Efficient Suppression of Solvent Resonances in HR-MAS of Resin-Supported Molecules

Elucidation of the chemical and morphological structure of double-network (DN) hydrogels by high-resolution magic angle spinning (HRMAS) NMR spectroscopy

(1)H HRMAS NMR spectroscopy is applied to gain insight into the chemical and morphological structure of double-network (DN) hydrogels, prepared from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(acrylamide) (PAAm). The method enables one to obtain detailed information at the molecular level about the formation of covalent bonds between the two polymer networks through non-reacted double bonds of the cross-linker N,N'-methylene bis(acrylamide) (MBAA). Evidence to the existence of strong hydrogen-bond interactions based on the N-H group of the PAMPS as a hydrogen-bond donor and the C=O group of the PAAm as a hydrogen-bond acceptor is also provided. These findings clarify the origin of the toughening mechanism and the exceptionally strong mechanical properties of DN gels, further supported by microhardness data.

Application of diffusion-edited NMR spectroscopy for selective suppression of water signal in the determination of monomer composition in alginates

Alginate is a linear copolymer of 1-4 linked β-D-mannuronic acid (M) and 1-4 linked α-L-guluronic acid (G). The physical properties of these polysaccharides such as gel properties and viscosity are largely correlated to the monomer composition (M/G ratio), the sequence of the polymer and the molecular weight. Determination of the M/G ratio is therefore important and NMR spectroscopy is among the most common methods used to accurately obtain this ratio. Instead of using time consuming, possibly sample altering, acid hydrolysis to reduce the viscosity of the alginate sample prior to analysis, samples of low concentrations can be used. However, this results in a water peak in the NMR spectrum that is several orders of magnitude larger than the alginate signals and water suppression is required. In this article, a diffusion-edited NMR experiment that suppresses the water peak while retaining the signals of interest has been used to enable correct M/G ratio determination. This approach exploits the difference in translational diffusion between the larger alginate molecules and the smaller water molecules. Using this method, the monomer composition of 20 different alginate powders was determined. The diffusion parameters were optimized to allow measurement for samples covering a large range of M/G ratios and viscosities. Thus, such method should be useful for analyzing large numbers of unknown alginate samples using, for example, automation procedures.

Application of HR-MAS NMR in the solid-phase synthesis of a glycopeptide using Sieber amide resin

The solid-phase synthesis (SPS) of a structurally complex glycopeptide, using Sieber amide resin, was monitored by high resolution magic angle spinning NMR, demonstrating the further application of this technique. A synthetic peptidoglycan derivative, a precursor of a biologically active PGN, known to be involved in the cellular recognition, was prepared by SPS. The synthesis involved the preparation of an N-alloc glucosamine moiety and the synthesis of a simple amino acid sequence L-Ala-D-Glu-L-Lys-D-Ala-D-Ala. Last step consisted the coupling, on solid-phase, of the protected muramyl unit to the peptide chain. Proton spectra with good suppression of the polystyrene signals in swollen resin samples were obtained in DMF-d(7) as a solvent and by using a nonselective 1D TOCSY/DIPSI-2 scheme, thus allowing to follow the SPS without losses of compound and cleavage from the resin. The assignment of the proton spectra of the resin-bound amino acid sequence and of the bound glycopeptide was achieved through the combination of MAS COSY, TOCSY and NOESY.

Identification and quantification of water-soluble metabolites by cryoprobe-assisted nuclear magnetic resonance spectroscopy applied to microbial fermentation

