Last updated July 2009.  Copyright © 2000 – 2010 Rice Research Group.

Teichoic Acid STudies

The cell wall of Gram-positive bacteria is mainly composed of peptidoglycan (peptide cross-linked disaccharide polymers) and teichoic acids (long phosphodiester polymers), which are essential to bacterial health.  Teichoic acids are suggested to participate in several different aspects of bacterial survival, one being the coordination of metal cations.  Of the different functional groups present in teichoic acids (phosphate, alanine, N-acetylglucosamine, hydroxyl), the phosphates are the primary chelation site and have been the subject of numerous investigations into the metal binding chemistry.  It is well known that bacteria, especially the Gram-positive variety, carry an enormous capacity for metal chelation.  Studies have also shown that both the carboxyl sites of peptidoglycan and the phosphoryl sites of teichoic acid show affinity for metal chelation over a broad range of pH values.  However, little is currently known about the exact mechanism in which the bacteria binds and intercalates these metal ions.  Due to the amorphous nature (inability to be crystallized), the variable polymer length, and the tendency of the individual monomers to aggregate in solution, conventional liquid state NMR, X-ray crystallography, and mass spectrometry have failed to yield structural data about the binding environment of teichoic acid to metals and to the peptidoglycan matrix.  Therefore, we have turned to solid-state NMR (SSNMR) to uncover the binding interactions of teichoic acid in the endeavor to discover novel chemical targets for antibacterial research.

The SSNMR technique used initially in this project was an indirect analysis method.  Through the use of cross-polarization magic-angle spinning (CPMAS) at low spin rate, the chemical shift anisotropy (CSA) tensor data was collected and extracted via the Herzfeld-Berger method.  Data from lipoteichoic acid and pep-wall (peptidoglycan with wall teichoic acid) samples were collected with and without metals added to measure the change in the CSA tensor, reflecting a change in the chemical shielding environment around the phosphorus nuclei.  Density Functional Theory (DFT) computational calculations based on the phosphodiester repeat units were also performed in tandem with the tensor analyses.  Through the use of Gaussian 03 software, the optimized bound and unbound model structures were analyzed for NMR tensor data as well.  These data were entered into the SpecTrum Analysis of Rotating Solids (STARS) software in order to produce simulated spectra, which could then be compared visually to the spectra collected experimentally.  These data supported a bidentate inner-sphere binding mechanism contrary to the current paradigm.