Salt-induced micellization of a triblock copolymer in aqueous solution: a 1H nuclear magnetic resonance spectroscopy study

An aqueous micellar solution of a PEO-PPO-PEO triblock copolymer, pluronic F88 (EO103PO39EO103), in the presence of salt (KCl) has been investigated by 1H NMR spectroscopy. The hydrogen-bonding structure in water is directly changed by the strong polarization effect of added salt, which indirectly weakens the interaction of polymer molecules with water. Both EO and PO blocks are dehydrated by the addition of salt in a similar way, whereas the solubility of the PO blocks may be affected in a more pronounced way, which results in the decrease of the critical micellization temperature (CMT). It is found that the addition of salt favors a more compact micellar core, where the water content is decreased and an effective PO-PO interaction is increased. Increasing the salt concentration would result in a decrease in the number of gauche conformers in the PPO chain, which may be the deeper reason for the decreasing solubility of PPO segments in aqueous salt solution. The temperature region over which the micellization occurs is broad, indicating that micelles and unimers coexist over an extended temperature range, whereas this transition region is significantly narrowed by the addition of salt. The addition of salt offers a good substitute way of changing the temperature to induce micellization. The critical micellization salt concentration (CMSC) is determined to be 1.0 mol l-1 for KCl in 2.5% aqueous pluronic F88 solution at 25 degrees C, and the transition region in which both free and associated copolymer molecules coexist is defined to range from 1 to 2 mol L-1.     

Accurate size determination of PS latex nanoparticles in aqueous solution using pulsed field gradient nuclear magnetic resonance spectroscopy

The accurate size determination of nanoparticles in solution is an important subject in nano/bio-technologies. However, interactions between particles induce a significant misinterpretation of the size determination by PFG-NMR method. In the sodium dodecyl sulfate (SDS) aqueous solution, we observed the significant change of the apparent size of PS-latex that depended on the concentration of SDS. In this Letter, accurate size determination of PS latex was carried out by extrapolation methods varying both the concentrations of the PS latex and the SDS in aqueous solution. The proposed method makes it possible to compare the determined sizes of nanoparticles in the liquid-phase by PFG-NMR to those by differential mobility analyzer (DMA) in the gas-phase.     

Covariance nuclear magnetic resonance spectroscopy

Covariance nuclear magnetic resonance (NMR) spectroscopy is introduced, which is a new scheme for establishing nuclear spin correlations from NMR experiments. In this method correlated spin dynamics is directly displayed in terms of a covariance matrix of a series of one-dimensional (1D) spectra. In contrast to two-dimensional (2D) Fourier transform NMR, in a covariance spectrum the spectral resolution along the indirect dimension is determined by the favorable spectral resolution obtainable along the detection dimension, thereby reducing the time-consuming sampling requirement along the indirect dimension. The covariance method neither involves a second Fourier transformation nor does it require separate phase correction or apodization along the indirect dimension. The new scheme is demonstrated for cross-relaxation (NOESY) and J-coupling based magnetization transfer (TOCSY) experiments.     

Shiftless nuclear magnetic resonance spectroscopy

The acquisition and analysis of high resolution one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectra without chemical shift frequencies are described. Many variations of shiftless NMR spectroscopy are feasible. A two-dimensional experiment that correlates the dipole-dipole and dipole-dipole couplings in the model peptide , (15)N labeled N-acetyl-leucine is demonstrated. In addition to the resolution of resonances from individual sites in a single crystal sample, the bond lengths and angles are characterized by the two-dimensional powder pattern obtained from a polycrystalline sample.     

Theory of covariance nuclear magnetic resonance spectroscopy

Covariance nuclear magnetic resonance (NMR) spectroscopy provides an effective way for establishing nuclear spin connectivities in molecular systems. The method, which identifies correlated spin dynamics in terms of covariances between 1D spectra, benefits from a high spectral resolution along the indirect dimension without requiring apodization and Fourier transformation along this dimension. The theoretical treatment of covariance NMR spectroscopy is given for NOESY and TOCSY experiments. It is shown that for a large class of 2D NMR experiments the covariance spectrum and the 2D Fourier transform spectrum can be related to each other by means of Parseval's theorem. A general procedure is presented for the construction of a symmetric spectrum with improved resolution along the indirect frequency domain as compared to the 2D FT spectrum.     