We highlight a range of cryoprobe-assisted NMR methods for studying metabolite production by cyanobacteria, which should be valuable for a wide range of biological applications requiring ultrasensitivity and precise concentration determination over a large dynamic range. Cyroprobe-assisted (1)H and (13)C NMR have been applied to precise determination of metabolic products excreted during autofermentation in two cyanobacterial species: filamentous Arthrospira (Spirulina) maxima CS-328 and unicellular Synechococcus sp. PCC 7002. Several fermentative end products were identified and quantified in concentrations ranging from 50 to 3000 microM in cell-free media (a direct measurement of native-like samples) with less than 5.5% relative error in under 10 min of acquisition per sample with the assistance of an efficient water-suppression protocol. Relaxation times (T1) of these metabolites in aqueous ((1)H(2)O) solution were measured and found to vary by nearly threefold, necessitating generation of individual calibration curves for each species for highest precision. However, using a 4.5 x longer overall recycle delay between scans, the metabolite concentrations can be predicted within 25% error by calibrating only to a single calibration standard (succinate); other metabolites are then calculated on the basis of their signal integrals and known proton degeneracies. Precise ratios of concentrations of (13)C-labeled versus unlabeled metabolites were determined from integral ratios of (1)H peaks that exhibit (13)C-(1)H J-couplings and independently confirmed by direct measurement of areas of corresponding (13)C resonances. (13)C NMR was used to identify and quantify production of osmolytes, trehalose, and glucosylglycerol by A. maxima.

Inverse H-C ex situ HRMAS NMR experiments for solid-phase peptide synthesis

The growing importance of solid-phase peptide synthesis (SPPS) has necessitated the development of spectroscopic experiments that can be used to obtain structural and conformational information on resin-bound peptides. Despite the utility of two-dimensional high-resolution magic angle spinning (HRMAS) NMR experiments that provide homonuclear shift correlations, experiments that provide heteronuclear shift correlations are necessary for complex conformational and structural elucidatory problems. Here we report the optimization and implementation of non-gradient inverse NMR experiments for acquiring the 1H-13C shift correlations of resin-bound peptides. The use of non-gradient experiments is advantageous as many magic angle spinning (MAS) probes do not possess gradient coils. An HRMAS BIRD-HMQC experiment with a reduced 1JCH constant has proven very suitable for obtaining one-bond correlations. Long-range correlations can be interpolated by using a non-gradient HRMAS CT-HMBC-1 experiment where the resulting data is processed with forward linear prediction. It has been shown that removing the effects of 1H-1H J-modulation is crucial in order to view cross peaks that correspond to long-range correlations. Additionally, both experiments prove extremely useful over routine one-dimensional 13C HRMAS experiments for extracting carbon chemical shift data. The non-gradient HRMAS BIRD-HMQC and CT-HMBC-1 experiments can be used to assist in conformational analysis and to identify and deconvolute situations where accidental equivalence and seemingly correlated isochronous signals arise.

Pulsed field gradient magic angle spinning NMR self-diffusion measurements in liquids

Several investigations have recently reported the combined use of pulsed field gradient (PFG) with magic angle spinning (MAS) for the analysis of molecular mobility in heterogeneous materials. In contrast, little attention has been devoted so far to delimiting the role of the extra force field induced by sample rotation on the significance and reliability of self-diffusivity measurements. The main purpose of this work is to examine this phenomenon by focusing on pure liquids for which its impact is expected to be largest. Specifically, we show that self-diffusion coefficients can be accurately determined by PFG MAS NMR diffusion measurements in liquids, provided that specific experimental conditions are met. First, the methodology to estimate the gradient uniformity and to properly calibrate its absolute strength is briefly reviewed and applied on a MAS probe equipped with a gradient coil aligned along the rotor spinning axis, the so-called 'magic angle gradient' coil. Second, the influence of MAS on the outcome of PFG MAS diffusion measurements in liquids is investigated for two distinct typical rotors of different active volumes, 12 and 50 microL. While the latter rotor led to totally unreliable results, especially for low viscosity compounds, the former allowed for the determination of accurate self-diffusion coefficients both for fast and slowly diffusing species. Potential implications of this work are the possibility to measure accurate self-diffusion coefficients of sample-limited mixtures or to avoid radiation damping interferences in NMR diffusion measurements. Overall, the outlined methodology should be of interest to anyone who strives to improve the reliability of MAS diffusion studies, both in homogeneous and heterogeneous media.

