Conformations and 13C NMR chemical shifts of some polyamides in the solid state as studied by high-resolution 13C NMR spectroscopy

High-resolution ¹³Carbon nuclear magnetic resonance (NMR) spectra of Nylons 4, 6, and 66 in the solid state were measured over a wide range of temperature. From the results, it was found that resonance lines of crystalline and noncrystalline components were separable and their chemical shifts were determined. The ¹³C chemical shift behavior is closely related to their conformation. The origin of the conformational effects on the chemical shifts is discussed.     

Assignment of the absolute configuration of hydroxy- and aminophosphonates by NMR spectroscopy

Phosphonate analogues of amino- and hydroxy acids have received considerable attention in bioorganic and medicinal chemistry due to their unique activities as peptidomimetics, being known as inhibitors of such enzymes as human renin, HIV protease and polymerase, leucine aminopeptidase and serine proteases. They have also been exploited as haptens for catalytic antibody research, herbicides, antibiotics, antiviral and anticancer agents and neuromodulators. Therefore, the demand for the asymmetric synthesis of hydroxy- and aminophosphonates should be accompanied by reliable methods for their absolute configuration assignment. NMR spectroscopy is one of the most commonly used techniques for the assignment of absolute configuration of different classes of compounds. This report describes the principles and practical aspects of applying chiral discriminating agents for the assignment of absolute configuration of 1- and 2-hydroxyphosphonates and 1- and 2-aminophosphonates by NMR spectroscopy. The report is organized in sections discussing the types of the chiral discriminating agents (including the models used for configuration assignment, if this was proposed) and the scope of their applications (with the list of all the examples of hydroxy- and aminophosphonates examined by this method). The application of the chiral derivatizing agents (CDA) and chiral solvating agents (CSA) used for these purposes, such as α-methoxy-α-(trifluoromethyl)phenylacetic acid (MTPA), α-methoxyphenylacetic acid (MPA), amino acids, diazaphospholidine, camphanic acid, naproxen, quinine and t-butylphenylphosphinothioic acid is discussed. Easy access to the selected values of the NMR chemical shifts observed for the diastereomeric species of the tested hydroxy- and aminophosphonates examined, will enable the reader to compare trends observed in spectra and subsequent absolute configuration assignment. In addition, any available complementary data confirming the configuration established by NMR (X-ray, chemical correlations, optical rotation) is also provided.     

Quantitative monitoring of solid phase organic reactions by high-resolution magic angle spinning NMR spectroscopy

Three possible high-resolution magic angle spinning (HR MAS) NMR experiments to quantitatively monitor a solid phase supported Horner-Emmons reaction are presented. In the first experiment we follow the solid phase reaction in deuterated solvent directly in the NMR rotor. The second quantification is done by reconditioning of a few milligrams of resin from an undefined reaction vessel by washing, drying, and reswelling in deuterated solvent, and the evaluation of the amount of resin bound structures by comparing to an external standard. The third experiment represents the first analytical quantification of resin-bound structures without any sample preparation, except the transfer of resin-solvent suspension (large excess of reagents in protonated dimethylformamide) from the reaction vessel to the NMR rotor.     

Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.     

Study of the behaviour of amino acids in aqueous solution by time-domain NMR and high-resolution NMR

The study of protein hydration by time-domain NMR is complicated by the great number of interactions involved, resulting from the presence of several amino acids and the possible modifications produced by the various structures. Moreover, a good comprehension of the molecular interactions of the simple amino acids in solution is essential to elucidate the mechanism of the biological functions of proteins. Measurements of transverse relaxation rates of the protons of water (R(2) = 1/T(2)) in aqueous solutions of amino acids such as L-glycine, L-asparagine, L-arginine and L-tryptophan were carried out in order to study the effects of chemical exchange and molecular diffusion on the amplitude of R(2). The values of R(2) measured by the Carr-Purcell-Meiboom-Gill (CPMG) sequence were studied while varying the solution pH and the parameters of the CPMG sequence. The dependence of R(2) on pH and tau (inter-pulse delay between the first and the second pulses of the CPMG sequence) is interpreted in terms of chemical exchange between the protons of water and those of the labile amino acid groups. This interpretation was confirmed by the analysis of the proton spectra acquired using a 300 MHz NMR spectrometer.     

