Latest Research Output

Solution NMR methods for the study of ligand-nanocrystal interactions.

In the Journal of the American Chemical Society, we introduce transfer NOE spectroscopy for the characterization of ligand nanocrystal interactions in situ. This technique, well known from biomolecular NMR was ‘hijacked’ to the field of colloidal nanocrystals, and represents the latest addition to the NMR Toolbox that our group has developed in collaboration with the FCN group at UGent (Prof. Z. Hens). Using transfer NOE spectroscopy, information regarding which and to what extent ligands spend time at the nanocrystal surface can be obtained. In complex solvent mixtures, its potential as a screening method to detect binders from non binders is demonstrated.

The work complements techniques already developed by both groups for the case where ligand molecules are in no or slow exchange with the nanocrystal surface, work that culminated in the fall of 2008 with a report on the surface chemistry of PbSe colloidal nanocrystals, also published in the Journal of the American Chemical Society.
Fritzinger et al., J. Am. Chem. Soc. 131,3024-3032 (2009)
Moreels et al., J. Am. Chem. Soc. 130, 15081-15086 (2008)

Structure and Catalytic Performance of Indenylidene Complexes of Ruthenium with NHC ligands

In this years fifth issue of the European Journal of Organic Chemistry, the complete 1H, 13C and 31P resonance assignment and the procedure applied to obtain these from a combination of 1D and 2D NMR techniques, is also reported for 6 ruthenium complexes of interest for olefin metathesis reactions. Using ROESY several conformational processes involving rotations around N-phenyl and C–Ru bonds on the millisecond to second timescale have been characterized.

A clear correlation is demonstrated between the bulkiness of the axial ligand (L) and the rotational freedom of the SIMe(s) ligand. A qualitative analysis also suggests that the extra para-methyl of SIMes leads to additional steric interactions with the 3-phenylinden-1-ylidene ligand. The data demonstrates that the substitution patterns present on the N-heterocyclic carbene ligand have a significant influence on the activity of the second generation indenylidene catalysts for a given metathesis reaction. The work was performed in a collaboration with the COMOC group at UGent (Prof. F. Verpoort).
Monsaert el al. Eur. J. Org. Chem. 5, 655-665 (2009)

Rapid Identification of Common pyranose Monosaccharide Units by TOCSY Matching

A novel TOCSY-based matching scheme is presented as a new NMR tool for discrimination between glucose-, galactose- and mannose-type stereochemistries in hexapyranoses.
Application of the scheme allows the assignment process of complex oligo- and polysaccharides to be directed. Moreover, this approach allows simple and sensitive discrimination between the a- and b-stereochemistry in D-mannose and its derivatives, a feat otherwise difficult to achieve by NMR.

Gheysen, K., Mihai, C., Conrath, K., and , Martins, J. C.
Chem. Eur. J. 14, 8869-8878 (2008)

How low can you go

The double sandwich ELISE technique for sample volume restriction, developed by the Lippens NMR group at Lille I University, allows to restrict the sample of interest to the actual detection volume. Together with the Lille group, the 1mm probe on the Interuniversitary NMR Facility hosted by our lab, is used to establish an unofficial world record : measurement of a 1H-15N HSQC of 8 mg of labeled Cyclophilin B protein. This represents less than 1 nanomole of material. In the double sandwich ELISE, the volume used to dissolve the compound of interest is limited to the detection volume by sandwiching the aqueous solution of interest between chloroform (density larger than water) and mineral oil (density smaller than water). This is nicely seen in the photograph, where the aqueous phase was coloured with a blue dye. An alternative view of the 1.0 µL aqueous sample volume is provided via imaging methods,as seen in the 3D representations below.

Wieruszeski, J. M., Fritzinger, B., Hanoulle X., Martins, J. C. and Lippens, G.
J. Magn. Reson. 193, 37-40 (2008)

‘Click’ functionalization of Cryogels Conveniently Verified and Quantified Using
High-Resolution MAS NMR

Chemical modification reactions of alkyne containing polyHEMA-based macroporous network structures (cryogels) by Cu(I) catalyzed azide-alkyne “click” cycloaddition reactions and their monitoring and quantification with high-resolution magic angle spinning (hr-MAS) NMR spectroscopy are reported. Complete conversion is obtained when benzylazide is reacted with the grafted alkyne function, but only partial conversion is observed when using azide-modified poly(ethylene glycol) (PEG-N3). Subsequent addition of benzylazide consumes all remaining alkyne groups. All chemical modifications are easily monitored at each stage using hr-MAS NMR. The alkyne functionality and the resulting triazole ring provide well resolved 1H resonances to monitor and quantify the progress of such click reactions in general.

Van Camp, W.; Dispinar, T.; Dervaux, B.; Du Prez, F. E.; Martins, J. C.; Fritzinger, B.
Macromolecular Rapid Communications , 30, 1328-1333 (2009)

Structure and X-ray conformation of pseudodesmins A and B, two new cyclic lipodepsipeptides from Pseudomonas bacteria

Two new cyclic lipodepsipeptides named pseudodesmins A and B have been isolated from Pseudomonas bacteria collected from the mucus layer in the skin of the black belly salamander. Both compounds show moderate antibacterial activity against Gram positive bacteria, including MRSA. Complete 1H, 13C and 15N NMR assignment of both compounds afforded their covalent structure and served to guide the analysis of LC-MS and X-ray diffraction data from which the final stereochemistry could be established. Both molecules Call be categorized as new members of the viscosin group of cyclic lipodepsipeptides.

