Halogenated 2,5-pyrrolidinediones: synthesis, bacterial mutagenicity in Ames tester strain TA-100 and semi-empirical molecular orbital calculations

The chloroimide 3,3-dichloro-4-(dichloromethylene)-2,5-pyrrolidinedione, a tetrachloroitaconimide, is the principal mutagen produced by chlorination of simulated poultry chiller water. It is the second most potent mutagenic disinfection by-product of chlorination ever reported. Six of seven new synthetic analogs of this compound are direct-acting mutagens in Ames tester strain TA-100. Computed energies of the lowest unoccupied molecular orbital (E_LUMO) and of the radical anion stability (ΔH_f^rad−ΔH_f) from MNDO-PM3 for the chloroimides show a quantitative correlation with the Ames TA-100 bacterial mutagenicity values. The molar mutagenicities of these direct acting mutagenic imides having an exocyclic double bond fit the same linear correlation (ln M_m vs. E_LUMO; ln M_m vs. ΔH_f^rad−ΔH_f) as the chlorinated 2(5H)-furanones, including the potent mutagen MX, 3-chloro-4-(dichloro-methyl)-5-hydroxy-2(5H)-furanone, a by-product of water chlorination and paper bleaching with chlorine. Mutagenicity data for related haloimides having endocyclic double bonds are also given. For the same number of chlorine atoms, the imides with endocyclic double bonds have significantly higher Ames mutagenicity compared to their structural analogs with exocyclic double bonds, but do not follow the same E_LUMO or ΔH_f^rad−ΔH_f correlation as the exocyclic chloroimides and the chlorinated 2(5H)-furanones.     

Differential scanning calorimetry of dipalmitoyl phosphatidylcholine analogues and of their interaction products with basic polypeptides

The thermotropic behaviour of dipalmitoyl phosphatidylcholine analogues with a varying number (n) of CH2 groups between the phosphate and the quaternary ammonium has been investigated. The temperature (Tm) and the enthalpy (deltaH) of the phase transition are non-monotonous functions of the number of CH2 groups. Tm oscillates between 40 and 45 degrees C and deltaH between 7 and 13 kcal/mol for a variation of n between 2 and 11. It is concluded that the hydrocarbon chains in the head groups do not penetrate the hydrocarbon region and do not contribute directly to the melting of the acyl chains. It is suggested that their length may affect the critical balance between the attractive and the repulsive forces within the bidimensional lattice of the head groups. Copolypeptides of lysine with phenylalanine do not appreciably affect the Tm but have a pronounced effect on deltaH of the lipid phase transition, which depends strongly on the ratio of the two amino acids in the polypeptide. The effect of copolypeptide of any defined composition on deltaH is also a non-monotonous function of the number of CH2 groups in the phosphatidylcholine head group, but it does not parallel completely the oscilations in the Tm and deltaH of the pure lipids.     

Chemical enthalpy-entropy compensation effect in the hydrolysis of p-nitrophenyl carboxylates catalyzed by alkaline mesentericopeptidase

The temperature dependence of the hydrolysis of p-nitrophenyl carboxylates with general formula H(CH₂)_nCOOC₆H₄NO₂ catalyzed by alkaline mesentericopeptidase has been studied (n varying from 1 to 7, temperature range 2–30°C, pH 8.80, 5 vol% dimethylsulfoxide). The activation parameters of the deacylation step depend on the length of the hydrophobic side chain of the substrate molecule ( , , and decrease by 2.0 kcal/mol, 4.9 kcal/mol, and 10 eu, respectively, as the length of the acyl carbon chain increases from n = 1 to n = 4). The following criteria were applied to establish a chemical enthalpy-entropy compensation effect: (a) Exner's plot of log vs : (b) Petersen's plot of log, k/T vs 1/T; (c) Exner's statistical treatment in coordinates log k vs 1/T; (d) according to Krug et al. (ΔH^≠ vs ΔG_Thm^≠). By use of all the above-mentioned criteria the existence of a chemical enthalpy-entropy compensation effect was proved with an isokinetic temperature β of about 470°K, which is significantly higher than the average experimental temperature.     

