Shasad Sharif

Pyridine-15N: A mobile NMR sensor for surface acidity and surface defects of mesoporous silica

The hydrogen bond interaction of pyridine with the silanol groups of the inner surfaces of MCM-41 and SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The pore diameters were varied between 3 and 4 nm for MCM-41 and between 7 and 9 nm for SBA-15. 1 H MAS experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d 5 reveal that the large majority of silanol groups are located in the inner surfaces, isolated from each other but able to form hydrogen bonds with pyridine. On the other hand, low- and room-temperature 1 5 N CPMAS and MAS experiments (CP ≡ cross-polarization) performed on pyridine- 1 5 N show that at low concentrations all pyridine molecules are involved in hydrogen bonds with the surface silanol groups. In the presence of an excess of pyridine, a non-hydrogen-bonded pyridine phase is observed at 120 K in the slow hydrogen bond exchange regime and associates with an inner core phase. From these measurements, the number of pyridine molecules bound to the inner surfaces corresponding to the number of silanol groups could be determined to be n O H 3 nm - 2 for MCM-41 and 3.7 nm - 2 for SBA-15. At room temperature and low concentrations, the pyridine molecules jump rapidly between the hydrogen-bonded sites. In the presence of an excess of pyridine, the hydrogen-bonded binding sites are depleted as compared to low temperatures, leading to smaller apparent numbers n O H . Using a correlation established previously between the 1 5 N and 1 H chemical shifts and the NHO hydrogen bond geometries, as well as with the acidity of the proton donors, the distances in the pyridine-hydroxyl pairs were found to be about r H N = 1.68 A, r O H = 1.01 A, and r O N = 2.69 A. This geometry corresponds in the organic solid state to acids exhibiting in water a pK a of about 4. Roomtemperature 1 5 N experiments on static samples of pyridine- 1 5 N in MCM-41 at low coverage show a residual 1 5 N chemical shift anisotropy, indicating that the jumps of pyridine between different different silanol hydrogen bond sites is accompanied by an anisotropic reorientational diffusion. A quantitative analysis reveals that in this regime the rotation of pyridine around the molecular C 2 axis is suppressed even at room temperature, and that the angle between the Si-O axes and the OH axes of the isolated silanol groups is about 47°. These results are corroborated by 2 H NMR experiments performed on pyridine-4-d 1 . In contrast, in the case of SBA-15 with the larger pore diameters, the hydrogen bond jumps of pyridine are associated with an isotropic rotational diffusion, indicating a high degree of roughness of the inner surfaces. This finding is correlated with the finding by 2 9 Si CPMAS of a substantial amount of Si(OH) 2 groups in SBA-15. in contrast to the MCM-41 materials. The Si(OH) 2 groups are associated with surface defects, exhibiting not only silanol groups pointing into the pore center but also silanol groups pointing into other directions of space including the pore axes, leading to the isotropic surface diffusion. All results are used to develop molecular models for the inner surface structure of mesoporous silica which may be a basis for future simulations of the surfaces of mesoporous silica.

Cross-Link Formation and Peptidoglycan Lattice Assembly in the FemA Mutant of Staphylococcus aureus

Staphylococcus aureus FemA mutant grown in the presence of an alanine-racemase inhibitor was labeled with d-[1-13C]alanine, l-[3-13C]alanine, [2-13C]glycine, and l-[5-19F]lysine to characterize some details of the peptidoglycan tertiary structure. Rotational-echo double-resonance (REDOR) NMR of isolated cell walls was used to measure internuclear distances between 13C-labeled alanines and 19F-labeled lysine incorporated in the peptidoglycan. The alanyl 13C labels were preselected for REDOR measurement by their proximity to the glycine label using 13C–13C spin diffusion. The observed 13C–13C and 13C–19F distances are consistent with a tightly packed, hybrid architecture containing both parallel and perpendicular stems in a repeating structural motif within the peptidoglycan.

Desleucyl-Oritavancin with a Damaged D-Ala-D-Ala Binding Site Inhibits the Transpeptidation Step of Cell-Wall Biosynthesis in Whole Cells of Staphylococcus aureus.

We have used solid-state nuclear magnetic resonance to characterize the exact nature of the dual mode of action of oritavancin in preventing cell-wall assembly in Staphylococcus aureus. Measurements performed on whole cells labeled selectively in vivo have established that des-N-methylleucyl-N-4-(4-fluorophenyl)benzyl-chloroeremomycin, an Edman degradation product of [19F]oritavancin, which has a damaged d-Ala-d-Ala binding aglycon, is a potent inhibitor of the transpeptidase activity of cell-wall biosynthesis. The desleucyl drug binds to partially cross-linked peptidoglycan by a cleft formed between the drug aglycon and its biphenyl hydrophobic side chain. This type of binding site is present in other oritavancin-like glycopeptides, which suggests that for these drugs a similar transpeptidase inhibition occurs.