Investigation of the molecular dynamics of some phenols and their acetyl isomers in solutions by NMR relaxation

The dynamics of relaxations and Fries rearrangements in phenol acetates and their acetyl isomers was studied by the NMR relaxation technique in acetone-d ₆. The results of ¹³C and ¹H spin-lattice nuclear relaxation measurements show that these experiments can be used for determining the mobility and activation energies of the molecular motions of compounds in different systems.     

Characterization of protein-ligand interactions by high-resolution solid-state NMR spectroscopy

A novel approach for detection of ligand binding to a protein in solid samples is described. Hydrated precipitates of the anti-apoptotic protein Bcl-xL show well-resolved (13)C-(13)C 2D solid-state NMR spectra that allow site-specific assignment of resonances for many residues in uniformly (13)C-enriched samples. Binding of a small peptide or drug-like organic molecule leads to changes in the chemical shift of resonances from multiple residues in the protein that can be monitored to characterize binding. Differential chemical shifts can be used to distinguish between direct protein-ligand contacts and small conformational changes of the protein induced by ligand binding. The agreement with prior solution-state NMR results indicates that the binding pocket in solid and liquid samples is similar for this protein. Advantages of different labeling schemes involving selective (13)C enrichment of methyl groups of Ala, Val, Leu, and Ile (Cdelta1) for characterizing protein-ligand interactions are also discussed. It is demonstrated that high-resolution solid-state NMR spectroscopy on uniformly or extensively (13)C-enriched samples has the potential to screen proteins of moderate size ( approximately 20 kDa) for ligand binding as hydrated solids. The results presented here suggest the possibility of using solid-state NMR to study ligand binding in proteins not amenable to solution NMR.     

Structural studies of sialon ceramics by high-resolution solid-state NMR

High-resolution solide-state ²⁷Al NMR with magic-angle spinning (MASNMR) readily monitors the quantity and coordination (four- and six-fold) of aluminium in two ceramic materials of the SiAlON system. Sialon X-phase, of approximate composition Si₃Al₆O₁₂N₂, contains aluminium-centred octahedra and tetrahedra in the ratio ca. 1.9:1.0, while another sample containing a mixture of sialon polytypoids shows AlO₆ octahedra and a large quantity of what is most probably nitrogen-coordinated tetrahedral aluminium. In addition, ²⁹Si MASNMR detects two different kinds of silicon in the latter sample in a 2:1 ratio. These observations are interpreted satisfactorily in terms of the crystal structures of the compounds and provide further examples of the potential of MASNMR in the investigation of complex ceramic systems.     

Fine-tuned characterization at the solid/solution interface of organotin compounds grafted onto cross-linked polystyrene by using high-resolution MAS NMR spectroscopy

The structural characterization of organotin compounds that are grafted onto insoluble cross-linked polymers has necessarily been limited to elemental analysis, infrared spectroscopy, and in a few instances, solid-state NMR spectroscopy. This important bottleneck in the development of such grafted systems has been addressed by using high-resolution magic angle spinning (hr-MAS) NMR spectroscopy. The great potential of this technique is demonstrated through the structural characterization of diphenylbutyl-(3,4) and dichlorobutylstannanes (5,6), grafted onto divinylbenzene cross-linked polystyrene by means of a suitable linker (1, 2). First, conditions suitable for the application of hr-MAS NMR spectroscopy were identified by characterizing the (1)H resonance line widths of the grafted organotin moiety following swelling of the functionalized beads in eight representative solvents. The presence of clearly identifiable tin coupling patterns in both the 1D (13)C and 2D (1)H-(13)C HSQC spectra, and the incorporation of (119)Sn chemical shift and connectivity information from hr-MAS 1D (119)Sn and 2D (1)H-(119)Sn HMQC spectra, provide an unprecedented level of characterization of grafted organotins directly at the solid/liquid interface. In addition, the use of hr-MAS (119)Sn NMR for reaction monitoring, impurity detection, and quantification and assessment of the extent of coordination reveals its promise as a novel tool for the investigation of polymer-grafted organotin compounds. The approach described here should be sufficiently general for extension to a variety of other nuclei of interest in polymer-supported organometallic chemistry.     