Selective intracellular accumulation of the major metabolite issued from the activation of the prodrug ethionamide in mycobacteria

BACKGROUND

Ethionamide is one of the most widely used drugs for the treatment of multidrug-resistant tuberculosis (MDR-TB). Like isoniazid, and pyrazinamide, ethionamide is a prodrug that needs to be activated by a mycobacterial enzyme. Activation pathways of prodrugs are generally problematic to uncover as they produce intermediates potentially difficult to characterize, to purify and that might prove unstable outside of their cellular context.

OBJECTIVES AND METHODS

We have used high resolution magic angle spinning-NMR (HRMAS-NMR) to follow ethionamide activation directly within living mycobacterial cells.

RESULTS

Data indicated that the intracellular metabolization of ethionamide strictly depends on the presence of the monooxygenase EthA and that EthA-dependent activation of ethionamide is coupled to a precise molecular sorting mechanism of the ethionamide metabolites. We found that the previously identified ethionamide metabolite 2-ethyl-4-hydroxymethylpyridine is produced in substantial amounts by the ethionamide-treated mycobacteria and that it is present exclusively outside of the bacteria. In contrast, the still unidentified ethionamide metabolite ETH* is the only ethionamide derivative detected within the bacterial cell. Moreover, ETH* appears to be unable to cross the bacterial envelope and consequently accumulates within the cytoplasm of the ethionamide-treated mycobacteria.

CONCLUSIONS

These results strongly suggest that ETH* is the active antimycobacterial ethionamide derivative and open new perspectives for the understanding of the mode of action of prodrugs.

Classification of spectroscopically encoded resins by Raman mapping and infrared hyperspectral imaging

Barcoded resins (BCRs) were recently introduced as a potential platform for pre-encoded multiplexed synthesis, screening, and biomedical diagnostics. A key step toward the development of this strategy is the ability to rapidly interrogate and classify the BCRs in a high-throughput, noninvasive manner. Here, we describe a one-step strategy based on Raman mapping and Fourier transform infrared imaging to classify and spatially resolve randomly distributed BCRs. To illustrate this methodology, mixtures of up to 25 different BCRs were imaged and classified with 100% confidence. This strategy can be readily extended to a larger pool of resins, provided each BCR features a unique vibrational fingerprint (spectroscopic barcode). We have also established that reliable single-bead Raman spectra can be recorded in 10 ms, thus confirming that Raman mapping, in particular, could be a very fast method to classify the BCRs.

Optimization of Diffusion-Filtered NMR Experiments for Selective Suppression of Residual Nondeuterated Solvent and Water Signals from 1H NMR Spectra of Organic Compounds

Interpretation of 1H NMR spectra of organic compounds is sometimes hampered by the presence of strong peaks arising from residual nondeuterated solvent and water that obscure compound signals. Classical solvent suppression techniques such as presaturation or those based on pulsed field gradients are not effective in this regard because they also remove the compound resonances that overlap with the solvent signal being suppressed. Here, we propose an alternative scheme by using an optimized NMR diffusion filter that eliminates the nondesired peaks while retaining the signals of interest. This strategy has proved to be useful in three common deuterated solvents, namely, CDCl3, DMSO-d6, and CD3OD, resulting in clean spectra with no interference from solvent or water peaks.

Spectroscopically encoded resins for high throughput imaging time-of-flight secondary ion mass spectrometry

Spectroscopic barcoding was recently introduced as a new pre-encoding strategy wherein the resin beads are not just carriers for solid phase synthesis, but are, in addition, the repository of the synthetic scheme to which they were subjected. To expand the repertoire of spectroscopically barcoded resins (BCRs), here we introduce a new family of halogenated polystyrene-based polymers designed for high-throughput combinatorial analysis using not only infrared and Raman spectroscopy but also imaging time-of-flight secondary ion mass spectrometry (ToF-SIMS). In particular, we have established that (a) the halogen content of these new resins can be used as an encoding element in quantitative imaging ToF-SIMS and (b) the number of styrene monomers used to generate unique vibrational fingerprints can be significantly reduced by using monomers in different molar ratios. The combination of quantitative imaging ToF-SIMS and vibrational spectroscopy is anticipated to dramatically increase the repertoire of possible BCRs from a few hundreds to several thousands.

