Membrane Transport Experiments Research Articles

Unique Selectivity Reversal in the Complexation of Uranium(VI) and Plutonium(IV) with Two Novel C-Pivoted Tripodal Amide Ligands: Extraction and Liquid Membrane Transport Studies

ABSTRACT Two new carbon pivoted tripodal amide-containing ligands with different spacers (L I : -CH2- spacer; L II : -CH2-CH2- spacer) have been investigated for the liquid–liquid extraction of tetravalent plutonium and hexavalent uranium from nitric acid feed solutions. The studies were carried out using the ligand solutions prepared in 95% n-dodecane + 5% isodecanol. Pu(IV) extraction was higher than that of U(VI) at 3 M HNO3 for ligand L I which was just the opposite for ligand L II . Increase of the nitric acid concentration resulted in increasing extraction of both metal ions. Isodecanol variation studies exhibited decrease in the extraction of Pu(IV) and U(VI) with increasing isodecanol content in the organic phase. Greater that 98% stripping of Pu(IV) and U(VI) was obtained from the complexes of L I and L II using 0.5 M HNO3 + 0.1 M HEDTA and 1 M Na2CO3, respectively. The slope analysis study suggested the formation of 1:2 (ML2) species with Pu(IV) as well as U(VI) with both ligands at 3 M HNO3. Temperature variation studies showed that the extraction of U(VI) and Pu(IV) with both ligands is exothermic in nature. Supported liquid membrane transport experiments using 0.11 M ligands (L I and L II ) in 95% n-dodecane + 5% isodecanol indicated poor transport (<10% after 2 h) of U(VI) and Pu(IV).

Short-chain diamines are the physiological substrates of PACE family efflux pumps

Acinetobacter baumannii has rapidly emerged as a major cause of gram-negative hospital infections worldwide. A. baumannii encodes for the transport protein AceI, which confers resistance to chlorhexidine, a widely used antiseptic. AceI is also the prototype for the recently discovered proteobacterial antimicrobial compound efflux (PACE) family of transport proteins that confer resistance to a range of antibiotics and antiseptics in many gram-negative bacteria, including pathogens. The gene encoding AceI is conserved in the core genome of A. baumannii, suggesting that it has an important primordial function. This is incongruous with the sole characterized substrate of AceI, chlorhexidine, an entirely synthetic biocide produced only during the last century. Here we investigated a potential primordial function of AceI and other members of the PACE family in the transport of naturally occurring polyamines. Polyamines are abundant in living cells, where they have physiologically important functions and play multifaceted roles in bacterial infection. Gene expression studies revealed that the aceI gene is induced in A. baumannii by the short-chain diamines cadaverine and putrescine. Membrane transport experiments conducted in whole cells of A. baumannii and Escherichia coli and also in proteoliposomes showed that AceI mediates the efflux of these short-chain diamines when energized by an electrochemical gradient. Assays conducted using 8 additional diverse PACE family proteins identified 3 that also catalyze cadaverine transport. Taken together, these results demonstrate that short-chain diamines are common substrates for the PACE family of transport proteins, adding to their broad significance as a novel family of efflux pumps.

Selective Separation and Recovery of Pt(IV) from Pd(II) through an Imidazolium-ionic-liquid-based Supported Liquid Membrane.

A supported liquid membrane (SLM) system for the selective separation of platinum(IV) from palladium(II) has been developed. The SLM was prepared using imidazolium-based IL 1-octy-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The initial flux and separation factor of Pt(IV) from Pd(II) were found to significantly depend on the composition of the receiving solution. Based on the performance, a 0.1 mol dm-3 NaClO4 solution was selected as a suitable receiving solution. Membrane transport experiments showed that 93% of Pt(IV) can be selectively transported into the receiving solution, whereas most Pd(II) remains in the feed solution.

