Cryogen-free 400-MHz nuclear magnetic resonance spectrometer as a versatile tool for pharmaceutical process analytical technology

The discovery of new ceramic materials containing Ba-La-Cu oxides in 1986 that exhibited superconducting properties at high temperatures in the range of 35 K or higher, recognized with the Nobel Prize in Physics in 1987, opened a new world of opportunities for nuclear magnetic resonance (NMRs) and magnetic resonance imaging (MRIs) to move away from liquid cryogens. This discovery expands the application of high temperature superconducting (HTS) materials to fields beyond the chemical and medical industries, including electrical power grids, energy, and aerospace. The prototype 400-MHz cryofree HTS NMR spectrometer installed at Amgen's chemistry laboratory has been vital for a variety of applications such as structure analysis, reaction monitoring, and CASE-3D studies with RDCs. The spectrometer has been integrated with Amgen's chemistry and analytical workflows, providing pipeline project support in tandem with other Kinetic Analysis Platform technologies. The 400-MHz cryofree HTS NMR spectrometer, as the name implies, does not require liquid cryogens refills and has smaller footprint that facilitates installation into a chemistry laboratory fume hood, sharing the hood with a process chemistry reactor. Our evaluation of its performance for structural analysis with CASE-3D protocol and for reaction monitoring of Amgen's pipeline chemistry was successful. We envision that the HTS magnets would become part of the standard NMR and MRI spectrometers in the future. We believe that while the technology is being developed, there is room for all magnet options, including HTS, low temperature superconducting (LTS) magnets, and low field benchtop NMRs with permanent magnets, where utilization will be dependent on application type and costs.

Residual Dipolar Coupling Based Conformational Analysis Allows the Configurational Assessment of Steroids with up to Eight Stereocenters

Residual dipolar couplings (RDCs) induced by anisotropic media have been proved as a powerful tool for the structure elucidation of organic molecules in solution in nuclear magnetic resonance (NMR) based analysis. The value of dipolar couplings to solve complex conformational and configurational problems represents indeed an appealing analytical tool for the pharmaceutical industry particularly focusing on the stereochemistry characterization of NCEs since the early phase of the drug development process. In our work, RDCs were used for the conformational and configurational study of synthetic steroids with multiple stereocenters - prednisone and beclomethasone dipropionate (BDP) -. For both molecules the correct relative configuration was identified among all the possible diastereoisomers (32 and 128 respectively) arising from the compounds stereogenic carbons. Only for prednisone the use of additional experimental data (i. e. rOes) was necessary to resolve the right stereochemical structure.

Chain Entropy Beats Hydrogen Bonds to Unfold and Thread Dialcohol Phosphates inside Cyanostar Macrocycles To Form [3]Pseudorotaxanes

The recognition of substituted phosphates underpins many processes including DNA binding, enantioselective catalysis, and recently template-directed rotaxane synthesis. Beyond ATP and a few commercial substrates, however, little is known about how substituents effect organophosphate recognition. Here, we examined alcohol substituents and their impact on recognition by cyanostar macrocycles. The organophosphates were disubstituted by alcohols of various chain lengths, dipropanol, dihexanol, and didecanol phosphate, each accessed using modular solid-phases syntheses. Based on the known size-selective binding of phosphates by π-stacked dimers of cyanostars, threaded [3]pseudorotaxanes were anticipated. While seen with butyl substituents, pseudorotaxane formation was disrupted by competitive OH···O- hydrogen bonding between both terminal hydroxyls and the anionic phosphate unit. Crystallography also showed formation of a backfolded propanol conformation resulting in an 8-membered ring and a perched cyanostar assembly. Motivated by established entropic penalties accompanying ring formation, we reinstated [3]pseudorotaxanes by extending the size of the substituent to hexanol and decanol. Chain entropy overcomes the enthalpically favored OH···O- contacts to favor random-coil conformations required for seamless, high-fidelity threading of dihexanol and didecanol phosphates inside cyanostars. These studies highlight how chain length and functional groups on phosphate's substituents can be powerful design tools to regulate binding and control assembly formation during phosphate recognition.