Analysis of glycans in glycoproteins by diffusion-ordered nuclear magnetic resonance spectroscopy

Enzymatically cleaved glycans from sub-milligram quantities of erythropoietin (EPO) and ovalbumin have been analyzed, without further purification, by two-dimensional diffusion-ordered nuclear magnetic resonance spectroscopy. At NMR sample concentrations below 50 μmol L⁻¹ the major components of the oligosaccharide fractions could be distinguished by their anomeric proton chemical shift and their size-dependent diffusion coefficients. Figure ¹H NMR diffusion decay curves of anomeric protons in the EPO glycan fraction     

Analysis of biological fluids by high-field nuclear magnetic resonance spectroscopy

The application of high-field Fourier transform nuclear magnetic resonance (NMR) spectroscopy to the analysis of biological fluids such as urine, plasma and bile is described. Applications include areas such as clinical chemistry, experimental and clinical toxicology and drug metabolism studies. In the case of proton NMR some means of attenuating or eliminating the interference due to water protons is required and suitable strategies for achieving this are discussed. The use of 2-dimensional NMR or solid-phase extraction/chromatography to enable the identification of unknowns is discussed and the potential usefulness of ¹⁹F NMR for studying the metabolism of fluorinated xenobiotics is highlighted.     

Computer-aided spectral assignment in nuclear magnetic resonance spectroscopy

The proton-carbon correlation spectra, HMBC (heteronuclear multiple bond correlation) and HMQC (heteronuclear multiple quantum correlation), respectively, provide direct and remote connectivity information with high sensitivity. Their combination enables carbon-carbon proximity relationships to be deduced, which are formally identical to those produced by a fictitious INADEQUATE-2D experiment, where correlations would be established exclusively between atoms linked by one or two bonds. The CASA program uses these relationships, as well as DEPT spectra and elementary chemical-shift considerations to assign the ¹³C spectrum of a compound if its structure is known or assumed. If the structure conflicts with the experimental data, no assignment is produced. The CASA program serves as an aid to either spectral assignment or structural elucidation.     

Ultrafast-based projection-reconstruction three-dimensional nuclear magnetic resonance spectroscopy

Recent years have witnessed increased efforts toward the accelerated acquisition of multidimensional nuclear magnetic resonance (nD NMR) spectra. Among the methods proposed to speed up these NMR experiments is "projection reconstruction," a scheme based on the acquisition of a reduced number of two-dimensional (2D) NMR data sets constituting cross sections of the nD time domain being sought. Another proposition involves "ultrafast" spectroscopy, capable of completing nD NMR acquisitions within a single scan. Potential limitations of these approaches include the need for a relatively slow 2D-type serial data collection procedure in the former case, and a need for at least n high-performance, linearly independent gradients and a sufficiently high sensitivity in the latter. The present study introduces a new scheme that comes to address these limitations, by combining the basic features of the projection reconstruction and the ultrafast approaches into a single, unified nD NMR experiment. In the resulting method each member within the series of 2D cross sections required by projection reconstruction to deliver the nD NMR spectrum being sought, is acquired within a single scan with the aid of the 2D ultrafast protocol. Full nD NMR spectra can thus become available by backprojecting a small number of 2D sets, collected using a minimum number of scans. Principles, opportunities, and limitations of the resulting approach, together with demonstrations of its practical advantages, are here discussed and illustrated with a series of three-dimensional homo- and heteronuclear NMR correlation experiments.     

Assignment of juvabione diastereomers by 13C nuclear magnetic resonance spectroscopy

The determination of structures and partial assignments of stereochemistry of juvabione and some of its analogues can be made on the basis of ¹³C nuclear magnetic resonance studies. The complete ¹³C n.m.r. spectral assignments for juvabione and five analogues are reported.     

Molecular signal suppression by in situ microextraction in nuclear magnetic resonance spectroscopy

The detailed characterization of complex mixtures by NMR is often hampered by the presence of signals from uninformative compounds, the resonances of which overlap with those of the molecules of interest. We provide here a proof of principle for an approach to NMR signal suppression in complex samples using Molecularly Imprinted Polymers (MIPS). Addition of a few milligrams of polymer to a solution traps the target molecule in typical micromolar to millimolar concentration, thus achieving in situ signal suppression, without altering any other spectral features. This method minimized any manipulation or perturbation of the spectrum and was applied to a complex mixture of known compounds and to a plant extract, in both cases spiked with a compound (bisphenol A), which was subsequently removed by selective binding to a complementary MIP. What is described in this report is comparable with microextraction and may in due course be applied to a large number of analytical challenges. Copyright © 2014 John Wiley & Sons, Ltd.