Recovery of gold ions from discarded mobile phone leachate by solvent extraction and polymer inclusion membrane (PIM) based separation using an amic acid extractant

This paper reports on the selective separation and recovery of gold ions from leachates of discarded mobile phones using liquid-liquid extraction and a polymer inclusion membrane (PIM) transport system. The collected mobile phones were crushed by a mill and the obtained powder was calcinated. After leaching with aqua regia, the metal composition of the leachate was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The analysis results confirmed that the mobile phone waste contained 397 g/ton of gold. Liquid-liquid extraction and PIM-based separation procedures for the selective recovery of gold(III) from synthetic and actual leachates were developed. The extracting organic solution and the PIM incorporated the newly synthesized extractant N-[N,N-di(2-ethylhexyl)aminocarbonylmethyl]glycine (D2EHAG) which exhibited high selectivity for the gold(III) ion over the other metal ions present in much higher concentrations in the leachates. The compositions of the feed and receiving solutions in both the liquid-liquid and PIM based extraction and back-extraction of the gold(III) ions were optimized. It was established that optimal extraction required a HCl concentration in the feed solution of 2 mol/L and that a receiving solution containing 0.1 M thiourea in 1 M HCl was capable of back-extracting gold(III) quantitatively. Membrane transport experiments with a synthetic leachate as the feed solution demonstrated that 96% of the gold(III) ions was selectively transported into the receiving solution of the transport cell thus leaving all other metal ions in the leachate.

Guanidinium export is the primal function of SMR family transporters

The small multidrug resistance (SMR) family of membrane proteins is prominent because of its rare dual topology architecture, simplicity, and small size. Its best studied member, EmrE, is an important model system in several fields related to membrane protein biology, from evolution to mechanism. But despite decades of work on these multidrug transporters, the native function of the SMR family has remained a mystery, and many highly similar SMR homologs do not transport drugs at all. Here we establish that representative SMR proteins, selected from each of the major clades in the phylogeny, function as guanidinium ion exporters. Drug-exporting SMRs are all clustered in a single minority clade. Using membrane transport experiments, we show that these guanidinium exporters, which we term Gdx, are very selective for guanidinium and strictly and stoichiometrically couple its export with the import of two protons. These findings draw important mechanistic distinctions with the notably promiscuous and weakly coupled drug exporters like EmrE.

Multi-stage chiral separation with electrospun chitin nanofiber membranes

Chitin nanofiber membranes were fabricated via electrospray deposition. The nanofiber membrane adsorbed l-isomer from racemic mixture of Glu and Phe in preference to the corresponding antipodes, while d-Lys was preferentially incorporated into the membrane. The adsorption isotherms revealed that all enantiomers studied in the present study were incorporated into the nanofiber membrane without any specific interaction, in other words, those were simply adsorbed. In membrane transport experiments, d-isomers were preferentially transported through the nanofiber membrane. In order to attain higher permselectivity, a multi-stage cascade separation was applied in the present study. As expected, the permselectivity toward d-enantiomer was enhanced exponentially with the increase in number of stages from one to five stages via three by applying a multi-stage cascade membrane separation.

Fluoride resistance and transport by riboswitch-controlled CLC antiporters

A subclass of bacterial CLC anion-transporting proteins, phylogenetically distant from long-studied CLCs, was recently shown to be specifically up-regulated by F(-). We establish here that a set of randomly selected representatives from this "CLC(F)" clade protect Escherichia coli from F(-) toxicity, and that the purified proteins catalyze transport of F(-) in liposomes. Sequence alignments and membrane transport experiments using (19)F NMR, osmotic response assays, and planar lipid bilayer recordings reveal four mechanistic traits that set CLC(F) proteins apart from all other known CLCs. First, CLC(F)s lack conserved residues that form the anion binding site in canonical CLCs. Second, CLC(F)s exhibit high anion selectivity for F(-) over Cl(-). Third, at a residue thought to distinguish CLC channels and transporters, CLC(F)s bear a channel-like valine rather than a transporter-like glutamate, and yet are F(-)/H(+) antiporters. Finally, F(-)/H(+) exchange occurs with 1:1 stoichiometry, in contrast to the usual value of 2:1.