Conformational Investigations in Flexible Molecules Using Orientational NMR Constraints in Combination with 3J-Couplings and NOE Distances

The downscaling of NMR tensorial interactions, such as dipolar couplings, from tens of kilohertz to a few hertz in low-order media is the result of dynamics spanning several orders of magnitudes, including vibrational modes (~ns-fs), whole-molecule reorientation (~ns) and higher barrier internal conformational exchange (<ms). In this work, we propose to employ these dynamically averaged interactions to drive an “alignment-tensor-free” molecular dynamic simulation with orientation constraints (MDOC) in order to efficiently access the conformational space sampled by flexible small molecules such as natural products. Key to this approach is the application of tensorial pseudo-force restraints which simultaneously guide the overall reorientation and conformational fluctuations based on defined memory function over the running trajectory. With the molecular mechanics force-field, which includes bond polarization theory (BPT), and complemented with other available NMR parameters such as NOEs and scalar J-couplings, MDOC efficiently arrives at dynamic ensembles that reproduce the entire NMR dataset with exquisite accuracy and theoretically reveal the systems conformational space and equilibrium. The method as well as its potential towards configurational elucidation is presented on diastereomeric pairs of flexible molecules: a small 1,4-diketone 1 with a single rotatable bond as well as a 24-ring macrolide related to the natural product mandelalide A 2.

Configuration determination by residual dipolar couplings: accessing the full conformational space by molecular dynamics with tensorial constraints

Residual dipolar couplings (RDCs) and other residual anisotropic NMR parameters provide valuable structural information of high quality and quantity, bringing detailed structural models of flexible molecules in solution in reach. The corresponding data interpretation so far is directly or indirectly based on the concept of a molecular alignment tensor, which, however, is ill-defined for flexible molecules. The concept is typically applied to a single or a small set of lowest energy structures, ignoring the effect of vibrational averaging. Here, we introduce an entirely different approach based on time averaged molecular dynamics with dipolar couplings as tensorial orientational restraints that can be used to solve structural problems in molecules of any size without the need of introducing an explicit molecular alignment tensor into the computation. RDC restraints are represented by their full 3D interaction tensor in the laboratory frame, for which pseudo forces are calculated using a secular dipolar Hamiltonian as the target. The resulting rotational averaging of each individual tensorial restraint leads to structural ensembles that best fulfil the experimental data. Using one-bond RDCs, the approach has been implemented in the force field procedures of the program COSMOS and extensively tested. A concise theoretical introduction, including the special treatment of force fields for stable and fast MD simulations, as well as applications regarding configurational analyses of small to medium-sized organic molecules with different degrees of flexibility, is given. The observed results are discussed in detail.

Molecular Dynamics with Orientational Tensorial Constraints: A New Approach to Probe the Torsional Angle Distributions of Small Rotationally Flexible Molecules

The potential of residual dipolar couplings (RDCs) in conformational studies of small molecules is now widely recognized, but current theoretical approaches for their interpretation have several limitations and there is still the need for a general method to probe the torsional angle distributions applicable to any rotationally flexible molecule. Molecular dynamics simulations with RDC-based orientational tensorial constraints (MDOC), implemented in the software COSMOS, are presented here as a conceptually new strategy. For the cases of the fluorinated anti-inflammatory drug diflunisal and the disaccharide cellobiose, we demonstrate that MDOC simulations with one-bond RDCs as tensorial constraints unveil torsion distributions and allow the determination of relative configuration in the presence of rotational flexibility. The independence of the initial structure or any a priori assumption as well as the possibility to combine different experimental constraints represent features, which make the COSMOS software a promising tool for the investigation of torsional angle distributions of flexible molecules, regardless of their size and degree of freedom.

Structure elucidation and biosynthetic gene cluster analysis of caniferolides A–D, new bioactive 36-membered macrolides from the marine-derived Streptomyces caniferus CA-271066

The structures of caniferolides A–D have been determined combining NMR and bioinformatics prediction of the absolute configuration.

