SECONDARY AND TERTIARY STRUCTURE OF MUREIN - LOW AND MEDIUM-ANGLE X-RAY EVIDENCE AGAINST CHITIN-BASED CONFORMATIONS OF BACTERIAL PEPTIDOGLYCAN

X‐ray diffraction, density measurements, and stereochemical data were used in order to disclose the architecture of murein, the rigid component of almost all bacterial cell walls. Dry densities of 1.38–1.39 g/cm3 were observed for Micrococcus luteus and Staphylococcus aureus. The X‐ray data for gram‐positive (S. aureus, M. luteus) and gram‐negative (Escherichia coli) strains were almost identical, showing, in addition to some diffuse scattering with broad maxima corresponding to Bragg values of 0.22 and 0.45 nm, two sharp peaks indicating periodicities of about 0.7 and 0.94 nm. These latter periodicties were not found in whole bacteria but emerge immediately after breakage of the whole cells. In gram‐positive species a weak oriented reflex at 4.2 nm appeared, while all other reflexes remain ring‐shaped. Diffraction patterns obtained under the influence of humidity and certain metal ions showed strong variations in the position of the 0.45‐nm halo. These data, together with stereochemical considerations, invalidate the hiterto advanced models of a chitin‐like structure of the glycan chains in murein. Instead, the following model proposed and discussed here is consistent with the experimental data. The structure is made up of layers, which in S. aureus and M. luteus are separated by about 4.2 nm. The layers are more or less randomly rotated against each other. The sugar chains run parallel to these layers and, hence, to the surface of the cell wall. They do not possess a twofold screw axis as in chitin or cellulose. Thus, the peptide strands protude from the glycan chain axis in different directions. The peptide strands seem to assume fairly rigid conformations and to be mainly responsible for the regular layer‐like arrangement of murein. Copyright © 1979, Wiley Blackwell. All rights reserved