Detection of localized water clusters in a charged peptidyl resin.
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Nuclear Magnetic Resonance, Biomolecular↗
Biomedical subjects
G Chessari
Publications and source records attributed to G Chessari.
Quantitative determination of intermolecular interactions with fluorinated aromatic rings.
The chemical double mutant cycle approach has been used to investigate substituent effects on intermolecular interactions between aromatic rings and pentafluorophenyl pi-systems. The complexes have been characterised using 1H and 19F NMR titrations, X-ray crystal structures of model compounds and molecular mechanics calculations. In the molecular zipper system used for these experiments, H-bonds and the geometries of the interacting surfaces favour the approach of the edge of the aromatic ring with the face of the pentafluorophenyl pi-system. The interactions are generally repulsive and this repulsion increases with more electron-withdrawing substituents up to a limit of +2.2 kJ mol(-1), when the complex distorts to minimise the unfavourable interaction. Strongly electron-donating groups cause a change in the geometry of the aromatic interaction and attractive stacking interactions are found (-1.6 kJ mol(-1) for NMe2). These results are generally consistent with an electrostatic model: the polarisation of the pentafluorophenyl ring leads to a partial positive charge located at the centre and this leads to repulsive interactions with the positive charges on the protons on the edge of the aromatic ring; when the aromatic ring has a high pi-electron density there is a large electrostatic driving force in favour of the stacked geometry which places this pi-electron density over the centre of the positive charge on the pentafluorophenyl group.
Crystallography, X-Ray↗
Rotor synchronization of radiofrequency and gradient pulses in high-resolution magic angle spinning NMR.
We have investigated the extent to which rotor synchronization of radiofrequency pulses leads to spectral improvement in high-resolution magic angle spinning NMR experiments. Several pulse sequences were tested, and the effect was found to be maximal in homonuclear TOCSY spectra. The physicochemical nature of the sample plays a role in the phenomenon, as rotor synchronization allows the refocusing of residual anisotropic interactions. However, even in a liquid sample the effects were visible. Radial inhomogeneities of the radiofrequency field were identified as an important source of the problem.
Magnetic Resonance Spectroscopy↗
High resolution magic angle spinning NMR in combinatorial chemistry.
Solid phase organic chemistry coupled with combinatorial methods promises to increase dramatically the diversity and number of small molecules available for medical and biological applications. However, optimizing the reaction conditions can be a time consuming step, especially since analytical tools to monitor reaction progress and detect impurities for solid phase chemistry are less developed than for solution chemistry. The use of high resolution magic angle spinning (HRMAS) NMR is described here as such an analytical tool. Whereas initial applications of molecular identification using deuterated organic solvents to swell the resins presented a significant gain in time over the cleave-and-analysis methods, the introduction of a differential diffusion filter has made immediate recording of spectra possible without any sample treatment. The applications of HRMAS NMR to different solid supports that are used in combinatorial chemistry will be described in terms of rapidity, robustness and sensitivity.
Chemistry, Organic↗
Singly bridged calix
Crowned calix[8]arenes are obtained by direct alkylation of p-tert-butylcalix[8]arene (1) with poly(ethylene glycol) ditosylates in the presence of various bases. K2CO3 promotes the preferential formation of 1,3-calix[8]crowns. Cs2CO3 mainly gives the 1,5-isomers, which are selectively obtained in high yields when shorter chains are used (1,5-crown-2, 88%; 1,5-crown-3, 78%). NaH affords the 1,4-isomers in yields up to 46%, often as the sole crown derivative, besides unreacted 1. 1,2-Calix[8]crowns are also obtained in appreciable amount in some instances. The observed regioselectivity is rationalized in terms of preferential formation of specific anions in dependence of the base strength. Dynamic NMR and modeling studies prove that the polyether chain, depending on its bridging mode, may significantly reduce the available space for the through the annulus passages leading to derivatives conformationally blocked (on the NMR time scale).
Journal Article↗