Dynamic structure of whole cells probed by nuclear Overhauser enhanced nitrogen-15 nuclear magnetic resonance spectroscopy

Abstract

The proton-decoupled 15N Fourier transform nuclear magnetic resonance (NMR) spectra of 15N-enriched Escherichia coli, Bacillus licheniformis, baker's yeast, and Friend leukemic cells were obtained. The 15N NMR spectra of whole cells displayed 15N resonances originating from (i) protein backbones with lysine, arginine, and histidine side chains, (ii) ribonucleic acids, (iii) peptidoglycan, and (iv) phospholipids. Several additional amino and amide resonances were observed but not identified. In bacteria and yeast, the cell wall was found to be the site of a relatively mobile group of molecules, whose resonances dominate the proton-decoupled 15N NMR spectra of whole cells. 15N NMR chemical shifts and nuclear Overhauser effects have provided information on the in vivo structure of cell wall peptidoglycan. In Staphylococcus aureus the pentaglycine cross-bridge of cell wall peptidoglycan was found to have a random coil conformation. In B. licheniformis considerable segmental motional freedom was detected in teichuronic acid and peptidoglycan polysaccharide chains in the wall of the intact cell.