Flow NMR applications in combinatorial chemistry

What affects the accuracy and applicability of determining wastewater sludge water content via low-field nuclear magnetic resonance?

The accurate determination of waster sludge water content is crucial to sludge dewatering treatment and its disposal management. Though previous studies highlight the great advantages of low-field nuclear magnetic resonance (LF-NMR) in the determination of sludge water content, its accuracy and applicability are not well studied. Herein, this study investigated the settling of operating parameters and the properties of sludge samples on the accuracy and applicability of LF-NMR method in measuring sludge water content. The results showed that the setting of basic parameters such as standard curve, number of scanning times (NS) and sample weight affected the accuracy of sludge water content by LF-NMR. The standard calibration curve constructed by 3 g/L CuSO₄, NS = 8 and the sample weight of about 5 g, were suitable for the accurate determination of sludge water content. Furthermore, the existence of magnetic substances in sludge can affect the distribution gradient of main magnetic field, and thus restricted the applicability of LF-NMR. The saturation magnetization of chemical reagents strongly correlated with the measured relative errors of sludge water content (r = 0.995, p < 0.01), the greater the saturation magnetization of the magnetic material, the greater the error of the test results. On the whole, it is necessary to fully consider the influence of process parameters and sludge properties to evaluate the accuracy and applicability of the LF-NMR method, rather than simply copying the parameters in literatures.

Reactions in NMR Tubes as Key Weapon in Rational Drug Design

NMR spectroscopy is a powerful technique suitable for obtaining detailed structural and dynamic data at atomic resolution. Progress in NMR instrumentation has led the scientific community to produce novel techniques which provide valuable information to resolve demanding and crucial questions of molecular biology and rational drug design. This chapter outlines the progress of NMR spectroscopy in the rational drug design. In addition, it offers an example of a reaction in NMR tube for achieving rational drug design.

Assessment of dietary exposure and effect in humans: The role of NMR

In human nutritional science progress has always depended strongly on analytical measurements for establishing relationships between diet and health. This field has undergone significant changes as a result of the development of NMR and mass spectrometry methods for large scale detection, identification and quantification of metabolites in body fluids. This has allowed systematic studies of the metabolic fingerprints that biological processes leave behind, and has become the research field of metabolomics. As a metabolic profiling technique, NMR is at its best when its unbiased nature, linearity and reproducibility are exploited in well-controlled nutritional intervention and cross-sectional population screening studies. Although its sensitivity is less good than that of mass spectrometry, NMR has maintained a strong position in metabolomics through implementation of standardisation protocols, hyphenation with mass spectrometry and chromatographic techniques, accurate quantification and spectral deconvolution approaches, and high-throughput automation. Thus, NMR-based metabolomics has contributed uniquely to new insights into dietary exposure, in particular by unravelling the metabolic fates of phytochemicals and the discovery of dietary intake markers. NMR profiling has also contributed to the understanding of the subtle effects of diet on central metabolism and lipoprotein metabolism. In order to hold its ground in nutritional metabolomics, NMR will need to step up its performance in sensitivity and resolution; the most promising routes forward are the analytical use of dynamic nuclear polarisation and developments in microcoil construction and automated fractionation.

Spatially resolved solid-state 1H NMR for evaluation of gradient-composition polymeric libraries

Polyurethane libraries consisting of films with composition gradients of aliphatic polyisocyanate and hydroxy-terminated polyacrylate resin were characterized using methods of (1)H NMR microimaging (i.e., magnetic resonance imaging, (MRI)) and solid-state NMR. Molecular mobilities and underlying structural information were extracted as a function of the relative content of each of the two components. Routine NMR microimaging using the spin-echo sequence only allows investigations of transverse relaxation of magnetization at echo times >2 ms. A single-exponential decay was found, which is likely due to free, noncross-linked polymer chains. The mobility of these chains decreases with increasing content of the aliphatic polyisocyanate. The concept of a 1D NMR profiler is introduced as a novel modality for library screening, which allows the convenient measurement of static solid-state NMR spectra as a function of spatial location along a library sample that is repositioned in the rf coil between experiments. With this setup the complete transverse relaxation function was measured using Bloch decays and spin echoes. For all positions within the gradient-composition film, relaxation data consisted of at least three components that were attributed to a rigid highly cross-linked resin, an intermediate cross-linked but mobile constituent, and the highly mobile free polymer chains (the latter is also detectable by MRI). Analysis of this overall relaxation function measured via Bloch decays and spin echoes revealed only minor changes in the mobilities of the individual fractions. Findings with respect to the most mobile components are consistent with the results obtained by NMR microimaging. The major effect is the significant increase in the rigid-component fraction with the addition of the hydroxy-terminated polyacrylate resin.