Residual Dipolar Couplings in Structure Determination of Natural Products

The determination of natural products stereochemistry remains a formidable task. Residual dipolar couplings (RDCs) induced by anisotropic media are a powerful tool for determination of the stereochemistry of organic molecule in solution. This review will provide a short introduction on RDCs-based methodology for the structural elucidation of natural products. Special attention is given to the current availability of alignment media in organic solvents. The applications of RDCs for structural analysis of some examples of natural products were discussed and summarized. This review provides a short introduction on RDCs-based methodology for the structural elucidation of natural products. Special attention is given to the current availability of alignment media in organic solvents. The applications of RDCs for structural analysis of some examples of natural products were discussed and summarized.

Determination of the conformational states of strychnine in solution using NMR residual dipolar couplings in a tensor-free approach

Small molecules with rotatable bonds can occupy different conformational states in solution as a consequence of their thermal fluctuations. The accurate determination of the structures of such states, as well as of their statistical weights, has been challenging because of the technical difficulties in extracting information from experimental measurements, which are normally averaged over the conformational space available. Here, to achieve this objective, we present an approach based on a recently proposed tensor-free method for incorporating NMR residual dipolar couplings as structural restraints in replica-averaged molecular dynamics simulations. This approach enables the information provided by the experimental data to be used in the spirit of the maximum entropy principle to determine the structural ensembles of small molecules. Furthermore, in order to enhance the sampling of the conformational space we incorporated the metadynamics method in the simulations. We illustrate the method in the case of strychnine, determining the three major conformational states of this small molecule and their associated occupation probabilities.

Progressive Stereo Locking (PSL): A Residual Dipolar Coupling Based Force Field Method for Determining the Relative Configuration of Natural Products and Other Small Molecules

Establishing the relative configuration of a bioactive natural product represents the most challenging part in determining its structure. Residual dipolar couplings (RDCs) are sensitive probes of the relative spatial orientation of internuclear vectors. We adapted a force field structure calculation methodology to allow free sampling of both R and S configurations of the stereocenters of interest. The algorithm uses a floating alignment tensor in a simulated annealing protocol to identify the conformations and configurations that best fit experimental RDC and distance restraints (from NOE and J-coupling data). A unique configuration (for rigid molecules) or a very small number of configurations (for less rigid molecules) of the structural models having the lowest chiral angle energies and reasonable magnitudes of the alignment tensor are provided as the best predictions of the unknown configuration. For highly flexible molecules, the progressive locking of their stereocenters into their statistically dominant R or S state dramatically reduces the number of possible relative configurations. The result is verified by checking that the same configuration is obtained by initiating the locking from different regions of the molecule. For all molecules tested having known configurations (with conformations ranging from mostly rigid to highly flexible), the method accurately determined the correct configuration.

Total syntheses of the archazolids: an emerging class of novel anticancer drugs

V-ATPase has recently emerged as a promising novel anticancer target based on extensive in vitro and in vivo studies with the archazolids, complex polyketide macrolides which present the most potent V-ATPase inhibitors known to date, rendering these macrolides important lead structures for the development of novel anticancer agents. The limited natural supply of these metabolites from their myxobacterial source renders total synthesis of vital importance for the further preclinical development. This review describes in detail the various tactics and strategies employed so far in archazolid syntheses that culminated in three total syntheses and discusses the future synthetic challenges that have to be addressed.

Complete NMR assignment and conformational analysis of 17-α-ethinylestradiol by using RDCs obtained in grafted graphene oxide

The ¹ H and ¹³ C NMR spectra of 17-α-ethinylestradiol (EE2), a well-known contraceptive, including diastereotopic methylene groups, were fully assigned with the help of residual dipolar couplings (RDC) measured in the recently developed grafted graphene oxide orienting medium. RDC analysis, which included all ¹ D_CH couplings and the long-range ² D_CH¹ H-C≡¹³ C coupling, also pointed to the presence of a minor conformation arising from pseudo-rotation of the steroid B ring. Saturation-transfer difference (STD) measurements revealed that the most likely interaction between EE2 and orienting medium occurred on the C and D ring. Copyright © 2016 John Wiley & Sons, Ltd.