Monitoring of the ethionamide pro-drug activation in mycobacteria by 1H high resolution magic angle spinning NMR

In this study, we use HRMAS NMR as a non-invasive technique to monitor the in vivo metabolism of a xenobiotic. The antituberculosis Ethionamide is a pro-drug that has to be activated in mycobacteria before inhibiting its cellular target. The use of (1)H HRMAS NMR has allowed to detect a metabolite (ETH*) of the drug directly in living bacteria, even with a spectrometer operating at the relatively low magnetic field of 300MHz. We show that metabolism monitoring of an unlabelled drug at a therapeutically relevant concentration as low as 5mug/ml is within reach of the technique. (1)H HRMAS NMR in combination with diffusion filtering leads to the conclusion that the metabolite is located inside the intact cells. The comparison of the metabolite NMR signature with that of synthetic molecules proves the non-identity of ETH* with the ETH derivatives described previously in the literature.

Polystyrene-Supported Organotin Dichloride as a Recyclable Catalyst in Lactone Ring-Opening Polymerization: Assessment and Catalysis Monitoring by High-Resolution Magic-Angle-Spinning NMR Spectroscopy

Dialkyltin dichloride grafted to a cross-linked polystyrene, with the formula [P-H]((1-t))[P-(CH2)nSnBuCl2]t (P=[CH2CH(pC6H4)], t=the degree of functionalization, and n=6 or 11), is investigated as a recyclable catalyst in the ring-opening polymerization (ROP) of epsilon-caprolactone (CL). It is demonstrated that high-resolution magic-angle-spinning (HR-MAS) NMR spectroscopy is an invaluable tool to characterize completely the supported catalyst. The 2D 1H-13C HSQC HR-MAS spectrum, in particular, allowed extensive assignment of the 1H and 13C resonances, as well as accurate measurement of the (n)J((1)H-(117/119)Sn) and (n)J((13)C-(117/119)Sn) coupling constants. 1H and 119Sn HR-MAS NMR spectroscopy is presented as a monitoring tool for catalytic processes based on organotin compounds, particularly for the investigation of the extent to which polymerization residues are observable in situ in the material pores and for the assessment of the chemical integrity and recycling conditions of the grafted catalyst. From polymerization experiments with CL, initiated by n-propanol and with [P-H]((1-t))[P-(CH2)nSnBuCl2]t of various compositions as the supported catalyst, it appears that a partial 'burst' of the polystyrene support occurs when the length of the alkyl spacer is limited to n=6, as a result of polymer chains growing within the pores of the support. However, extension of the length of the aliphatic polymethylene spacer from 6 to 11 carbon atoms preserves the support integrity and allows the production of catalyst-deprived poly(epsilon-caprolactone) (PCL) oligomers. A preliminary attempt to recycle the heterogeneous catalyst has shown that very good reproducibility can be obtained, in terms of both catalyst activity and molecular-weight parameters of the as-recovered PCL polyester chains.

Development of a high-resolution magic-angle spinning nuclear magnetic resonance identity assay of the capsular polysaccharide from Haemophilus influenzae type b present in cetavlon precipitate

We describe the use of high-resolution magic-angle spinning nuclear magnetic resonance to control the identity of the capsular polysaccharide from Haemophilus influenzae type b (Hib) present in the cetavlon precipitate. This step is one of the earliest in the purification of this polysaccharide, which is further used in the production of Hib polysaccharide-protein conjugate vaccine. The effects of sample procedure and magnetic field strength have been investigated. Since this assay is rapid and simple, it may represent a useful technique for characterization of polysaccharides present in complex and insoluble matrices. Moreover, it allows a rapid evaluation of the structure of the produced polysaccharides very early on during the production process and is as such an essential analytical tool before starting the purification process.