Adaptative response to enhanced basal oxidative damage in sod mutants from Saccharomyces cerevisiae

We investigated the adaptative response of S. cerevisiae in sod mutants (sod1Δ, sod2Δ and sod1Δsod2Δ) after H₂O₂ treatment in the stationary phase. sod2Δ and sod1Δsod2Δ demonstrated the highest levels of GSH in the control, suggesting that pathways which include GSH protect these double mutants against oxidative stress. In addition, sod1Δ and sod1Δsod2Δ had higher iron levels than the wild-type, independently of H₂O₂ stress. Fe levels were increased in sod2Δ following H₂O₂ In addition, the sod2Δ mutant was more sensitive to H₂O₂ treatment than the wild-type. These results suggest that sod2Δ sensibility may be associated with •OH production by the Fenton reaction. This increased iron demand in the sod2Δ mutant may be a reflection of the cells’ efforts to reconstitute proteins that are inactivated in conditions of excess superoxide. MDA levels were assayed by HPLC in these mutants. The highest MDA levels could be observed after 10mM H₂O₂ treatment in the sod1Δsod2Δ double mutant. After treatment with a GSH inhibitor, the MDA level was still higher in the same strain. Thus, both direct and indirect GSH pathways are involved in the protection of lipid membranes and proteins in these mutants and may constitute an adaptative response to enhanced basal oxidative damage produced by superoxide.     

Synthesis and crystal structure of the MgCl2(CH3COOC2H5)2· (CH3COOC2H5) adduct

The reaction of α-MgCl₂ with boiling ethyl acetate affords MgCI₂(CH₃COOC₂H₅)₂· (CH₃COOC₂H₅), which is obtained as crystals suitable for X-ray analysis only from the mother liquor. M=315.5, orthorhombic, space group P2₁22₁ (No. 18), a=25.077(3), b=8.616(1), c=7.345(1) Å, V=1587.0(3) ų, Z=4, D_x=1.32 g cm⁻³,λ A(Mo Kα)=0.71069 Å, μ=4.17 cm⁻¹, F(000)=664, T=298 K, observed reflections: 1667, R=0.059 and R_w=0.069. The structure is composed of polymeric chains of MgCl₂(CH₃COOC₂H₅₎₂ and the ethyl acetate molecules occupy a mutually trans position.     

Yeast sterol C24-methyltransferase: role of highly conserved tyrosine-81 in catalytic competence studied by site-directed mutagenesis and thermodynamic analysis

The role of Try-81 in the reaction catalyzed by Saccharomyces cerevisiae sterol 24-C-methyltransferase (Erg6p) was investigated kinetically and for product differences against a panel of position-81 mutants in which Tyr was substituted with Trp, Phe, Ile, Leu, Val and Ala. The residue chosen for mutation is one that was reported previously to accept fecosterol and yield a set 24-ethyl (idene) sterol products typical of plants, showing the amino acid residue is located close to the transient C25 carbocation intermediate in the active site. One group of mutants (aromatic) tested with the natural substrate zymosterol accelerated the C-methylation reaction (k_cat/K_m) whereas the other group of mutants (aliphatics) decreased catalytic competence as the amino acid side chain was downsized. Mutating to aromatic and assaying with the substrate analog designed as a suicide substrate 26,27-dehydrozymosterol favored C26-monol formation, whereas mutating to the aliphatic of smaller size favored C26-diol formation (a measure of enzyme alkylation). In no case was zymosterol converted to an intermediate that formed a C25-diol. Thermodynamic analysis (determination of E_a, ΔG^‡, ΔH^‡ and TΔS^‡) for the C-methylation reaction performed by these enzymes assayed with the substrate and its analog or zymosterol paired with the “charged’ high energy intermediate (HEI) analogs 24(R,S)25,epiminolanosterol and 25-azalanosterol or “neutral” membrane insert ergosterol showed that mutation to aromatics can reduce inhibitor potency (measured as K_m/K_i), yet catalysis can improve in Trp81 by the introduction of a gain in free energy associated with stabilization of the transition state of a rate-controlling step directed toward turnover. Alternatively, mutation to the smaller aliphatic amino acid side chains led to a destabilization in the active site structure which was accompanied by increases in the partition ratios associated with abortive complex formation. The results are explained by consideration of the functional differences attributed to Tyr81 substitution to aromatics and aliphatics of different size involved with cation-π or hydrogen bonding interactions and in the activation barriers required of differing side chain conformations to orient the reactants in the direction of turnover versus enzyme inactivation.     