Alcohol enhanced permeation in model membranes. Part II. Thermodynamic analysis of membrane partitioning

The role of solvents in drug transport has not been properly addressed in the literature, despite its well known influence on drug permeation. Previously we have conducted thermodynamic and kinetic analyses to probe the molecular mechanisms of alcohol enhanced permeation. In the present study, the influence of temperature on the partitioning of methyl paraben into silicone membranes is investigated. In line with previous membrane transport studies of methyl paraben in silicone membranes, butanol and heptanol are used as representative alcohols. The results show higher amounts of methyl paraben extracted from the silicone membrane following equilibration with butanol, at all experimental temperatures. This was in line with alcohol uptake data. In fact, a linear correlation ( r 2 ∼0.97) was found between the amount of methyl paraben in the silicone membrane and the corresponding alcohol uptake. Calculated “specific” vehicle–membrane partition coefficients for both alcohols were approximately one, suggesting that the effective concentrations of methyl paraben inside and outside the membrane were the same. Thermodynamic analysis of the alcohol–membrane partition coefficients as a function of temperature showed no apparent trend for butanol, with an associated enthalpy change of approximately zero. Conversely, there was a positive trend in the van’t Hoff plot for methyl paraben in heptanol, indicative of an exothermic process. Moreover, the partitioning trends of methyl paraben in silicone membranes obtained from membrane transport and equilibrium experiments were not the same. This reflects the fundamental differences between the calculated vehicle–membrane partition coefficients in the two studies. Finally, the findings from membrane transport and equilibrium experiments support a model of alcohol enhanced permeation where high solvent sorption promotes high solute concentrations in the overall volume of the membrane (i.e. K), thus leading to modified solute transport (i.e. increased flux). The same model also accounts for changes in membrane diffusivity (i.e. D) related to the properties of the imbibed alcohol.

Novel lariat calix[4]-1,3-aza-crowns with two branched chains — The excellent phase transfer catalysts for nucleophilic substitution reaction

By reacting calix[4]-1,3-diethoxylaminoethyl derivative (2) with phenyl isothiocyanate, novel dendritic calix[4]arene derivative (3) with two different branched chains was synthesized in a yield of 78%. By reacting compound 2 with 1,6-diisocyanatohexane or N,N′-bis(2-chloracetamide)ethylene in a “1 + 1” intermolecular addition mode, novel lariat calix[4]-1,3-aza-crowns (4 and 5, respectively) with two branched ethoxyl chains were prepared in reasonable yields. The composition, structures, and conformations of all new compounds were confirmed by elemental analyses, IR, ESI-MS, 1H NMR, and so forth. The liquid–liquid extraction experiments showed that all new hosts possessed good extraction abilities towards soft and hard metal cations. The liquid membrane transport experiments suggested that they had good transport abilities for K+ and Ag+. The experiments of phase transfer catalysis indicated that they possessed excellent catalytic properties of aromatic nucleophilic substitution reaction and benzyl nucleophilic substitution. The optimum yields of products in catalytic reaction were as high as approximately 100%.

Anion Selectivity in Zwitterionic Amide‐Functionalised Metal Salt Extractants

Amide-functionalised salen ligands capable of extracting metal salts have been synthesised and characterised. Single-crystal X-ray structure determinations of complexes of NiSO4, [Ni(L)(SO4)], confirm that the ionophores are in a zwitterionic form with Ni(II) bound in the deprotonated salen moiety and the SO4(2-) ion associated with protonated pendant N'-amidopiperazine groups. Treatment of [Ni(L)(SO4)] with base removes the protons from the pendant amido-amine group resulting in loss of the SO4(2-) ion and formation of metal-only complexes of type [Ni(L-2H)], which have been characterized by single-crystal X-ray diffraction. Three of the ligands with solubilities suitable for solvent extraction studies show loading and stripping pH-profiles that are suitable for the recovery of CuSO4 or CuCl2 from industrial leach solutions. The copper-only complexes, [Cu(L-2H)], are selective for Cl- over SO4(2-) in both solvent extraction and bulk liquid membrane transport experiments and were found to bind Cl- in two steps via the formation of a 1:1:1 [Cu(L-H)Cl] assembly, followed by a 1:1:2 [Cu(L)Cl2] assembly as the pH of the aqueous phase is lowered. The anion transport selectivity was evaluated for a number of other mono-charged anions and interestingly the ligands were found to display a preference for the Br- ion. To probe the influence of the Hofmeister bias on the selectivity of anion complexation, single-phase potentiometric titration experiments were employed to investigate the binding of SO4(2-) and Cl- by one of the copper only complexes, [Cu(L-2H)] in 95 %/5 % MeOH/water. Under these conditions selectivity was reversed (SO4(2-)>Cl-) confirming that the Hofmeister bias, which reflects the relative hydration energies of the anions, dominates the selectivity of anion extraction from aqueous media into CHCl3.