NMR-based structural glycomics for high-throughput screening of carbohydrate-active enzyme specificity

We report here the development of a straightforward, sensitive, and quantitative NMR-based method for high-throughput characterization of carbohydrate structure and screening of carbohydrate active enzyme (CAZyme) specificity. Automated assays starting from gene library expression to carbohydrate structure determination directly from crude reaction media have been established and successfully used to screen a library of 4032 CAZymes obtained by combinatorial engineering, at a rate of 480 enzyme variants per day. This allowed one to accurately discriminate 303 enzyme variants with altered specificity. The results demonstrate the potential of high-throughput NMR technology in glycomics, to mine artificial and natural enzyme diversity for novel biocatalysts.

Microscale LC-MS-NMR platform applied to the identification of active cyanobacterial metabolites

An LC-MS-NMR platform is demonstrated, which combines two innovations in microscale analysis, nanoSplitter LC-MS and microdroplet NMR, for the identification of unknown compounds found at low concentrations in complex sample matrixes as frequently encountered in metabolomics or natural products discovery. The nanoSplitter provides the high sensitivity of nanoelectrospray MS while allowing 98% of the HPLC effluent from a large-bore LC column to be collected and concentrated for NMR. Microdroplet NMR is a droplet microfluidic NMR loading method providing severalfold higher sample efficiency than conventional flow injection methods. Performing NMR offline from LC-UV-MS accommodates the disparity between MS and NMR in their sample mass and time requirements, as well as allowing NMR spectra to be requested retrospectively, after review of the LC-MS data. Interpretable 1D NMR spectra were obtained from analytes at the 200-ng level, in 1 h/well automated NMR data acquisitions. The system also showed excellent intra- and interdetector reproducibility with retention time RSD values less than 2% and sample recovery on the order of 93%. When applied to a cyanobacterial extract showing antibacterial activity, the platform recognized several previously known metabolites, down to the 1% level, in a single 30-mug injection, and prioritized one unknown for further study.

Implementation of real-time two-dimensional nuclear magnetic resonance spectroscopy for on-flow high-performance liquid chromatography

Directly coupled HPLC-NMR has become a powerful tool for separation and structural elucidation of unknown compounds. However, there are only a few reports on application of on-flow two-dimensional (2D) NMR in HPLC-NMR. Here we present an alternative method for recording real-time 2D-NMR spectrum (total correlation spectroscopy, TOCSY) on a commercial HPLC-NMR system. The method is based on well-established Hadamard matrix for 2D-NMR frequency encoding. In addition, a modified/improved solvent suppression approach is incorporated. This makes it possible to carry out the experiment with both polar and gradient eluents, the widely used chromatographic conditions. The method is example using a synthesized mixture of three amino acids (His, Phe and Try) and a human urine sample. The method demonstrated here may be utilized for high-throughput structural or unknown component identification and fast dynamic study in a variety of applications.

Extending the scope of NMR spectroscopy with microcoil probes

Capillary NMR (CapNMR) spectroscopy has emerged as a major breakthrough for increasing the mass-sensitivity of NMR spectroscopic analysis and enabling the combination of NMR spectroscopy with other analytical techniques. Not only is the acquisition of high-sensitivity spectra getting easier but the quality of CapNMR spectra obtained in many small-molecule applications exceeds what can be accomplished with conventional designs. This Minireview discusses current CapNMR technology and its applications for the characterization of mass-limited, small-molecule and protein samples, the rapid screening of small-molecule or protein libraries, as well as hyphenated techniques that combine CapNMR with other analytical methods.