1H high-resolution magic-angle spinning (HR-MAS) NMR analysis of ligand density on resins using a resin internal standard

We recently attempted to generate an affinity chromatography adsorbent to purify cytochrome P450 4A1 by coupling 11-(1'-imidazolyl)-3,6,9-trioxaundecanoic acid to Toyopearl AF-Amino 650 M resin. Variations in ligand density for several resin batches were quantified by high-resolution magic-angle spinning (HR-MAS) NMR spectroscopy using a novel resin internal standard. The uniquely designed ImQ internal resin standard yields its signature resonance in a transparent region of the analyte spectrum making suppression of the polymer background unnecessary. This method enabled us to target a reasonable ligand density for enzyme purification and provides an advantageous alternative to quantitation against soluble standards or protonated solvent.

Solid-phase functionalization of peptides by an alpha-hydrazinoacetyl group

A novel procedure for the preparation of alpha-hydrazinoacetyl peptides is reported on the basis of the solid-phase coupling of partially or fully Boc-protected hydrazino acetic acid derivatives. The degree of unwanted polymerization of the activated ester during both activation and coupling was found to be significant for the monoprotected derivative BocNHNHCH(2)CO(2)H but could be minimized with the diprotected derivative BocNHNH(Boc)CH(2)CO(2)H and suppressed with the fully protected acid. Despite the instability of the imidocarbonate group toward acids and bases, a low-cost and effective route was sought for the preparation of the tris(Boc)-protected derivative. The N,N,N'-tris(Boc)hydrazinoacetic acid could be introduced on the solid phase after or before peptide elongation using Fmoc/tert-butyl chemistry. In this latter case, HR MAS NMR analysis of model solid supports demonstrated the partial loss of one Boc group during the repetitive piperidine treatments. Despite this slight instability, N,N,N'-tris(Boc)hydrazinoacetic acid was found to be a highly convenient reagent for the robust and easily scalable preparation of hydrazinopeptides in good yield and high purity.

Preparation, physical properties, on-bead binding assay and spectroscopic reliability of 25 barcoded polystyrene-poly(ethylene glycol) graft copolymers

Here we describe the preparation of 25 beaded polystyrene-poly(ethylene glycol) graft copolymers from six spectroscopically active styrene monomers: styrene, 2,5-dimethylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 4-tert-butylstyrene, and 3-methylstyrene. These polymers were thoroughly characterized by Raman, infrared, and (1)H/(13)C NMR spectroscopies, and differential scanning calorimetry. Determination of the swelling properties, peptide synthesis, and on-bead streptavidin-alkaline phosphatase (SAP) binding assay further established that their physical and chemical properties where not significantly altered by the diversity of their encoded polystyrene core. Each of the 25 resins displayed a unique Raman and infrared vibrational fingerprint, which was converted into a "spectroscopic barcode". The position of each bar matches the peak wavenumber in the corresponding spectrum but is independent of its intensity. From this simplified representation similarity maps comparing 35 000 resin pairs were generated to establish the spectroscopic barcoding as a reliable encoding methodology. In effect, in 99% of the cases, the highest similarity coefficients were obtained for resin pairs prepared from the same styrene derivatives even after SAP binding assay. We have also shown that a small but unique combination of a resin's vibrations (30-40%) is sufficient for its identification. However, in rare cases where a resin's vibrational signature has been severely compromised, both the Raman and infrared barcodes were synergistically and reliably utilized to unequivocally identify its chemical make up.

Enhanced diffusion-edited NMR spectroscopy of mixtures using chromatographic stationary phases

We introduce an analytical method that combines in one pot the advantages of column chromatography separation and NMR structural analysis. The separation of the NMR spectra of the components of a mixture can be achieved according to their apparent diffusion rates [James, T. L. and McDonald, G. G. (1973) J. Magn. Reson. 58, 58-61]. We show that the separation of the spectral components, corresponding to single molecular species, can be enhanced by order of magnitudes upon addition of a typical stationary phase used in HPLC. The solid phase imbibed by the mixture for analysis is an heterogeneous ensemble, so that solid-state NMR methods (high-resolution magic angle spinning) are necessary to recover high-resolution spectra. We demonstrate applications of this combination of high-resolution magic angle spinning and NMR diffusometry on test mixtures for direct (silica gel) and inverse (C18) columns. However, many common chromatographic supports available for HPLC should be readily adaptable for use with this technique.

