Molecular dynamics simulations of hydrated alkaline and alkaline-earth metal cations at room temperature (T ) 300 K) were carried out using the CHARMM22 force field. Dynamic and static properties of systems containing one ion and 123 or 525 water molecules were investigated by analysis of trajectories of 1 ns duration and compared to experimental and theoretical results. In addition, the size and the direction of the elementary motions of both the ions and the water molecules were investigated on the scale of the integration time step of 1 fs. Comparison between systems of different size revealed that for the larger system the diffusion coefficient and the number of hydrogen bonds were increased. Radial pair distribution functions and coordination numbers are in good agreement with X-ray and neutron scattering data. The diffusion coefficient D of bulk TIP3P water in a system with 528 water molecules was by one-fourth higher than the experimental value. Minor differences of approximately 10% between experimental and simulated diffusion coefficients were found for Li + , Na + , K + , and Mg 2+ . On the other hand, D was underestimated by the simulation for Ca 2+ and Sr 2+ by as much as 30%. On the average, 2.9 hydrogen bonds per bulk water molecule were found. The observed order of residence times for the monovalent ions, τ(Li + ) > τ(Na + ) > τ(K + ), is in good agreement with the literature. Although τ was expected to increase with decreasing mass of the ion, the exchange of water molecules from the solvation shell of Mg 2+ occurred much faster than for Ca 2+ .

Molecular modelling techniques have been applied to calculate the three-dimensional architecture and the conformational flexibility of a complete bacterial S-form lipopolysaccharide (LPS) consisting of a hexaacyl lipid A identical to Escherichia coli lipid A, a complete Salmonella typhimurium core oligosaccharide portion, and four repeating units of the Salmonella serogroup B O-specific chain. X-ray powder diffraction experiments on dried samples of LPS were carried out to obtain information on the dimensions of the various LPS partial structures. Up to the Ra-LPS structure, the calculated model dimensions were in good agreement with experimental data and were 2.4 nm for lipid A, 2.8 nm for Re-LPS, 3.5 nm for Rd-LPS, and 4.4 nm for Ra-LPS. The maximum length of a stretched S-form LPS model bearing four repeating units was evaluated to be 9.6 nm; however, energetically favored LPS conformations showed the O-specific chain bent with respect to the Ra-LPS portion and significantly smaller dimensions (about 5.0 to 5.5 nm). According to the calculations, the Ra-LPS moiety has an approximately cylindrical shape and is conformationally well defined, in contrast to the O-specific chain, which was found to be the most flexible portion within the molecule.

The conformational properties of the isolated S form of Salmonella sp. lipopolysaccharide (LPS), of Re mutant LPS, and of free lipid A were investigated by using X-ray diffraction and conformational energy calculations. The data obtained showed that LPS in a dried, in a hydrated, and probably also in an aqueous dispersion state is capable of forming bilayered lamellar arrangements similar to phospholipids. From the bilayer packing periodicities, a geometrical model of the extensions of the LPS regions lipid A, 2-keto-3deoxyoctulosonic acid, and 0-specific chain along the membrane normal could be calculated. Furthermore, the lipid A component was found to assume a remarkably high ordered conformation: its fatty acid chains were tightly packed in a dense hexagonal lattice with a center-to-center distance of 0.49 nm. The hydrophilic backbone of lipid A showed a strong tendency to form domains in the membrane, resulting in a more or less parallel arrangement of lipid A units. According to model calculations, the hydrophilic backbone of lipid A appears to be oriented-45°to the membrane surface, which would lead to a shed roof-like appearance of the surface structure in the indentations of which the 2-keto-3-deoxyoctulosonic acid moiety would fit. In contrast, the 0-specifiv chains assume a low ordered, heavily coiled conformation. Comparison of these structural properties with those known for natural phospholipids in biological membranes indicates that the high state of order of the lipid A portion of LPS might be an important factor in the structural role and permeation barrier functions of LPS in the outer membrane of gram-negative bacteria.