Conformational analysis of small organic molecules using NOE and RDC data: A discussion of strychnine and α-methylene-γ-butyrolactone

To understand the properties and/or reactivity of an organic molecule, an understanding of its three-dimensional structure is necessary. Simultaneous determination of configuration and conformation often poses a daunting challenge. Thus, the more information accessible for a given molecule, the better. Additionally to (3)J-couplings, two sources of information, quantitative NOE and more recently also RDCs, are used for conformational analysis by NMR spectroscopy. In this paper, we compare these sources of conformational information in two molecules: the configurationally well-characterized strychnine 1, and the only recently configurationally and conformationally characterized α-methylene-γ-butyrolactone 2. We discuss possible sources of error in the measurement and analysis process, and how to exclude them. By this means, we are able to bolster the previously proposed flexibility for these two molecules.

Long-Range Residual Dipolar Couplings: A Tool for Determining the Configuration of Small Molecules

Together with NOE and J coupling, one-bond residual dipolar coupling (RDC), which reports on the three-dimensional orientation of an internuclear vector in the molecular frame, plays an important role in the conformation and configuration analysis of small molecules in solution by NMR spectroscopy. When the molecule has few C-H bonds, or too many bonds are in parallel, the available RDCs may not be sufficient to obtain the alignment tensor used for structure elucidation. Long-range RDCs that connect nuclei over multiple bonds are normally not parallel to the single bonds and therefore complement one-bond RDCs. Herein we present a method for extracting the long-range RDC of a chosen proton or group of protons to all remotely connected carbon atoms, including non-protonated carbon atoms. Alignment tensors fitted directly to the total long-range couplings (T=J+D) enabled straightforward analysis of both the long-range and one-bond RDCs for strychnine.

Machine learning estimates of natural product conformational energies

Machine learning has been used for estimation of potential energy surfaces to speed up molecular dynamics simulations of small systems. We demonstrate that this approach is feasible for significantly larger, structurally complex molecules, taking the natural product Archazolid A, a potent inhibitor of vacuolar-type ATPase, from the myxobacterium Archangium gephyra as an example. Our model estimates energies of new conformations by exploiting information from previous calculations via Gaussian process regression. Predictive variance is used to assess whether a conformation is in the interpolation region, allowing a controlled trade-off between prediction accuracy and computational speed-up. For energies of relaxed conformations at the density functional level of theory (implicit solvent, DFT/BLYP-disp3/def2-TZVP), mean absolute errors of less than 1 kcal/mol were achieved. The study demonstrates that predictive machine learning models can be developed for structurally complex, pharmaceutically relevant compounds, potentially enabling considerable speed-ups in simulations of larger molecular structures.

Molecular dynamics simulations provide insight into the dynamic behavior of molecules, e.g., into the adopted spatial arrangements of its atoms over time. Methods differ in the approximations they employ, resulting in a trade-off between accuracy and speed that ranges from highly accurate but expensive quantum mechanical calculations to fast but more inaccurate molecular mechanics force fields. Machine learning, a sub-discipline of artificial intelligence, provides algorithms that learn from data, that is, make predictions based on previously seen examples. By starting with a few expensive quantum mechanical calculations, training a machine learning algorithm on them, and then using the resulting model to carry out the molecular dynamics simulation, one can improve the accuracy/speed trade-off. We have developed and applied such a hybrid quantum mechanics/machine learning approach to Archazolid A, a natural product from the myxobacterium Archangium gephyra and a potent inhibitor of vacuolar-type ATPase. By dynamically refining our model over the course of the simulation, we achieve errors of less than 1 kcal/mol while saving over 40% of the quantum mechanical calculations. Our study demonstrates the feasibility of predictive machine learning models for the dynamics of structurally complex, pharmaceutically relevant compounds, potentially enabling considerable speed-ups in simulations of even larger biomolecular structures.

