Jonathan Farjon

Enantiomeric excess measurements in weakly oriented chiral liquid crystal solvents through 2D 1H selective refocusing experiments

Abstract In this article, a simple and robust method is proposed for simplifying the analysis of proton spectra of molecules dissolved in weakly oriented chiral media. The NMR approach investigated is based on the use of proton selective refocusing 2D experiments (SERF) to measure proton–proton dipolar couplings from unresolved lines. This technique is applied to the case of enantiomers dissolved in chiral polypeptide liquid crystals. It is shown that an accurate determination of enantiomeric excess is possible within a short experimental time.

Longitudinal-relaxation-enhanced NMR experiments for the study of nucleic acids in solution.

Atomic-resolution information on the structure and dynamics of nucleic acids is essential for a better understanding of the mechanistic basis of many cellular processes. NMR spectroscopy is a powerful method for studying the structure and dynamics of nucleic acids; however, solution NMR studies are currently limited to relatively small nucleic acids at high concentrations. Thus, technological and methodological improvements that increase the experimental sensitivity and spectral resolution of NMR spectroscopy are required for studies of larger nucleic acids or protein-nucleic acid complexes. Here we introduce a series of imino-proton-detected NMR experiments that yield an over 2-fold increase in sensitivity compared to conventional pulse schemes. These methods can be applied to the detection of base pair interactions, RNA-ligand titration experiments, measurement of residual dipolar (15)N-(1)H couplings, and direct measurements of conformational transitions. These NMR experiments employ longitudinal spin relaxation enhancement techniques that have proven useful in protein NMR spectroscopy. The performance of these new experiments is demonstrated for a 10 kDa TAR-TAR*(GA) RNA kissing complex and a 26 kDa tRNA.

Iron Coordination Chemistry with New Ligands Containing Triazole and Pyridine Moieties. Comparison of the Coordination Ability of the N-Donors

We report the synthesis, characterization, and solution chemistry of a series of new Fe(II) complexes based on the tetradentate ligand N-methyl-N,N-bis(2-pyridyl-methyl)-1,2-diaminoethane or the pentadentate ones N,N,N-tris(2-pyridyl-methyl)-1,2-diaminoethane and N,N,N-tris(2-pyridyl-methyl)-1,3-diaminopropane, modified by propynyl or methoxyphenyltriazolyl groups on the amino functions. Six of these complexes are characterized by X-ray crystallography. In particular, two of them exhibit an hexadentate coordination environment around Fe(II) with two amino, three pyridyl, and one triazolyl groups. UV-visible and cyclic voltammetry experiments of acetonitrile solutions of the complexes allow to deduce accurately the structure of all Fe(II) species in equilibrium. The stability of the complexes could be ranked as follows: [L(5)Fe(II)-py](2+) > [L(5)Fe(II)-Cl](+) > [L(5)Fe(II)-triazolyl](2+) > [L(5)Fe(II)-(NCMe)](2+), where L(5) designates a pentadentate coordination sphere composed of the two amines of ethanediamine and three pyridines. For complexes based on propanediamine, the hierarchy determined is [L(5)Fe(II)-Cl](+) > [L(5)Fe(II)(OTf)](+) > [L(5)Fe(II)-(NCMe)](2+), and no ligand exchange could be evidenced for [L(5)Fe(II)-triazolyl](2+). Reactivity of the [L(5)Fe(II)-triazolyl](2+) complexes with hydrogen peroxide and PhIO is similar to the one of the parent complexes that lack this peculiar group, that is, generation of Fe(III)(OOH) and Fe(IV)(O), respectively. Accordingly, the ability of these complexes at catalyzing the oxidation of small organic molecules by these oxidants follows the tendencies of their previously reported counterparts. Noteworthy is the remarkable cyclooctene epoxidation activity by these complexes in the presence of PhIO.

Fully resolved NMR correlation spectroscopy.

A new correlation experiment cited as push-G-SERF is reported. In the resulting phased 2D spectrum, the chemical shift information is selected along the direct dimension, whereas scalar couplings involving a selected proton nucleus are edited in the indirect domain. The robustness of this pulse sequence is demonstrated on compounds with increasing structural and spectral complexity, using state-of-the-art spectrometers. It allows for full resolution of both dimensions of the spectrum, yielding a straightforward assignment and measurement of the coupling network around a given proton in the molecule. This experiment is intended for chemists who want to address efficiently the structural analysis of molecules with an overcrowded spectrum.

Effect of the Solvent on the Conformation of a Depsipeptide: NMR-Derived Solution Structure of Hormaomycin in DMSO from Residual Dipolar Couplings in a Novel DMSO-Compatible Alignment Medium.

The macrocyclic compound hormaomycin has been investigated by NMR spectroscopy and by restrained molecular‐dynamics simulations. Measurement of residual dipolar couplings induced by dissolving the depsipeptide in a polyacrylamide gel compatible with DMSO and their incorporation into the structure calculation of the title compound improved the precision of the family of structures. In DMSO the macrocyclic ring shows two β‐turns, whose positions in the sequence differ from those found in the CDCl3 solution structure and in the crystal structure obtained from hexylene glycol/H2O (50:50). The bulky side chain consisting of a 3‐(2‐nitrocyclopropyl)alanine and a chlorinated N‐hydroxypyrrole moiety is flexible in DMSO.