Probing proton-proton proximities in the solid state: high-resolution two-dimensional 1H-1H double-quantum CRAMPS NMR spectroscopy

A new 1H DQ (double-quantum) CRAMPS (combined rotation and multiple-pulse sequence) solid-state nuclear magnetic resonance experiment incorporating DUMBO homonuclear 1H dipolar decoupling is presented. The major resolution enhancement enables DQ peaks corresponding to all 22 close (<3.5 A) proton-proton proximities in the dipeptide beta-AspAla to be observed. In particular, the DQ CRAMPS spectrum provides access to the alkyl region of the spectrum and yields a clear assignment of the two CH and two diastereotopic CH2 proton resonances.     

Determination of chlorine distribution in chlorosulfonated polyethylenes by high-resolution NMR spectroscopy

High-resolution proton magnetic resonance (NMR) spectroscopy was used to determine the sequence distribution of chlorines in elastomeric chlorosulfonated polyethylenes. The determination is based on measuring the relative amounts of methylene groups that are α, β, and γ (or greater) from chlorine containing groups (CHCl groups) in chlorosulfonated polyethylenes. The results obtained from the NMR examination at 220 MHz were compared with the theoretical predictions based on the statistics of substitution polymers. The comparison showed that polyethylenes chlorosulfonated by a solution reaction with gaseous chlorine and sulfur dioxide show a random chlorine distribution.     

Investigation of N-nitrohydrazines by the NMR method

Functionally substituted N-nitrohydrazines were studied by heteronuclear NMR. It was shown that all the investigated products from nitration of the trimethylsilyl derivatives of functionally substituted hydrazines contain the N-NO₂ fragment. The chemical shift of the nitrogen atoms of the hydrazine fragment in the¹⁵N NMR spectra and the spin-spin coupling constants were used as the main tests for structural identification. It was established that the number of recordable conformers decreases in the transition from the trimethylsilyl phenylhydrazine derivatives to the nitrohydrazines as a result of the conformational flexibility of the N(NO₂)CO fragment.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2024–2031, September, 1991.     

Protein 3D structure determination by high-resolution solid-state NMR

Developments in NMR technology, sample preparation, pulse sequence methodology and structure calculation protocols have recently allowed one to progress towards structure determination at high-resolution of proteins by solid-state NMR spectroscopy. We here report solid-state NMR protocols based on magic-angle-spinning experiments, combined with modified structure calculation protocols, for structure determination of uniformly ¹³C, ¹⁵N isotopically labeled proteins. We demonstrate the use of these protocols to obtain high-resolution structures for the example of the microcrystalline Crh protein. The CHHC, DARR and PAR solid-state NMR experiments, as well as the calculation protocols using the program ARIA, are presented.     

The fine structure of high-resolution solid state 29Si NMR spectra of zeolites X and Y

Examination of ²⁹Si magic-angle-spinning NMR spectra of zeolites with the faujasite structure reveals that the characteristic pattern of linewidths and chemical shifts is due to effects in the first coordination shell of silicon.     

Identification of hardeners in solid epoxy resins by high-resolution 13C-NMR

CPMAS ¹³C-NMR spectra at 75 MHz with spinning side band suppression are presented for a series of epoxy resins cured with a variety of commercially important hardeners. The hardeners can be readily identified from the spectra; the high-field is important for distinguishing between related amine hardeners.