The identification of plant derived structures in humic materials using three-dimensional NMR spectroscopy

Here we demonstrate the application of 3-D NMR spectroscopy to structural studies of humic substances, the most abundant of organic compounds on earth. The increased spectral dispersion provided by the additional dimension is proven to be highly advantageous in separating the overlapping signals observed in 2-D spectra. Assignments of the major aliphatic structures and selected aromatic moieties are given as examples. We find that in a forest soil fulvic acid the major aliphatic materials are likely derived from leaf cuticles and further demonstrate that lignin signatures can be identified among the aromatic species. Once identified from the 3-D spectra, these structures can be assigned using the partial information available in 2-D, and in some cases, in the 1-D spectra. These signals are demonstrated to be characteristic to given samples of natural organic matter, and the case is made for their use as indicators of terrestrial biomarkers in mixtures of compounds with unknown origins.

Solid-phase enolate chemistry investigated using HR-MAS NMR spectroscopy

Supported P4-t-Bu enolate chemistry of phenylacetyloxymethyl polystyrene (PS) resin was investigated using high-resolution magic angle spinning (HR-MAS) NMR spectroscopy. Direct analysis of the crude reaction suspensions through the use of a diffusion filter (DF) allowed a rapid selection of the optimal experimental conditions, but also the characterization of the enolate on the solid phase. Comparison with solution experiments and literature data allowed us to address partially the structure of the enolate. HR-MAS NMR spectra of the enolate revealed also a tight interaction of P4-t-Bu base with the polymer matrix.

Rotor synchronization of radiofrequency and gradient pulses in high-resolution magic angle spinning NMR

We have investigated the extent to which rotor synchronization of radiofrequency pulses leads to spectral improvement in high-resolution magic angle spinning NMR experiments. Several pulse sequences were tested, and the effect was found to be maximal in homonuclear TOCSY spectra. The physicochemical nature of the sample plays a role in the phenomenon, as rotor synchronization allows the refocusing of residual anisotropic interactions. However, even in a liquid sample the effects were visible. Radial inhomogeneities of the radiofrequency field were identified as an important source of the problem.

In vivo detection of the cyclic osmoregulated periplasmic glucan of Ralstonia solanacearum by high-resolution magic angle spinning NMR

We investigate the mobility of the osmoregulated periplasmic glucans of Ralstonia solanacearum in the bacterial periplasm through the use of high-resolution (HR) NMR spectroscopy under static and magic angle spinning (MAS) conditions. Because the nature of periplasm is far from an isotropic aqueous solution, the molecules could be freely diffusing or rather associated to a periplasmic protein, a membrane protein, a lipid, or the peptidoglycan. HR MAS NMR spectroscopy leads to more reproducible results and allows the in vivo detection and characterization of the complex molecule.

NMR spectroscopy in drug discovery: tools for combinatorial chemistry, natural products, and metabolism research

NMR spectroscopy has enjoyed many advances recently, and the pace of development shows no signs of slowing. This article focuses on advances that have affected solution-state NMR. These advances fall into three general categories: new experimental techniques (new pulse sequence tools), improved hardware and more powerful software. These advances are allowing NMR to help solve important problems in the field of drug discovery. Their impact is widespread. NMR spectroscopy is now being used to determine protein structures, to monitor ligand-receptor binding, to study diffusion, to analyze mixtures using LC-NMR, to analyze solid-phase synthesis resins and to determine the structures of organic small molecules. NMR spectroscopy can provide both qualitative and quantitative information, and can be used in both routine analytical applications and demanding research applications. The applications described here can benefit numerous disciplines in drug discovery, including natural products research, synthetic medicinal chemistry, metabolism studies, drug production, quality control, rational drug design and combinatorial chemistry.