Diversity in amylopectin structure, thermal and pasting properties of starches from wheat varieties/lines

Structural, thermal and pasting diversity of starches from Indian and exotic lines of wheat was studied. Majority of the starches showed amylose content ranging between 22% and 28%. Endotherm temperatures (T_o, T_p and T_c) of the starches showed a range between 56–57, 60 –61 and 65.5–66.5 °C, respectively. Exotherms with T_p between 87.0 and 88.2 °C were observed during cooling of heated starches, indicating the presence of amylose–lipid complexes. Exotherm temperatures were negatively correlated to swelling power. Amylopectin unit chains with different degree of polymerization (DP) were observed to be associated with pasting temperature, setback and thermal (endothermic T_o, T_p, and T_c) parameters. Amylopectin unit chains of DP 13–24 showed positive relationship with endothermic T_o, T_p and T_c. Pasting temperature showed positive correlation with short chains (DP 6–12) while negative correlation with medium chain (DP 13–24) amylopectins. Setback was positively correlated to DP 16–18 and negatively to DSC amylose–lipid parameters.     

Effect of structural factors on the stability of duplexes formed by oligonucleotide conjugates with minor groove binders

The effect of structural factors on the stability of duplexes formed by DNA minor groove binders conjugated with oligonucleotide mono- or diphosphoramidates of the general formula Oligo-MGB_m (where Oligo is an oligonucleotide; m = 1 or 2; MGB is -L(Py)₂R, -L(Py)₄R, -L(Im)₄R, or -L(Py)₄NH(CH₂)₃CO(Py)₄R; Py is a 4-aminopyrrole-2-carboxylic acid residue; L is a -aminobutyric acid or an -aminocaproic acid residue, R = OEt, NH(CH₂)₆NEt₂, or NH(CH₂)₆N⁺Me₃) was studied by the method of thermal denaturation. The mode of binder interaction with the minor groove depends on the conjugate structure; it may be of the parallel head to head type for bisphosphoramidates and of the antiparallel head to tail type for monophosphoramidates of a hairpin structure. The effects of the duplexes with parallel orientation (bisphosphoramidates, MGB is L(Py)₄R, m = 2) and those of the hairpin structure with the antiparallel orientation (monophosphoramidates, MGB is L(Py)₄(CH₂)₃CO(Py)₄R, m = 1) on T _m values were close. The influence of the linker (L) and substituent (R) structures upon T _m was more pronounced for monophosphoramidate (MGB is L(Py)_nR, m = 1) than for bisphosphoramidate (MGB is L(Py)_nR, m = 2). No more than two oligopyrrolecarboxamide residues (either in parallel or antiparallel orientations) can be incorporated into the duplex minor groove. Moreover, it was shown by the example of monophosphoramidates (Oligo-L(Py)₄R and Oligo-L(Py)₄NH(CH₂)₃CO(Py)₄R) that the addition of a second ligand capable of incorporation into the minor groove increased T _m of the corresponding duplex in comparison with the duplex formed by the starting monophosphoramidate. At the same time, the introduction of a ligand incapable of incorporating decreased the T _m value. The mode of interaction of the conjugated binder with the oligonucleotide duplex is determined by its structure. For example, dipyrrolecarboxamide containing an ethoxy group at the binder C-end stabilizes the duplex due to stacking interaction with the terminal A · T pair, whereas tetrapyrrolecarboxamides stabilize the duplex by incorporation into the minor groove.__________Translated from Bioorganicheskaya Khimiya, Vol. 31, No. 2, 2005, pp. 159–166.Original Russian Text Copyright © 2005 by Ryabinin, Butorin, Elen, Denisov, Pyshnyi, Sinyakov.     

Evidence of α fluctuations in myoglobin's denaturation in the high temperature region: Average relaxation time from an Adam–Gibbs perspective

In this work, we derive an analytical expression for the relaxation time τ as a function of temperature T for myoglobin protein (Mb, PDB:1MBN) in the high temperature limit (T > T_g = 200 K). The method is based on a modified version of the Adam–Gibbs theory (AG theory) for the glass transition in supercooled liquids and an implementation of differential geometry techniques. This modified version of the AG theory takes into account that the entropic component in protein's denaturation has two major sources: a configurational contribution ΔS_c due to the unfolding of the highly ordered native state N and a hydration contribution ΔS^hyd arising from the exposure of non-polar residues to direct contact with solvent polar molecules. Our results show that the configurational contribution ΔS_c is temperature-independent and one order of magnitude smaller than its hydration counterpart ΔS^hyd in the temperature range considered. The profile obtained for log τ(T) from T = 200 K to T = 300 K exhibits a non-Arrhenius behavior characteristic of α relaxation mechanisms in hydrated proteins and glassy systems. This result is in agreement with recent dielectric spectroscopy data obtained for hydrated myoglobin, where at least two fast relaxation processes in the high temperature limit have been observed. The connection between the relaxation process calculated here and the experimental results is outlined.     