Fine Tuning of β‐Peptide Foldamers: a Single Atom Replacement Holds Back the Switch from an 8‐Helix to a 12‐Helix

Cyclic homologated amino acids are important building blocks for the construction of helical foldamers. N-aminoazetidine-2-carboxylic acid (AAzC), an aza analogue of trans-2-aminocyclobutanecarboxylic acid (tACBC), displays a strong hydrazino turn conformational feature, which is proposed to act as an 8-helix primer. tACBC oligomers bearing a single N-terminal AAzC residue were studied to evaluate the ability of AAzC to induce and support an 8-helix along the oligopeptide length. While tACBC homooligomers assume a dominant 12-helix conformation, the aza-primed oligomers preferentially adopt a stabilized 8-helix conformation for an oligomer length up to 6 residues. The (formal) single-atom exchange at the Nu2005terminus of a tACBC oligomer thus contributes to the sustainability of the 8-helix, which resists the switch to a 12-helix. This effect illustrates atomic-level programmable design for fine tuning of peptide foldamer architectures.

Selective NMR Excitations in Chiral Analysis

NMR spectroscopy using chiral liquid crystals as solvents is a powerful tool to visualize enantiomers. Using these oriented solvents, recent NMR studies with selective excitation have proven their effectiveness at simplifying enantiomeric visualization and enantiomeric excess determination through 1 H or 13 C spectra. In this review we present different straightforward and robust selective techniques dedicated to the spectral analysis of enantiomers dissolved in such chiral-oriented solvents.

Solution state conformational preferences of dipeptides derived from N-aminoazetidinecarboxylic acid: an assessment of the hydrazino turn.

Four model compounds and four dipeptides containing N-aminoazetidinecarboxylic acid (AAzC) and a particular stereoisomer of 2-aminocyclobutanecarboxylic acid (ACBC) were studied to establish their solution state conformational preferences, particularly regarding the ability of AAzC to induce a three-center hydrogen-bonded folding feature known as a hydrazino turn. On the basis of IR and NMR experiments, supported by molecular modeling, the AAzC residue adopted a trans configuration amenable to the formation of a cyclic eight-membered hydrogen bond conformation in solution, in all cases studied. The implication of the heterocyclic nitrogen atom of AAzC in the trans-like structure was demonstrated via a refined (1)H-(15)N HMBC experiment giving exploitable data at natural (15)N isotopic abundance, providing unprecedented evidence for the solution state hydrazino turn conformation. The predominance of this secondary structural feature depended on the configuration of the neighboring ACBC residue in the dipeptides: while the trans-ACBC derivatives prefer the hydrazino turn, the cis-ACBC derivatives may also populate low-energy 10-membered hydrogen-bonded ring structures. X-ray diffraction analysis of three compounds confirmed the presence of a solid state hydrazino turn in two cases, with geometries similar to those deduced from the solution state studies, but in the third compound, no intramolecular hydrogen-bonding feature was in evidence.

Synthesis of a Mycobacterium tuberculosis tetra-acylated sulfolipid analogue and characterization of the chiral acyl chains using anisotropic NAD 2D-NMR spectroscopy.

Tetra-O-acylated sulfolipids are metabolites found in the cell wall of Mycobacterium tuberculosis, the causative agent of tuberculosis. Their role in pathogenesis remains, however, undefined. Here we describe a novel access to model tetra-O-acylated trehalose sulfate derivatives having simple acyl chains. The trehalose core was regioselectively protected using a tandem procedure with catalytic iron(III) chloride hexahydrate and further desymmetrized. Model chiral fatty acids, prepared by a zinc-mediated cross-coupling, were incorporated into the trehalose core. The enantiomeric excess of the chiral fatty acids has been measured by natural abundance deuterium (NAD) 2D-NMR spectroscopy in a polypeptide based chiral liquid crystal. The synthetic approach established for the model compounds can easily be developed for the preparation of other analogues and natural sulfolipids.

SERF-filtered experiments: new enantio-selective tools for deciphering complex spectra of racemic mixtures dissolved in chiral oriented media.

The use of chiral liquid crystals as NMR solvent is a powerful tool to visualize and quantify enantiomers. This technique is much more robust than traditional liquid state techniques. Recent developments have been performed to simplify proton spectra obtained in these solvents which are usually complex to analyze. In the different published sequences the enantiomeric differentiation can be visualized but none of them rely the different (1)H-(1)H couplings to have a coherent set for each enantiomer especially in cases of racemic mixtures. In this work to overcome such limitations, we present a new NMR pulse sequence using semi-selective pulses to assign for each enantiomer the different (1)H-(1)H couplings. The aim is to obtain the signal of only one enantiomer, through a first Selective ReFocussing (SERF) filtering block implemented for the enantio-selection followed by a second SERF block for measuring a chosen coupling. The whole sequence is called (1)H SERF-filtered-SERF. These techniques permit for the first time to assign all dipolar couplings to each enantiomer of racemic mixtures dissolved in chiral oriented media. We demonstrate that using this experiment it is possible to visualize enantio-differentiation even in the case where the multiplets are not resolved in the SERF experiments, pushing back the limits of the enantiomeric visualization.