The Chemical Shift Index method applied to resin-bound peptides

The Chemical Shift Index (CSI) method proposed by Wishart et al. [Biochemistry (1992) 31, 1647-1651] to evaluate the secondary structure of peptides in aqueous solution uses as its reference the chemical shift values of each of the 20 natural amino acids (X) in a typical nonstructured sequence GGXAGG (17-20). In order to apply the CSI method to protected resin-bound peptides, we established a new database of chemical shift values for the same GGXAGG sequences in their protected form and anchored to a polystyrene resin swollen in DMF-d7. The predictive value of this new reference set in the CSI protocol was tested on different resin-bound peptides that were previously characterized by a full NOE analysis.

Preparation of fluorinated linkers: use of 19F NMR spectroscopy to establish conditions for solid-phase synthesis of pilicide libraries

Three fluorinated linkers which are analogues of linkers commonly used in solid-phase peptide synthesis have been prepared. One of the linkers was used in combination with gel-phase 19F NMR spectroscopy to develop conditions for solid-phase synthesis of two libraries of pilicides, i.e. compounds designed to inhibit assembly of adhesive pili in uropathogenic Escherichia coli. Attachment to and cleavage from the linker could be monitored based on the chemical shift of the fluorine atom of the linker. In addition, use of the linker as internal standard allowed quantification and optimization of reactions occurring further away from the linker when fluorinated building blocks were employed. Importantly, high-quality 19F NMR spectra were obtained for compounds linked to a TentaGel resin in a standard NMR tube using an ordinary NMR instrument.

Multistep synthesis of 2,5-diketopiperazines on different solid supports monitored by high resolution magic angle spinning NMR spectroscopy

The solid-phase synthesis of 2,5-diketopiperazines containing the trans-4-hydroxy-L-proline amino acid residue (Hyp) was performed on Ellman polystyrene, polyoxyethylene-polyoxypropylene (POEPOP), polystyrene-polyoxyethylene NovaSyn, and Wang resins, respectively. The reaction pathway allowed the introduction of different functional groups around the bicyclic scaffold in a combinatorial approach, and it generated mixtures of isomers. A detailed characterization of the single reaction steps by high resolution magic angle spinning (HRMAS) NMR spectroscopy was performed. The NMR spectral resolution of the resin-bound intermediates and final products was greatly influenced by the polymer matrix. The POEPOP resin permitted to obtain HRMAS NMR spectra with a resolution comparable with that of the spectra of the molecules in solution. Moreover, configurational and conformational isomers formed during the solid-phase reaction steps could be detected and easily assigned. Therefore, the combination of the HRMAS NMR technique with the use of nonaromatic resins may become an extremely powerful tool in solid-phase organic synthesis. This approach will allow the monitoring of multistep reactions and the conception of on-bead structural studies either on small molecules or on natural and/or synthetic oligomers.

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.

Chemically sensitive high throughput parallel analysis of solid phase supported library members

This paper introduces Fourier transform infrared imaging as a powerful spectroscopic tool for the parallel identification of members of resin-supported combinatorial libraries. This technique combines the chemical specificity and high sensitivity of FTIR with the ability to rapidly analyze multiple supported resin beads simultaneously. It is shown here that the chemical identity of ligands on a variety of supported resin beads can be identified in a single experiment without destroying or otherwise perturbing the system.

SPEEDY: spin-echo enhanced diffusion filtered spectroscopy. A new tool for high resolution MAS NMR

The pulsed field gradient diffusion edited experiment, bipolar LED, has been combined with the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence for the analysis of solid phase resin samples. Spin-echo enhanced diffusion filtered spectroscopy (SPEEDY), when optimized, filters both the compounds that demonstrate fast diffusion rates as well as the compounds that demonstrate fast T(2) relaxation rates. Using this technique, compounds that are not covalently attached to the resin are not observed and contributions from the resin matrix are greatly attenuated. The interpretation of the resulting spectrum is more readily accessible. This technique lessens the importance of completely removing reaction residues or the wash solvents simply for analytical evaluation. The utility of the combined filtering scheme was demonstrated by the implementation into a NOESY sequence.