Bioenergetics and mechanical actuation analysis with membrane transport experiments for use in biomimetic nastic structures

Nastic structures are synthetic constructs capable of controllable deformation and shape change similar to plant motility, designed to imitate the biological process of nastic movement found in plants. This paper considers the mechanics and bioenergetics of a prototype nastic structure system consisting of an array of cylindrical microhydraulic actuators embedded in a polymeric plate. Non-uniform expansion/contraction of the actuators in the array may yield an overall shape change resulting in structural morphing. Actuator expansion/contraction is achieved through pressure changes produced by active transport across a bilayer membrane. The active transport process relies on ion-channel proteins that pump sucrose and water molecules across a plasma membrane against the pressure gradient. The energy required by this process is supplied by the hydrolysis of adenosine triphosphate. After reviewing the biochemistry and bioenergetics of the active transport process, the paper presents an analysis of the microhydraulic actuator mechanics predicting the resulting displacement and output energy. Experimental demonstration of fluid transport through a protein transporter follows this discussion. The bilayer membrane is formed from 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-[Phospho-L-Serine] (Sodium Salt), 1-Palmitoyl-2-Oleoyl-sn-Glycero- 3-Phosphoethanolamine lipids to support the AtSUT4 H+-sucrose cotransporter.

Selective ω-thiocaprolactam-based recovery of Au(III) from chloride media in solvent extraction and polymer inclusion membrane systems

The selective transport of Au(III) over Cu(II) and Hg(II) using ω-thiocaprolactam as extractant and carrier in solvent extraction and polymer inclusion membrane transport experiments is discussed. Solvent extraction experiments were carried out using the extractant dissolved in chloroform to establish the chemical equilibria involved in the Au(III)-ω-thiocaprolactam separation system. Au(III) is extracted into the organic phase quantitatively ( E (%) > 99%) at low extractant concentrations from aqueous HCl solutions. Cu(II) extraction kinetics is slow compared to Au(III) extraction, allowing for the efficient separation of both metals. Polymer inclusion membranes (PIMs) are designed as alternatives to the use of high amounts of hazardous and expensive reagents. Several stripping phases were tested. Up to 70% of Au(III) can be recovered from 1 M HCl feed aqueous phase solutions. Interference from Cu(II) or Hg(II) is negligible at short times (160 min) in competitive ([M(II, III)] 0,feed = 0.1 mM) membrane transport experiments using a cellulose triacetate/NPOE/ω-thiocaprolactam PIM and a potassium iodide–hydrochloric acid aqueous stripping solution. Apparent diffusion coefficients in the membrane for Au(III) and Hg(II) in the system were estimated to be 2.2 × 10 −11 and 3.4 × 10 −15 m 2 s −1, respectively.

Silver(i) complexation of linked 2,2′-dipyridylamine derivatives. Synthetic, solvent extraction, membrane transport and X-ray structural studies

