REDOR and CODEX solid-state NMR for heterogeneous biological and synthetic polymers.

摘要

We used solid-state NMR spectroscopy to investigate the mechanism of vancomycin-intermediate resistance in Mu50, a clinical isolate of methicillin resistant Staphylococcus aureus. NMR analysis showed that compared to vancomycin-susceptible S. aureus, Mu50 had reduced crosslink density (57%) and up-regulated cell wall biosynthesis. These factors contribute to the increase in D-Ala-D-Ala terminated peptidoglycan stems (vancomycin binding sites) in the cell-walls of Mu50, enabling vancomycin sequestration. NMR investigation of the mechanism of resistance in Mu50 was consistent with affinity trapping but not with a proposed clogging mechanism in which the vancomycin bound to the exterior part of the cell-wall prevents further drug penetration.;We investigated the mechanism of secondary binding in oritavancin by destroying its primary binding site. Desleucyl-oritavancin (a glycopeptide antibiotic) is an Edman degradation product of oritavancin with a damaged D-Ala-D-Ala binding site. The addition of desleucyloritavancin during S. aureus growth (in vivo) resulted in transpeptidase inhibition, an effect comparable to that of oritavancin, despite the damaged D-Ala-D-Ala binding cleft. The results were consistent with desleucyl-oritavancin preferentially targeting the nascent and template peptidoglycan mediated by its secondary binding site.;We studied chain packing in glassy polymers using a 2D version of center-band only detection of exchange (CODEX). 2D CODEX is a constant-time experiment that avoids the problems associated with the variations in T1(C)'s due to dynamic site heterogeneity in the glass. We calibrated spin diffusion in polymers using two different mixing times of 200 and 1200 ms. For a short mixing time of 200 ms, only 13C-13C pairs separated by one or two bonds (2.5 A) were visible as cross peaks, providing reference to intrachain proximities. During the longer mixing time of 1200 ms, 5-A interchain proximities appeared. We used the resulting mixing-time dependent cross peaks from 2D CODEX to compare non-random chain packing in commercial polycarbonates with different mechanical properties.