Long-range proton-carbon coupling constants: NMR methods and applications

A general review of novel NMR methods to measure heteronuclear long-range proton-carbon coupling constants ((n)JCH; n>1) in small molecules is made. NMR experiments are classified in terms of NMR pulse scheme and cross-peak nature. A discussion about simplicity, general applicability and accuracy for each particular NMR experiment is presented and exemplified. Important aspects such as the sign determination and measurement of very small coupling values involving protonated and non-protonated carbons as well as the complementarity between different experiments are also discussed. Finally, a compilation of applications in structural and conformational analysis of different types of molecules since 2000 is surveyed.

Quantification of free ligand conformational preferences by NMR and their relationship to the bioactive conformation

Accurate unbound solution 3D-structures of ligands provide unique opportunities for medicinal chemistry and, in particular, a context to understand binding thermodynamics and kinetics. Previous methods of deriving these 3D-structures have had neither the accuracy nor resolution needed for drug design and have not yet realized their potential. Here, we describe and apply a NMR methodology to the aminoglycoside streptomycin that can accurately quantify accessible 3D-space and rank the occupancy of observed conformers to a resolution that enables medicinal chemistry understanding and design. Importantly, it is based upon conventional small molecule NMR techniques and can be performed in physiologically-relevant solvents. The methodology uses multiple datasets, an order of magnitude more experimental data than previous NMR approaches and a dynamic model during refinement, is independent of computational chemistry and avoids the problem of virtual conformations. The refined set of solution 3D-shapes for streptomycin can be grouped into two major families, of which the most populated is almost identical to the 30S ribosomal subunit bioactive shape. We therefore propose that accurate unbound ligand solution conformations may, in some cases, provide a subsidiary route to bioactive shape without crystallography. This experimental technique opens up new opportunities for drug design and more so when complemented with protein co-crystal structures, SAR data and pharmacophore modeling.

RDC-based determination of the relative configuration of the fungicidal cyclopentenone 4,6-diacetylhygrophorone A12

The hygrophorones, a class of cyclopentenones isolated from fruiting bodies of the genus Hygrophorus (basidiomycetes), show promising antifungal activity. While the constitution of 4,6-diacetylhygrophorone A(12) (3) and the relative configuration of the stereogenic centers in the cyclopentenone ring were elucidated using standard NMR and MS techniques, the relative configuration of the exocyclic stereogenic center could not be assigned. By introducing a sample of 3 into an alignment medium and measuring anisotropic NMR parameters, namely, residual dipolar couplings, we were able to unambiguously determine the relative configuration of all three stereogenic centers in 4,6-diacetylhygrophorone A(12) simultaneously by fitting several structure proposals to the experimental data.

Fast access to residual dipolar couplings by single-scan 2D NMR in oriented media

Residual dipolar couplings (RDCs) have revolutionized the structure determination of biomolecular and organic compounds. So far, their measurement has been rather time-consuming, but one might imagine that RDCs can one day also be useful in the investigation of compounds with limited stability or short lifetimes. For such applications, it is indispensable to shorten the experiment time. In this communication, we show the first measurement of RDCs from single-scan two-dimensional NMR. An ultrafast HSQC NMR pulse sequence is presented, which includes several of the recent improvements brought to ultrafast NMR in terms of sensitivity, resolution, and spectral width. Ultrafast spectra are obtained in as little time as 60 s on an organic compound at natural abundance, namely (+)-isopinocampheol. When extracting the RDCs from these ultrafast data, a good agreement with those extracted from conventional spectra (obtained in a much longer time) is observed. These results point out the efficiency of the ultrafast approach, particularly when considering the total experiment duration.