Clothing microclimate temperatures during thermal comfort in boys,young and older men

To examine the effects of age-related differences in thermoregulatory function on the clothing microclimate temperature (T _m) andT _m fluctuations while maintaining thermal comfort in daily life, 5 boys (group B, 10–11 years), 5 young men (group Y, 20–21 years) and 5 older men (group O, 60–65 years) volunteered to take part in this study. The subjects were asked to maintain thermal comfort as closely as possible in their daily lives.T _m (temperatures between the skin surface and the innermost garment) at four sites (chest, back, upper arm, and thigh), skin temperature on the chest (T _chest) and ambient temperature (T _a) were measured over a period of 8–12 h from morning to evening on one day in each of the seasons, spring, summer, autumn, and winter. Records of ability to maintain thermal comfort and of adjustment of their clothes were kept by each subject.T _a during periods of thermal comfort did not differ among the groups in any of the seasons. In group Y,T _m was significantly lower at the thigh than at the other sites in spring, autumn, and winter (P<0.05) and fluctuations (CV) ofT _m were significantly larger at the thigh than at other sites in autumn and winter (P<0.05). Similar tendencies were observed forT _m and CV ofT _m in group B. However,T _m and CV ofT _m in group O did not differ by site except for the autumnT _m. Group O had a smaller CV at the thigh in winter (P<0.05), compared to groups B and Y, suggesting a smaller regional difference inT _m fluctuation in group O. Group O adjusted their clothes even on the lower limbs (together with upper body) in order to maintain thermal comfort in accordance with changes inT _a, while groups B and Y did so only on their upper bodies. These results sugest that compared to boys and young men, lower thermoregulatory function in older men may affectT _m and CV ofT _m as a result of clothing on lower limbs being adjusted differently in order to maintain thermal comfort.     

Regulation of local sweating in sleep-deprived exercising humans

Thermoregulatory sweating [total body (m _sw,b), chest (m _sw,c) and thigh (m _sw,t) sweating], body temperatures [oesophageal (T _oes) and mean skin temperature (T _sk)] and heart rate were investigated in five sleep-deprived subjects (kept awake for 27 h) while exercising on a cycle (45 min at approximately 50% maximal oxygen consumption) in moderate heat (T _air andT _wall at 35° C. Them _sw,c andm _sw,t were measured under local thermal clamp (T _sk,1), set at 35.5° C. After sleep deprivation, neither the levels of body temperatures (T _oes,T _sk) nor the levels ofm _{sw, b},m _{sw, c} orm _{sw, t} differed from control at rest or during exercise steady state. During the transient phase of exercise (whenT _sk andT _sk,1 were unvarying), them _{sw, c} andm _{sw, t} changes were positively correlated with those ofT _oes. The slopes of them _{sw, c} versusT _oes, orm _{sw, t} versusT _oes relationships remained unchanged between control and sleep-loss experiments. Thus the slopes of the local sweating versusT _oes, relationships (m _{sw, c} andm _{sw, t} sweating data pooled which reached 1.05 (SEM 0.14) mg·cm^–2·min^–1°C^–1 and 1.14 (SEM 0.18) mg·cm^–2·min^–1·°C^–1 before and after sleep deprivation) respectively did not differ. However, in our experiment, sleep deprivation significantly increased theT _oes threshold for the onset of bothm _{sw, c} andm _{sw, t} (+0.3° C,P<0.001). From our investigations it would seem that the delayed core temperature for sweating onset in sleep-deprived humans, while exercising moderately in the heat, is likely to have been due to alterations occurring at the central level.     