Quantitative NMR in synthetic and combinatorial chemistry

The applications of quantitative NMR to synthetic organic chemistry are reviewed with taking into account both the small libraries (100-150 compounds) and the single, well-characterized substance. The precision and accuracy which are obtained with state of the art instrumentation--both around 1%--rival with other classical tools of quantitative analytics, and qNMR does not require a specific method setup or a standard of the same substance. This characteristic makes it the method of choice in an environment where many different molecules are investigated and reliable quantification is required. NMR may effectively replace other standard characterization tools, such as CHNS analysis, or even complex, multi-determination results as commonly required for the assessment of absolute purity or strength of a substance, when no specific standard is available. Finally, because of the high precision and intrinsic accuracy, quantitative NMR appears the ideal reference method for the validation of other, more rapid, generic techniques for quantitative analysis.

Development of Ultrahigh-Throughput NMR Spectroscopic Analysis Utilizing Capillary Flow NMR Technology

An ultrahigh-throughput method for acquiring 1H NMR spectra is described. By constructing a continuous flow system utilizing an HPLC pump, autosampler, and a capillary flow NMR probe, it was possible to inject samples into the NMR spectrometer every 30 s using a continuous flow rate of 30 microL/min. 1H NMR spectroscopic data were acquired continuously into a pseudo-2D data file, with a 96-well-plate completed in <50 min. Spectra in continuous flow mode were readily obtained from approximately 3.4 mug (500 MHz), while the LOD was <850 ng. There was found to be little variation in either sample broadening within the flow system or signal intensities between multiple injections. This system offers several advantages over more conventional NMR spectroscopic analyses, notably the limited solvent required, high sensitivity, high speed, and improved spectral quality as a result of reduced spectral "dead" regions resulting from residual solvent levels.

High-throughput characterization and quality control of small-molecule combinatorial libraries

To fully realize the potential of combinatorial synthesis and high-throughput screening for increasing the efficiency of the drug discovery and development process, issues related to compound purity must be addressed. Impurities, often present after synthesis, can lead to ambiguous screening results and inhibit the development of quality structure-activity relationships. The demand for high-throughput analytical characterization of combinatorial libraries has prompted the development of more rapid methods to keep pace with compound production. Recent progress has focused upon the development of parallel separation methods, multiplexed detector interfaces, and synergistic combinations of different detectors possessing complementary selectivities.

Applying combinatorial chemistry and biology to food research

In the past decade combinatorial chemistry has become a major focus of research activity in the pharmaceutical industry for accelerating the development of novel therapeutic compounds. The same combinatorial strategies could be applied to a broad spectrum of areas in agricultural and food research, including food safety and nutrition, development of product ingredients, and processing and conversion of natural products. In contrast to "rational design", the combinatorial approach relies on molecular diversity and high-throughput screening. The capability of exploring the structural and functional limits of a vast population of diverse chemical and biochemical molecules makes it possible to expedite the creation and isolation of compounds of desirable and useful properties. Several studies in recent years have demonstrated the utility of combinatorial methods for food research. These include the discovery of synthetic antimicrobial, antioxidative, and aflatoxin-binding peptides, the identification and analysis of unique flavor compounds, the generation of new enzyme inhibitors, the development of therapeutic antibodies for botulinum neurotoxins, the synthesis of unnatural polyketides and carotenoids, and the modification of food enzymes with novel properties. The results of such activities could open a large area of applications with potential benefits to the food industry. This review describes the current techniques of combinatorial chemistry and their applications, with emphasis on examples in food science research.