Configuration verification via RDCs on the example of a tetra-substituted pyrrolidine ring

The configurational analysis of organic compounds is an important application for high resolution NMR spectroscopy. In the present study, a tetra-substituted pyrrolidine with four chiral carbon atoms is analyzed using classical methods based on (3) J and NOE data in solution and compared and verified with recently introduced alternative approaches via residual dipolar couplings (RDCs) in two weak anisotropic alignment media. The molecule shows sufficient rigidity in the five-membered ring for the configurational characterization with the various techniques. However, the flexibility caused by the many freely rotating bonds potentially poses problems for the interpretation of data. It is shown that RDCs measured in poly-γ-benzyl-l-glutamate and a stretched polydimethylsiloxane gel provide useful information for the distinction of diastereomers, but the success varies with the data interpretation strategy used. Although a general improvement of corresponding correlation factors is observed when limiting data to a subset of dipolar couplings directly connected to the central ring, the distinction power is reduced because of the smaller number of RDCs available for potential model falsification. Singular value decomposition for fitting experimental RDCs is able to distinguish in most cases the correct from incorrect configurations, but the differences in correlation factors can be relatively small. Surprisingly, predicting RDCs using the rod model as implemented in PALES gives best results in distinguishing the eight possible diastereomers. It is also found that the use of proton-phosphorus and carbon-phosphorus RDCs helps with the configurational analysis of the model compound.

MSpin-RDC. A program for the use of residual dipolar couplings for structure elucidation of small molecules

We developed a new program, MSpin-RDC for the analysis of residual dipolar coupling data. This software, specially designed for small molecule analysis, can directly read many molecular-modeling and popular chemistry file formats and accept RDC values as a simple free-format table. Alignment tensor can then be computed by singular value decomposition, as well as predicted using inertia and gyration tensor-based methodologies. Trial structures are then ranked according to their Cornilescu's quality factor (Q) values. Analysis of multiconformational problems and fitting of RDC data to relative populations can be accomplished using the single-tensor approximation.

Influence of solvent and salt concentration on the alignment properties of acrylamide copolymer gels for the measurement of RDC

The dependence of molecular alignment with solvent nature and salt concentration has been investigated for mechanically stretched polyacrylamide copolymer gels. Residual dipolar couplings (RDCs) were recorded for D(2)O, DMSO-d(6), and DMSO-d(6)/D(2)O solutions containing different proportions of the solvents and different sodium chloride concentrations. Alignment tensors were determined by fitting the experimental RDCs to the DFT-computed structure of N-methylcodeinium ion. Analysis of the tensors shows that the degree of alignment decreases with the proportion of DMSO-d(6) as well as with the concentration of sodium chloride, most likely due to enhanced ion-pair aggregation. Furthermore, rotation of the alignment tensor is observed when increasing the salt concentration.

NMR in natural products: understanding conformation, configuration and receptor interactions

Covering: up to 2011. Natural products are of tremendous importance in both traditional and modern medicine. For medicinal chemistry natural products represent a challenge, as their chemical synthesis and modification are complex processes, which require many, often stereo-selective, synthetic steps. A prerequisite for the design of analogs of natural products, with more accessible synthetic routes, is the availability of their bioactive conformation. Nuclear Magnetic Resonance (NMR) spectroscopy and X-ray crystallography are the two techniques of choice to investigate the structure of natural products. In this review, I describe the most recent advances in NMR to study the conformation of natural products either free in solution or bound to their cellular receptors. In chapter 2, I focus on the use of residual dipolar couplings (RDC). On the basis of a few examples, I discuss the benefit of complementing classical NMR parameters, such as NOEs and scalar couplings, with dipolar couplings to simultaneously determine both the conformation and the relative configuration of natural products in solution. Chapter 3 is dedicated to the study of the structure of natural products in complex with their cellular receptors and is further divided in two sections. In the first section, I describe two solution-state NMR methodologies to investigate the binding mode of low-affinity ligands to macromolecular receptors. The first approach, INPHARMA (Interligand Noes for PHArmacophore Mapping), is based on the observation of interligand NOEs between two small molecules binding competitively to a common receptor. INPHARMA reveals the relative binding mode of the two ligands, thus allowing ligand superimposition. The second approach is based on paramagnetic relaxation enhancement (PRE) of ligand resonances in the presence of a receptor containing a paramagnetic center. In the second section, I focus on solid-state NMR spectroscopy as a tool to access the bioactive conformation of natural products in complex with macromolecular receptors.

