No abstract

Evidence is accumulating that in cell membranes microdomains exist, also referred to as rafts or detergent resistant membranes. In this study, atomic force microscopy is used to study supported lipid bilayers, consisting of a fluid phosphatidylcholine, sphingomyelin and cholesterol. Domains were visualized of which the morphology and size depended on the cholesterol concentration. The presence of cholesterol was found to induce bilayer coupling. At 30 mol% cholesterol, a change in percolation phase was observed, and at 50 mol%, when both fluid lipids and solid lipids are saturated with cholesterol, phase separation was still observed. In addition, we were able to directly visualize the resistance of domains against non-ionic detergent. ß

An intermolecular potential model of H2O with six interaction sites is proposed. The model is developed for the simulation of ice and water near the melting point. Parameters in the potential are determined to reproduce the real melting point of ice, and densities of ice and water near the melting point, which are predicted by calculating derivatives of the free energies and volumes of ice and water against potential parameters. Free energy calculations are carried out for several ice structures and water, and the results are compared with those obtained in four- and five-site models, which are currently in use. It is shown that, only in the present six-site model, the proton-disordered hexagonal ice is the stable structure at the melting point, as in real ice. The melting point of the proton-disordered hexagonal ice at 1 atm is estimated to be 271±9 K in the present model, which is in good agreement with the real melting point of 273.15 K. Moreover, results of Monte Carlo simulations of ice and water show that the present six-site model reproduces well the real structural and thermodynamic properties of ice and water near the melting point.

Lateral segregation in biological membranes leads to the formation of domains. We have studied the lateral segregation in gel-state model membranes consisting of supported dipalmitoylphosphatidylcholine (DPPC) bilayers with various model peptides, using atomic force microscopy (AFM). The model peptides are derivatives of the Ac-GWWL(AL) n WWA-Etn peptides (the so-called WALP peptides) and have instead of tryptophans, other flanking residues. In a previous study, we found that WALP peptides induce the formation of extremely ordered, striated domains in supported DPPC bilayers. In this study, we show that WALP analogues with other uncharged residues (tyrosine, phenylalanine, or histidine at pH 9) can also induce the formation of striated domains, albeit in some cases with a slightly different pattern. The WALP analogues with positively charged residues (lysine or histidine at low pH) cannot induce striated domains and give rise to a completely different morphology: they induce irregularly shaped depressions in DPPC bilayers. The latter morphology is explained by the fact that the positively charged peptides repel each other and hence are not able to form striated domains in which they would have to be in close vicinity. They would reside in disordered, fluidlike lipid areas, appearing below the level of the ordered gel-state lipid domains, which would account for the irregularly shaped depressions.Interactions between lipids and membrane proteins are generally known to have a major effect on the structure and function of biological membranes. An important aspect in the function and biogenesis of membrane proteins are the interactions between the amino acids flanking the hydrophobic transmembrane domain, and the hydrophobichydrophilic interfaces in the membrane.It has been found that in integral membrane proteins with single-or multiple-spanning transmembrane helices, the interfacial region is enriched in aromatic residues (1-3). Especially tryptophans and tyrosines are abundant in the interface, and they are generally thought to have an anchoring (4, 5) and stabilizing (6, 7) function. Moreover, it has been suggested that aromatic residues are essential for the proper folding and assembly (8), and functioning of integral membrane proteins (9, 10). Tryptophan residues were found to be located at the hydrophobic side of the interfacial region, near the carbonyl/glycerol groups of the surrounding phospholipids (5, 11).The positively charged residues lysine and arginine are enriched at the hydrophilic side of the interfacial region of transmembrane domains (1, 7) and in the segments flanking transmembrane R-helices (2, 3). Following the positive inside rule, they are preferentially located on the cis-side of membranes, (12), where they associate with anionic phospholipids (13), fulfilling a topology-determining role (14). When flanking a hydrophobic R-helix in transmembrane model peptides, lysines showed a distinct preference for the hydrophilic side of the interfacial region, close to the phosphate groups o...

We demonstrate that a six-site potential model of H 2 O, which we proposed recently, is suitable for simulation works on the crystal growth of ice from water. Using the model, a molecular dynamics simulation of a system including interfaces between the prismatic face of the proton-disordered hexagonal ice and water is carried out at a supercooled state and a pressure of 1 atm. The results clearly indicate that the ice grows at the interfaces continuously during the simulation. Moreover, the growth kinetics at the interfaces is analyzed on a molecular scale. r