Determination of regulatory phosphorylation sites in nanogram amounts of a synthetic fragment of ZAP-70 using microprobe NMR and on-line coupled capillary HPLC-NMR

From single to multiple microcoil flow probe NMR and related capillary techniques: a review

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important and powerful instrumental analytical techniques for structural elucidation of unknown small and large (complex) isolated and synthesized compounds in organic and inorganic chemistry. X-ray crystallography, neutron scattering (neutron diffraction), and NMR spectroscopy are the only suitable methods for three-dimensional structure determination at atomic resolution. Moreover, these methods are complementary. However, by means of NMR spectroscopy, reaction dynamics and interaction processes can also be investigated. Unfortunately, this technique is very insensitive in comparison with other spectrometric (e.g., mass spectrometry) and spectroscopic (e.g., infrared spectroscopy) methods. Mainly through the development of stronger magnets and more sensitive solenoidal microcoil flow probes, this drawback has been successfully counteracted. Capillary NMR spectroscopy increases the mass-based sensitivity of the NMR spectroscopic analysis up to 100-fold compared with conventional 5-mm NMR probes, and thus can be coupled online and off-line with other microseparation and detection techniques. It offers not only higher sensitivity, but in many cases provides better quality spectra than traditional methods. Owing to the immense number of compounds (e.g., of natural product extracts and compound libraries) to be examined, single microcoil flow probe NMR spectroscopy will soon be far from being sufficiently effective as a screening method. For this reason, an inevitable trend towards coupled microseparation-multiple microcoil flow probe NMR techniques, which allow simultaneous online and off-line detection of several compounds, will occur. In this review we describe the current status and possible future developments of single and multiple microcoil capillary flow probe NMR spectroscopy and its application as a high-throughput tool for the analysis of a large number of mass-limited samples. The advantages and drawbacks of different coupled microseparation-capillary NMR spectroscopy techniques, such as capillary high-performance liquid chromatography-NMR spectroscopy, capillary electrophoresis-NMR spectroscopy, and capillary gas chromatography-NMR spectroscopy, are discussed and demonstrated by specific applications. Another subject of discussion is the progress in parallel NMR detection techniques. Furthermore, the applicability and mixing capability of tiny reactor systems, termed "microreactors" or "micromixers," implemented in NMR probes is demonstrated by carbamate- and imine-forming reactions.

A microscale protein NMR sample screening pipeline

As part of efforts to develop improved methods for NMR protein sample preparation and structure determination, the Northeast Structural Genomics Consortium (NESG) has implemented an NMR screening pipeline for protein target selection, construct optimization, and buffer optimization, incorporating efficient microscale NMR screening of proteins using a micro-cryoprobe. The process is feasible because the newest generation probe requires only small amounts of protein, typically 30-200 microg in 8-35 microl volume. Extensive automation has been made possible by the combination of database tools, mechanization of key process steps, and the use of a micro-cryoprobe that gives excellent data while requiring little optimization and manual setup. In this perspective, we describe the overall process used by the NESG for screening NMR samples as part of a sample optimization process, assessing optimal construct design and solution conditions, as well as for determining protein rotational correlation times in order to assess protein oligomerization states. Database infrastructure has been developed to allow for flexible implementation of new screening protocols and harvesting of the resulting output. The NESG micro NMR screening pipeline has also been used for detergent screening of membrane proteins. Descriptions of the individual steps in the NESG NMR sample design, production, and screening pipeline are presented in the format of a standard operating procedure.

Evaluation of on-line solid-phase extraction parameters for hyphenated, high-performance liquid chromatography–solid-phase extraction–nuclear magnetic resonance applications