Challenge of large-scale motion for residual dipolar coupling based analysis of configuration: the case of fibrosterol sulfate A

Fibrosterol sulfate A is a polysulfated bis-steroid with an atypical side chain. Due to the flexibility of the linker, large-scale motions that change dramatically the shape of the entire molecule are expected. Such motions pose major challenges to the structure elucidation and the correct determination of configuration. In this study, we will describe the determination of the relative configuration of fibrosterol sulfate A through a residual dipolar coupling based multiple alignment tensor analysis complemented by molecular dynamics. For completeness, we applied also the single tensor approach which is unreliable due to the large-scale motions and compare the results.

Archazolid A-15-O-β-D-glucopyranoside and iso-archazolid B: potent V-ATPase inhibitory polyketides from the myxobacteria Cystobacter violaceus and Archangium gephyra

Two structurally novel analogues of the macrolides archazolids A and B, archazolid A-15-O-β-D-glucopyranoside (archazolid E, 5) and iso-archazolid B (archazolid F, 6), were isolated from the myxobacterium Cystobacter violaceus and Archangium gephyra, respectively. Macrolactone 5 represents the first 15-O-glycoside of the archazolids. iso-Archazolid B (6) incorporates a C-3 alkene and presents the first constitutional isomer reported for this natural product class. The structures of these polyketides were determined by spectroscopic analysis, in particular by HMBC, HMQC, and ROESY NMR investigations and by chemical degradation. iso-Archazolid B (6) demonstrated extremely high antiproliferative and V-ATPase inhibitory effects, with IC(50) values in the picomolar range, while only moderate activity was observed for glycoside 5. iso-Archazolid B presents the most potent archazolid known.

Variable angle NMR spectroscopy and its application to the measurement of residual chemical shift anisotropy

The successful measurement of anisotropic NMR parameters like residual dipolar couplings (RDCs), residual quadrupolar couplings (RQCs), or residual chemical shift anisotropy (RCSA) involves the partial alignment of solute molecules in an alignment medium. To avoid any influence of the change of environment from the isotropic to the anisotropic sample, the measurement of both datasets with a single sample is highly desirable. Here, we introduce the scaling of alignment for mechanically stretched polymer gels by varying the angle of the director of alignment relative to the static magnetic field, which we call variable angle NMR spectroscopy (VA-NMR). The technique is closely related to variable angle sample spinning NMR spectroscopy (VASS-NMR) of liquid crystalline samples, but due to the mechanical fixation of the director of alignment no sample spinning is necessary. Also, in contrast to VASS-NMR, VA-NMR works for the full range of sample inclinations between 0° and 90°. Isotropic spectra are obtained at the magic angle. As a demonstration of the approach we measure ¹³C-RCSA values for strychnine in a stretched PDMS/CDCl₃ gel and show their usefulness for assignment purposes. In this context special care has been taken with respect to the exact calibration of chemical shift data, for which three approaches have been derived and tested.

How different are diastereomorphous orientations of enantiomers in the liquid crystalline phases of PBLG and PBDG: a case study

The orientational properties of the two enantiomers of an example compound, namely isopinocampheol [(+)- and (-)-IPC] in the two enantiomers of a liquid crystalline phase, namely Poly-gamma-benzyl-L/D-glutamate (PBLG/PBDG) with the organic cosolvent CDCl(3), were investigated. The interactions can be either enantiomorphous, leading to equal orientations and residual dipolar couplings (RDCs), or diastereomorphous, leading to different orientations and RDCs. The difference between the two diastereomorphous orientations was determined to be rather small (5 degrees in the Euler angle beta). Furthermore, we investigated whether one of the two diastereomorphous interactions is favored.