Synthesis of the 2,2'-dipyridylamine derivatives di-2-pyridylaminomethylbenzene 1, 1,2-bis(di-2-pyridylaminomethyl)benzene 2, 1,3-bis(di-2-pyridylaminomethyl)benzene 3, 2,6-bis(di-2-pyridylaminomethyl)pyridine 4, 1,4-bis(di-2-pyridylaminomethyl)benzene 5, and 1,3,5-tris(di-2-pyridylaminomethyl)benzene 6 are reported together with the single-crystal X-ray structures of 2, 3, and 5. Reaction of individual salts of the type AgX (where X = NO(3)(-), PF(6)(-), ClO(4)(-), or BF(4)(-)) with the above ligands has led to the isolation of thirteen Ag(I) complexes, nine of which have also been characterised by X-ray diffraction. In part, the inherent flexibility of the respective ligands has resulted in the adoption of a range of coordination arrangements. A series of liquid-liquid (H(2)O/CHCl(3)) extraction experiments of Ag(I) with varying concentrations of 1-6 in the organic phase have been undertaken, with the counter ion in the aqueous phase being respectively picrate, perchlorate and nitrate. In general, extraction efficiencies for a given ionophore followed the Hofmeister order of picrate > perchlorate > nitrate; in each case the tris-dpa derivative 6 acting as the most efficient extractant of the six systems investigated. Competitive seven-metal bulk membrane transport experiments (H(2)O/CHCl(3)/H(2)O) employing the above ligands as the ionophore in the organic phase and equimolar concentrations of Co(II), Ni(II), Zn(II), Cu(II), Cd(II), Pb(II) and Ag(I) in the aqueous source phase were also undertaken, with transport occurring against a pH gradient. Under the conditions employed 1 and 5 yielded negligible transport of any of the metals present in the source phase while sole transport selectivity for Ag(I) was observed for 2-4 and 6.

Supramolecular Polymeric Macrocyclic Receptors – Hybrid Carrier versus Channel Transporters in Bulk Liquid Membranes

The synthesis of heteroditopic macrocyclic ureido-receptors is described. NMR studies show the formation of self-organized oligomeric superstructures. Membrane transport experiments show a direct relation between the self-assembling and the transport properties of these molecular information transfer devices. The self-organization of these receptors may provide the first evidence for the possible hybrid transport carrier versus channel mechanisms in liquid membranes.

Separation characteristics of tetrapropylammoniumbromide templating silica/alumina composite membrane in CO2/N2, CO2/H2 and CH4/H2 systems

Nanoporous silica membrane without any pinholes and cracks was synthesized by organic templating method. The tetrapropylammoniumbromide (TPABr)-templating silica sols were coated on tubular alumina composite support ( γ-Al2O3/ α-Al2O3 composite) by dip coating and then heat-treated at 550 °C. By using the prepared TPABr templating silica/alumina composite membrane, adsorption and membrane transport experiments were performed on the CO2/N2, CO2/H2 and CH4/H2 systems. Adsorption and permeation by using single gas and binary mixtures were measured in order to examine the transport mechanism in the membrane. In the single gas systems, adsorption characteristics on the α-Al2O3 support and nanoporous unsupport (TPABr templating SiO2/ γ-Al2O3 composite layer without α-Al2O3 support) were investigated at 20–40 °C conditions and 0.0–1.0 atm pressure range. The experimental adsorption equilibrium was well fitted with Langmuir or/and Langmuir-Freundlich isotherm models. The α-Al2O3 support had a little adsorption capacity compared to the unsupport which had relatively larger adsorption capacity for CO2 and CH4. While the adsorption rates in the unsupport showed in the order of H2> CO2> N2> CH4 at low pressure range, the permeate flux in the membrane was in the order of H2≫N2> CH4> CO2. Separation properties of the unsupport could be confirmed by the separation experiments of adsorbable/non-adsorbable mixed gases, such as CO2/H2 and CH4/H2 systems. Although light and non-adsorbable molecules, such as H2, showed the highest permeation in the single gas permeate experiments, heavier and strongly adsorbable molecules, such as CO2 and CH4, showed a higher separation factor (CO2/H2=5-7, CH4/H2=4-9). These results might be caused by the surface diffusion or/and blocking effects of adsorbed molecules in the unsupport. And these results could be explained by surface diffusion.