Sinnaeve, D.; Michaux, C.; Van Hemel, J.; Vandenkerckhove, J.; Peys, E.; Borremans, F. A. M.; Sas, B.; Wouters, J.; Martins, J. C. Tetrahedron, 65, 4173-4181, (2009) , DOI: 10.1016/j.tet.2009.03.045

The use of time-averaged 3JHH restrained molecular dynamics (tar-MD) simulations for the conformational analysis of five-membered ring systems: Methodology and applications

Because of its presence in many molecules of biological relevance, the conformational analysis of five-membered rings using 3JHH scalar coupling data from NMR is a topic of considerable interest. Typically, conformational analysis involves the use of a well-established mathematical procedure, originally developed by de Leeuw et al., that fits two rigid conformations to the available experimental data. This so-called pseudorotation analysis approach is not without problems, however, as chemically unrealistic conformations are sometimes generated from the data. Here, we present our investigations in the use of time-averaged restrained molecular dynamics simulations as a generic tool to determine the conformations that agree with experimental 3JHH scalar coupling data. For this purpose, a set of six ribose-based molecules has been used as model compounds. The influence of several modeling parameters is assessed and optimized values are proposed. The results obtained with the tar-MD approach are compared to those obtained from the two conformer fitting procedure. Interpretation of the latter is facilitated by the introduction of a fitting error analysis that allows mapping the solution space of the fitting procedure. The relative merits of both methods and the advantages that result from the use of a force field and a time-averaged restraint potential for the experimental data are discussed. When combined, both techniques allow an enhanced understanding of the molecules' conformational behavior and prevent possible overinterpretation. In view of the very reasonable computational burden of a tar-MD simulation for the systems investigated here, the approach should be generally applicable.

Pieter M. S. Hendrickx, Francisco Corzana, Stefaan Depraetere, Dirk A. Tourwé, Koen Augustyns, José C. Martins
J. Comput. Chem. 2009 (In Press) DOI:10.1002/jcc.21345

The Solution Structure and Self-Association Properties of the Cyclic Lipodepsipeptide Pseudodesmin A Support Its Pore-Forming Potential

Pseudodesmin A is a cyclic lipodepsipeptide (CLP) of the viscosin group with a moderate in vitro biological activity. For several CLPs, including members of this group, this activity has been related to the ability to form ion pores in cellular membranes. As their size does not allow individual CLPs to span the membrane bilayer, individual monomers must somehow assemble into a larger structure. Here, NMR has been used to demonstrate that in chloroform and other apolar organic solvents, pseudodesmin A monomers assemble into a supramolecular structure. These self-assembled structures can become sufficiently large to span the membrane bilayer as demonstrated with translational diffusion NMR measurements. With the aim to obtain more insight into the structural nature of this assembly, the solution conformation of pseudodesmin A was first determined using rOe restraints measured in acetonitrile, where no self-association occurs. The structure, which is found to be mostly similar to the previously described crystal structure, is shown to be retained within the supramolecular complex. Intermolecular rOe contacts obtained in chloroform together with chemical shift perturbation data provides structural insight into the organization of the self-associated complex. Based upon this analysis, a model for the organization of pseudodesmin A monomers in the supramolecular assembly is proposed, which is in agreement with the formation of bilayer spanning hydrophilic pores, providing the basis for a structure-function relationship for this type of CLPs. Finally, it is demonstrated that the differences previously reported between the crystal and solution conformation of WLIP, a close analogue of pseudodesmin A, are the result of the use of DMSO as solvent, whose strong hydrogen bonding capacity induces conformational exchange.

Sinnaeve, D.; Hendrickx P.M.S.; Van hemel, J.; Peys, E.; Kieffer, B.; Martins, J. C. Chemistry – A European Journal, 15, 12653-12662, (2009), DOI: 10.1002/chem.200901885

Widening the View on Dispersant-Pigment Interactions in Colloidal Dispersions with Saturation Transfer Difference NMR Spectroscopy

The application of Saturation Transfer Difference (STD) NMR spectroscopy for the characterization of dispersant-particle interactions is introduced. STD NMR has hitherto been applied, with great success, to the characterization of ligand−protein interactions and is currently a standard tool in biomolecular NMR spectroscopy. Nevertheless, the STD NMR technique has so far not yet crossed the boundaries of the biomolecular field. Here, we demonstrate that in spite of clear differences between a protein binding site and the surface of a pigment nanoparticle, the latter can also be subjected to STD NMR analysis, allowing us to detect (screen for) binding ligands, discriminate ligand from nonligand, and obtain information on the binding epitope. The approach should be generally applicable as long as the nanoparticle is comprised of a dense network of hydrogens, implicating almost all organic molecular nanocrystals. Thus it provides a novel investigative tool for the study of dispersions that is highly complementary to existing ones.

Agnieszka Szczygiel, Leo Timmermans, Bernd Fritzinger, and José C. Martins (J.Am.Chem.Soc. 2009) publication ASAP,DOI: 10.1021/ja905637y