2H2O has been monitored through self-diffusion measurements, using the Fourier transform NMR pulsed-gradient spin-echo multicomponent-self-diffusion technique. Also, purine, cytidine, uridine, purine with Mg2+ added and both cytidine and uridine with Mg2+, Zn2+ or Cd2+ added, were studied in the same way. The experimental data were fitted to two different aggregation models. For the mononucleotides with Mg2+ added a cooperative indefinite aggregation model, where the first (dimerization) aggregation constant is a magnitude lower than those for the higher aggregation step gives the best agreement between simulations and experiment. Typical values are 0.3 and 12 kg mol(-1), respectively. The latter value is about twice that found for the uncomplexed nucleotides. Also, purine and the nucleosides, cytidine and uridine, with divalent metal ions added fit best with this model. The degree of aggregation is increased upon metal ion addition, as previously shown for the mononucleotides. For purine, cytidine and uridine without metal ions added an 'isodesmic', indefinite aggregation model, with the aggregation constant for each step equal, fits the data as well. Here the application of the 'semi-isodesmic' model results in a higher first (dimerization) aggregation constant than is found for the nucleotides. The typical value is 2 kg mol(-1). In this case, the evaluated aggregation constants for the higher step become only about twice as large as those of the first step. The same measurements on isopropylcytidine, isopropyluridine and theophylline-7-acetic acid in water show that these three compounds aggregate to the same extent as the nucleosides, cytidine and uridine. Pyrimidine diffusion data reveal no aggregation at all; the application of either model results in essentially zero aggregation constants.

Nucleotide aggregation in aqueous solution. A multicomponent self-diffusion study.

The self-aggregation of the mononucleotides (AMP, CMP, GMP and UMP) and caffeine up to their solubility limit in 2H2O has been monitored through self-diffusion measurements, using the Fourier transform NMR pulsed-gradient spin-echo self-diffusion technique. The data were iteratively fitted to a number of aggregation models. It was concluded that the best agreement between simulations and experiment for the mononucleotides was obtained for a 'semi-isodesmic', indefinite aggregation model (also known as a Type III SEK or cooperative indefinite self-association model), where the first (dimerization) aggregation constant is a magnitude lower than those for the higher aggregation steps. Typical values were 0.4 and 6 l mol-1, respectively. Under these conditions, the main fraction of solute is monomeric throughout the concentration range and the distribution of higher oligomers is very broad. Caffeine self-aggregation is clearly different and is consistent with several aggregation models. The mixed aggregation of caffeine (at a low total concentration) and the mononucleotides was successfully monitored in an extension of the basic study. It was found that caffeine binding to mononucleotide aggregates increases in the series UMP, CMP, GMP and AMP.