The hyphenated technique HPLC-SPE-NMR is proving to be a useful analytical tool for structure elucidation of mixture components, particularly for mass-limited samples where traditional isolation procedures are either time consuming or challenging. In this work, we investigated SPE trapping performance of 25 model natural products within a format corresponding to that of HPLC-SPE-NMR hyphenation. Six different silica-based bonded phases and two polymeric phases were evaluated. The trapping efficiency of polystyrene/divinylbenzene polymers was generally superior compared to silica bonded phases, which showed variable results and performed well only with hydrophobic analytes. Acetonitrile concentration in the loading solvent was critical for trapping on polymeric phase (Resin GP), as small changes of the organic solvent concentration (+/-3%) could alter the trapping efficiency significantly. Flow rate changes of the loading solvent within 0.8-5.0 mL/min did not affect trapping kinetics. Simulation of multiple trapping showed excellent performance of this approach for hydrophobic analytes, and moderate gain for more polar analytes that do not trap quantitatively in a single trapping step. Determination of 50% breakthrough levels by frontal chromatography analysis showed feasibility of accumulation of analyte amounts corresponding to about 0.5 micromol (10 mm x 2 mm i.d. Resin GP cartridge).

Application of polymer based stationary phases in high performance liquid chromatography and capillary high performance liquid chromatography hyphenated to microcoil 1H nuclear magnetic resonance spectroscopy

The increased demand for chromatographic materials that are able to achieve a fast separation of large quantities of structure analogues is a great challenge. It is known that polymer based chromatographic materials have a higher loadability, compared to silica based sorbents. Unfortunately these polymer materials cannot be used under high pressure which is necessary in order to obtain high flow rates, and hence long times are needed to perform a separation. However, by immobilizing a polymer on a mechanically stable porous silica core, this problem can be circumvented and higher flows become feasible on these materials. Especially for capillary liquid chromatography hyphenated with nuclear magnetic resonance a high loadability is of great importance in order to obtain sharp, resolved, and concentrated peaks thus resulting in a good signal to noise ratio in the NMR experiment. Therefore, a highly shape selective chromatographic sorbent was developed by covalently immobilizing a poly(ethylene-co-acrylic) acid copolymer (-CH(2)CH(2)-)(x)[CH(2)CH(CO(2)H)-](y) (x=119, y=2.4) with a mass fraction of acrylic acid of 5% as stationary phase on silica via a spacer molecule (3-glycidoxypropyltrimethoxysilane). First, the loadability of this sorbent compared to C(30) is demonstrated by the HPLC separation of two xanthophyll isomers. Subsequently, it has been successfully employed in the hyphenation of capillary HPLC with microcoil (1)H NMR spectroscopy by separating and identifying a highly concentrated solution of the tocopherol homologues.

Microgram-scale protein structure determination by NMR

Using conventional triple-resonance nuclear magnetic resonance (NMR) experiments with a 1 mm triple-resonance microcoil NMR probe, we determined near complete resonance assignments and three-dimensional (3D) structure of the 68-residue Methanosarcina mazei TRAM protein using only 72 mug (6 microl, 1.4 mM) of protein. This first example of a complete solution NMR structure determined using microgram quantities of protein demonstrates the utility of microcoil-probe NMR technologies for protein samples that can be produced in only limited quantities.

Identification of natural products using HPLC-SPE combined with CapNMR

Two major development areas in HPLC-NMR hyphenation are postcolumn solid-phase extraction (HPLC-SPE-NMR) and capillary separations with NMR detection by means of solenoidal microcoils (CapNMR). These two techniques were combined off-line into HPLC-SPE-CapNMR, which combines the advantage of high loadability of normal-bore HPLC columns with high mass sensitivity of capillary NMR probes with an active volume of 1.5 microL. The technique was used for rapid identification of complex sesquiterpene lactones and esterified phenylpropanoids present in an essentially crude plant extract (toluene fraction of an ethanolic extract of Thapsia garganica fruits). Elution profiles of 10 x 1 mm i.d. SPE cartridges filled with poly(divinylbenzene) resin were found to be only marginally broader than those observed upon direct injection of 6-microL samples into the probe. Thus, the technique focuses analytes emerging in the HPLC elution bands of 0.5-1 mL into volumes of approximately 10 microL, compatible with the CapNMR probe. Using this technique, nine natural products (1-9) present in the plant extract in amounts varying from 0.1 to 20% were identified by means of 1D and 2D NMR spectra, supported by parallel HPLC-ESIMS measurements. Therefore, HPLC-SPE-CapNMR should be regarded as an attractive alternative to other applications of CapNMR for mixture analysis.

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.