Formate and glucose stimulation of methane and hydrogen production in rumen liquor

Membrane-inlet mass spectrometry was used to investigate the effects of increasing the concentration of the rumen metabolites, formate and glucose, upon CH₄ and H₂ production during fermentation by unfractionated rumen liquor. Additions of formate up to 3.6 mM stimulated CH₄ and then excess H₂ production. Each addition caused a large accumulation of H₂ (>40 µM), which returned to in situ concentrations after periods of more than 1 h. Glucose additions up to 2.0 mM gave linear increases in CH₄ and H₂ production. The conversion of substrate carbon into CH₄ was found to decrease from 34% to 9% for formate, as concentrations were increased (1.6–3.6 mM); approximately 13.5% of the glucose carbon was converted to CH₄.     

A Fourier-transform infrared spectroscopic study of the polymorphic phase behavior of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine; a polymerizable lipid which forms novel microstructures

We have investigated the phase characteristics of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (DC₂₃PC), a phosphatidylcholine with diacetylenic groups in the acyl chains, and its saturated analog 1,2-ditricosanoyl-sn-glycero-3-phosphocholine (DTPC), using Fourier-transform infrared spectroscopy (FTIR). Previous studies on the phase behavior of DC₂₃PC in H₂O have shown that DC₂₃PC exhibits: (1) formation of cylindrical structures (‘tubules’) by cooling fluid phase multilamellar vesicles (MLVs) through T_m (43° C), and 2) metastability of small unilamellar vesicles (SUVs) in the liquid-crystalline state some 40° C below T_m, with subsequent formation of a gel phase comprised of multilamellar sheets at 2° C. The sheets form tubules when heated and cooled through T_m. FTIR results presented here indicate that as metastable SUVs are cooled toward the transition to bilayer sheets, spectroscopic changes occur before the calorimetric transition as measured by a reduction in the CH₂ symmetric stretch frequency and bandwidth. In spite of the vastly different morphologies, the sheet gel phase formed from SUVs is spectroscopically similar to the tubule gel phase. The C-H stretch region of DC₂₃PC gel phase shows bands at 2937 and 2810 cm⁻¹ not observed in the saturated analog of DC₂₃PC, which may be related to perturbations in the acyl chains introduced by the diacetylenic moiety. The narrow CH₂ scissoring mode at 1470 cm⁻¹ and the prominent CH₂ wagging progression indicate that DC₂₃PC gel phase was highly ordered acyl chains with extended regions of all-trans methylene segments. In addition, the 13 cm⁻¹ reduction in the C  O stretch frequency (1733–1720 cm⁻¹) during the induction of DC₂₃PC gel phase indicates that the interfacial region is dehydrated and rigid in the gel phase.     

Biosynthesis of medium chain length poly(3-hydroxyalkanoates) (mcl PHAs) from cosmetic co-products by Pseudomonas raguenesii sp. nov., isolated from Tetiaroa, French Polynesia

A new bacterium, designated as strain TE9 was isolated from a microbial mat in French Polynesia and was studied for its ability to synthesize medium chain length poly-β-hydroxyalkanoates (mcl PHAs) during cultivation on cosmetics co-products. The composition of PHAs was analysed by coupled gas chromatography mass spectroscopy (GC/MS), nuclear magnetic resonance (NMR) and Fourier Transform InfraRed (FTIR) spectroscopy. PHAs were composed of C6–C14 3-hydroxyacids monomers, with a predominance of 3-hydroxyoctanoate (3HO), 3-hydroxydecanoate (3HD) and 3-hydroxydodecanoate (3HDD). Differential scanning calorimetry (DSC) experiments allowed the characterization of elastomeric materials with a melting point T_m near 50 °C, enthalpy of fusion ΔH_m from 27 to 32 J/g, and glass transition temperature T_g of −43 °C. Molecular weights ranged from 175,000 to 358,000 g/mol. On the basis of the phenotypical features and genotypic investigations, strain TE9 was assigned to the Pseudomonas genus and the name of Pseudomonas raguenesii sp. nov. is proposed.     