The role of natural product chemistry in drug discovery

Although traditionally natural products have played an important role in drug discovery, in the past few years most Big Pharma companies have either terminated or considerably scaled down their natural product operations. This is despite a significant number of natural product-derived drugs being ranked in the top 35 worldwide selling ethical drugs in 2000, 2001, and 2002. There were 15 new natural product-derived drugs launched from 2000 to 2003, as well as 15 natural product-derived compounds in Phase III clinical trials or registration at the end of 2003. Recently, there has been a renewed interest in natural product research due to the failure of alternative drug discovery methods to deliver many lead compounds in key therapeutic areas such as immunosuppression, anti-infectives, and metabolic diseases. To continue to be competitive with other drug discovery methods, natural product research needs to continually improve the speed of the screening, isolation, and structure elucidation processes, as well addressing the suitability of screens for natural product extracts and dealing with issues involved with large-scale compound supply.

High-Throughput Microcoil NMR of Compound Libraries Using Zero-Dispersion Segmented Flow Analysis

An automated system for loading samples into a microcoil NMR probe has been developed using segmented flow analysis. This approach enhanced 2-fold the throughput of the published direct injection and flow injection methods, improved sample utilization 3-fold, and was applicable to high-field NMR facilities with long transfer lines between the sample handler and NMR magnet. Sample volumes of 2 microL (10-30 mM, approximately 10 microg) were drawn from a 96-well microtiter plate by a sample handler, then pumped to a 0.5-microL microcoil NMR probe as a queue of closely spaced "plugs" separated by an immiscible fluorocarbon fluid. Individual sample plugs were detected by their NMR signal and automatically positioned for stopped-flow data acquisition. The sample in the NMR coil could be changed within 35 s by advancing the queue. The fluorocarbon liquid wetted the wall of the Teflon transfer line, preventing the DMSO samples from contacting the capillary wall and thus reducing sample losses to below 5% after passage through the 3-m transfer line. With a wash plug of solvent between samples, sample-to-sample carryover was <1%. Significantly, the samples did not disperse into the carrier liquid during loading or during acquisitions of several days for trace analysis. For automated high-throughput analysis using a 16-second acquisition time, spectra were recorded at a rate of 1.5 min/sample and total deuterated solvent consumption was <0.5 mL (1 US dollar) per 96-well plate.

Technological Advances in High-Throughput Screening

High-throughput screening (HTS) is the process of testing a large number of diverse chemical structures against disease targets to identify 'hits'. Compared to traditional drug screening methods, HTS is characterized by its simplicity, rapidness, low cost, and high efficiency, taking the ligand-target interactions as the principle, as well as leading to a higher information harvest. As a multidisciplinary field, HTS involves an automated operation-platform, highly sensitive testing system, specific screening model (in vitro), an abundant components library, and a data acquisition and processing system. Various technologies, especially the novel technologies such as fluorescence, nuclear-magnetic resonance, affinity chromatography, surface plasmon resonance, and DNA microarray, are now available, and the screening of more than 100,000 samples per day is already possible. Fluorescence-based assays include the scintillation proximity assay, time-resolved energy transfer, fluorescence anisotropy, fluorescence correlation spectroscopy, and fluorescence fluctuation spectroscopy. Fluorescence-based techniques are likely to be among the most important detection approaches used for HTS due to their high sensitivity and amenability to automation, giving the industry-wide drive to simplify, miniaturize, and speed up assays. The application of NMR technology to HTS is another recent trend in drug research. One advantage afforded by NMR technology is that it can provide direct information on the affinity of the screening compounds and the binding location of protein. The structure-activity relationship acquired from NMR analysis can sharpen the library design, which will be very important in furnishing HTS with well-defined drug candidates. Affinity chromatography used for library screening will provide the information on the fundamental processes of drug action, such as absorption, distribution, excretion, and receptor activation; also the eluting curve can give directly the possibility of candidate drug. SPR can measure the quantity of a complex formed between two molecules in real-time without the need for fluorescent or radioisotopic labels. SPR is capable of characterizing unmodified biopharmaceuticals, studying the interaction of drug candidates with macromolecular targets, and identifying binding partners during ligand fishing experiments. DNA microarrays can be used in HTS be used to further investigate the expression of biological targets associated with human disease, which then opens new and exciting opportunities for drug discovery. Without doubt, the addition of new technologies will further increase the application of HTS in drug screening and its related fields.