Molecular modelling techniques have been applied to compute the conformations accessible to bacterial deep rough lipopolysaccharide of Escherichia coli (Re-LPS). Analyses of the results showed that the models typically exhibit a tilt of the diglucosamine backbone with respect to the membrane normal of 53 7" while both the glucosamine ring planes are orientated approximately parallel to the membrane normal. Different models were found to show compact and elongated types of acyl chain arrangements, both producing anisotropic lateral dimensions of the models of 1.0-1.1 nm and 1.7-2.0 nm for the shorter and the longer side, respectively. The conformationally allowed range of the isolated dOclA(a-2-4)dOclA disaccharide (dOclA = 3-deoxy-~-mannooctulosonic acid) was found to be extremely limited. It appeared that the dOclA disaccharide (dOclA)2 is centred at the top of the Re-LPS molecule preferring two orientations stabilized by hydrogen bonds involving only one phosphate group of the lipid A moiety at a time.The effect of charges on the Re-LPS conformations has been studied in separate calculations. From these calculations it was obvious that charges have no significant effects on the conformations of the isolated lipid A and (dOclA)2 moieties. However, it was found that the orientation of (dOclA), with respect to the lipid A part is highly sensitive to charges, i. e. in the charged models the proximity of phosphate and carboxyl groups is prevented by strong electrostatic repulsion between these negatively charged groups.In order to rationalize the acyl chain packing of the models, a simple geometrical model which correlates the tilt of the diglucosamine backbone with the energetically favoured close packing of the acyl chains is proposed. Furthermore, the possibility of a chelate-like complexation of divalent cations and its contribution to head group mobility is discussed.Lipopolysaccharides (LPS) are the main constituent of the outer leaflet of the outer membrane of Gram-negative bacteria. As LPS are of special significance to bacterial viability and to interactions of bacteria with host organisms, many studies have been focused on the elucidation of their structure and function [l -51. Chemically, LPS consists of a lipid A and a polysaccharide component, of which the latter is usually subdivided into two regions [6], i.e. the 0-specific chain and the core oligosaccharide. Re-LPS is the smallest LPS found in viable Escherichia coli bacteria [2, 71. It consists of the lipid A part and a partial core built up by a dimer of the rare saccharide 3-deoxy-~-manno-octu~osonic acid (dOclA). Recent work has shown that LPS endotoxic activity is determined by structural components as they are present in E. coli lipid A, i.e. a 8-1-6 interlinked diglucosamine, two phosphoryl groups and at least five, but not more than six, fatty acids including one or two 3-acyloxyacyl groups [S]. However, as itCorrespondence to

Lipoteichoic acid, diglucosyldiacylglycerol, and phosphatidylglycerol isolated from Staphylococcus aureus were embedded in dipalmitoylglycerophosphoglycerol vesicles, and their thermotropic influence on this matrix was studied by differential scanning calorimetry. The natural fatty acids of phosphatidylglycerol effected peak broadening and a decrease in molar heat capacity. These effects were more pronounced with the glycolipid, which also increased the main transition temperature. With the lipoteichoic acid mixtures, two broad main transition peaks were observed, possibly due to different levels of lipoteichoic acid in vesicles. Both peaks showed a further upshift in transition temperatures and a pronounced decrease in molar heat capacity. Since the diacylglycerol moieties of all three amphiphiles were practically identical, the differences in the thermotropic effects have to be ascribed to the different structures of the head groups. Diglucosyldiacylglycerol is proposed to exert an additional effect by hydrogen bonding the hydroxyls of the sugar rings to their phospholipid neighbors. The stronger effect of lipoteichoic acid points to dynamic interactions of the long hydrophilic chain with the vesicle surface, which stabilize the membrane structure.Characteristic lipid components of the cytoplasmic membrane of many gram-positive bacteria are glycolipids and lipoteichoic acids (LTAs) (7). For example, the cytoplasmic membrane of Staphylococcus aureus contains LTA, diglucosyldiacylglycerol (DGDG), phosphatidylglycerol (PG), diacylglycerol, and lysylphosphatidylglycerol, which are present during exponential growth at 6, 8, 50, 24, and 10 mol%, respectively. Trace amounts of monoglucosyldiacylglycerol are also present, and toward the end of exponential growth, variable amounts of bisphosphatidylglycerol (cardiolipin) are formed (16). LTA is biosynthetically derived from DGDG, on which a linear chain of an average of 25 1,3-linked glycerophosphate residues is polymerized by the transfer of glycerophosphate from PG. Owing to the long hydrophilic chain, LTA contains more than 50% of the total membrane lipid glycerol. The concentration of LTA in the outer membrane layer is 12 mol%, which means that each LTA molecule is surrounded by eight lipid molecules.Although LTA was discovered 25 years ago and has been considered an essential cellular component, little is known about its physiological functions (2, 7). Several biological activities associated with the hydrophilic chain were described, but the structural diversity of the hydrophilic chain with regard to D-alanine ester content, substitution with glycosyl residues, and in particular net electric charge rendered a general function based on the chain structure unlikely. We therefore focused on LTA's role as a membrane constituent and searched for its influence on the physicochemical properties of the membrane.Recent X-ray scattering analysis revealed that LTAs are unique because they form micellar supramolecular structures in aqueous dispersion (18), in contrast to me...