NMR studies of glycerol permeability in lipid vesicles, erythrocytes and the alga Dunaliella

Glycerol diffusional permeabilities through the cytoplasmic cell membrane of Dunaliella salina, the cell envelope of pig erythrocyte and egg phosphattidylcholine vesicles were measured by NMR spectroscopy employing the spin-echo method and nuclear T₁ relaxation. The following permeability coefficients (P) and corresponding enthalpies of activation (ΔH^‡) were determined for glycerol at 25°C: for phosphatidylcholine vesicles 5·10⁻⁶ cm/s and 11±2 kcal/mol; for pig erythrocytes 7·10⁻⁸ cm/s and 18±3 kcal/mol, respectively; for the cytoplasmic membrane of D. salina the permeability at 17°C was found to be exceptionally low and only a lower limit (P<5·10⁻¹¹cm/s) could be calculated. At temperatures above 50°C a change in membrane permeability occurred leading to rapid leakage of glycerol accompanied by cell death. The data reinforce the notion that the cytoplasmic membrane of Dunaliella represents a genuine anomaly in its exceptional low permeability to glycerol.     

Proton/hydroxide conductance through lipid bilayer membranes

Summary A simple method of measuring proton/hydroxide conductance (G _H/OH) through planar lipid bilayer membranes is described. First the total conductance (G _m ) is measured electrically. Then the H⁺/OH^– transference number (T _H/OH) is estimated from the diffusion potential (V _m ) produced by a transmembrane pH gradient. The pH gradient is produced by a pair of buffered solutions which have identical concentrations of all ions except H⁺ and OH^–. Thus,V _m is due entirely to H⁺/OH^– diffusion andG _H/OH can be calculated from the relations,V _m =T _H/OH E _H/OH andG _H/OH=T _H/OH G _m , whereE _H/OH is the equilibrium potential for H⁺ and OH^–. In bilayers made from bacterial phosphatidylethanolamine (PE) inn-decane,G _H/OH is nearly independent of pH, ranging from about 10^–9 S cm^–2 at pH 1.6 to 10^–8 S cm^–2 at pH 10.5. BecauseG _H/OH is nearly independent of pH, the calculated permeability coefficients to H⁺ and/or OH^– are extremely pH dependent, which partly explains the wide range of values reported for phospholipid vesicles and biological membranes.G _H/OH appears to be independent of the membrane surface charge, because titrating either the phosphate or the amino group of PE has little effect onG _H/OH.G _H/OH is reduced about 10-fold when the water activity is reduced 33% by replacement with glycerol. Although the mechanism of H⁺/OH^– conductance is not known, the relation betweenG _H/OH and water activity suggests that several water molecules are involved in the H⁺/OH^– transport process.     

Morphological properties and thermoanalysis of micronized cassava starch

The granule morphology, microstructure, and thermal properties of micronized cassava starch prepared by a vacuum ball-grinding machine were investigated. Scanning electron microscopy (SEM) analysis indicated that the morphology of starch granule changes during the ball-grinding treatment. Differential scanning calorimetry (DSC) analysis indicated that the maximum peak temperature (T_p) of the gelatinization process, the glass transition (T_g), and peak height index (PHI) for the starch granules decreased when the size of micronized starch granules was reduced. When the size of starch granules was reduced beyond 9.11 μm, they have a tendency to agglomerate and their ΔH were increased. The granule size has a significant effect on the gelatinization properties of cassava starch. This study will provide useful information of the micronized starch for its potential industrial application.     

The structure of Escherichia coli membranes studied by fluorescence measurements of lipid phase transitions

Lipid phase transitions in Escherichia coli membranes and in dispersions of the extracted lipids were studied using the negatively charged fluorescence probe 1-anilinonaphthalene-8-sulfonate (ANS⁻) and the hydrophobic fluorescence probe N-phenyl-1-naphthylamine (NPN). The fluorescence change, ΔI, at the phase transition approaches a limiting value (ΔI)_lim with increasing dye concentration. A comparison of the limiting values (Δ)_lim^NPN obtained for membranes and the lipid standard allows us to estimate the lipid fraction, ρ, in the membrane that takes part in the phase transition (ρ = 80%). The same procedure carried out with ANS⁻ yields a value of 42.5% for the lipid fraction that is accessible from the aqueous phase. These values, combined with published freeze-etching data for the particle density within the fracture plane of membranes are used to quantify the Davson-Danielli-Robertson-Benson-Singer membrane model which assumes a fluid lipid bilayer with “integral” proteins embedded in the lipid matrix and surface proteins attached to the lipid head groups. It appears that on the average one “integral” membrane protein is surrounded by about 600 lipid molecules and that about 130 of these molecules are closely coupled to the protein molecule, forming an halo in which the chain-chain interaction between the lipids is disturbed. About half of the bilayer surface is covered with proteins; part of these seem to be stacked.