Macrocyclic ligand design. Structure–function relationships involving the interaction of pyridinyl-containing, oxygen–nitrogen donor macrocycles with selected transition and post transition metal ions on progressive N-benzylation of their secondary amines

Structure-function relationships underlying the interaction of progressively N-benzylated N(4)O(2)-donor macrocycles with cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II) have been probed using a range of techniques that include X-ray diffraction, DFT computations, solvent extraction, potentiometric stability constant determinations and competitive membrane transport experiments. Collectively, the results indicate that N-benzylation of the secondary amine donor groups of the parent macrocyclic ring results in an enhanced tendency towards selectivity for silver(I) relative to the other six metals investigated. The observed behaviour serves as additional exemplification of the previously proposed concept of selective 'detuning' as a mechanism for metal ion discrimination.

Arsenic(V) removal with polymer inclusion membranes from sulfuric acid media using DBBP as carrier.

Polymer inclusion membranes (PIMs) based on cellulose triacetate (CTA) and dibutyl butyl phosphonate (DBBP) were tested for arsenic(V) separation from H2SO4 for its recovery from copper electrolytes. Solvent extraction experiments allowed the determination of the As(V)-DBBP and H2SO4-DBBP complexes formed in the organic phase. Application of a transient model to membrane transport experiments in solutions containing only arsenic or H2SO4 indicated that it occurred under a kinetically controlled regime by formation of H3AsO4[DBBP]2 and H2SO4[DBBP] species, respectively. When arsenic and H2SO4 are simultaneously present, the existence of a third species, H3AsO4[DBBP][H2SO4], explains well the fact that As(V) flux decreases and that H2SO4 flux increases. In both cases, a limiting 50% recovery value was obtained. However, active arsenic transport (>50%) is achieved if the H2SO4 concentration gradient is assured (e.g., using a triple-cell configuration). In this way, high arsenic recovery factors (90% in 800 min) were obtained with initial concentrations of 5000 mg/L As(V) and 220 g/L H2SO4. In all membrane systems tested, good As(V) selectivity over copper (up to 30000 mg/L) was attained.

Self-organized heteroditopic macrocyclic superstructures.

[structure: see text] The synthesis of heteroditopic macrocyclic ureido receptors and of their NaX complexes is described. NMR studies and determination of the crystal structure show the formation of self-organized dimeric or polymeric superstructures by a cooperative macrocyclic cation complexation, anion-hydrogen bonding, and pi-pi stacking interactions. Membrane transport experiments show a direct relation between the synergetic ion-pair recognition and the transport properties of these molecular information transfer devices.

Metal ion recognition. Interaction of a series of successively N-benzylated derivatives of 1,4,8,11-tetraazacyclotetradecane (cyclam) with selected transition and post-transition metal ions

The interaction of a series of N-benzylated cyclam ligand derivatives incorporating from one to four N-benzyl groups with cobalt(II), zinc(II), cadmium(II), lead(II) and silver(I) nitrates is reported; the results are compared with those obtained in a prior study involving the formation of the corresponding complexes of nickel(II) and copper(II) with this ligand series. The isolation of a selection of 1 ∶ 1 (metal ∶ ligand) complexes has been carried out and X-ray diffraction structures of five such products have been determined. In each complex the metal ion is coordinated to the N4-donor set of the cyclam ring with the latter adopting either a trans-I or a trans-V configuration; monodentate or bidentate nitrato ligands complete the respective coordination spheres. Coordination numbers range from five in a zinc(II) complex, through six in two cadmium(II) complexes, to eight in two lead(II) complexes. Competitive bulk membrane transport experiments (water/chloroform/water) have been carried out in which each of the five benzylated ligands were in turn employed as the ionophore in the chloroform phase. The respective aqueous source phases (buffered at pH 4.9) contained an equimolar mixture of cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II) nitrates. The aqueous receiving phase was buffered at pH 3.0. Under these conditions, high selectivity for copper(II) was observed for each of the ionophores incorporating one or two N-benzyl groups whereas no transport for any of the seven metals present in the source phase was observed for the tribenzylated derivative. In marked contrast to the above behaviour, the tetra-N-benzyl derivative yielded sole transport selectivity for silver(I) under similar conditions. 1H NMR and 13C NMR titration experiments in CD3CN–CDCl3 confirmed the 1 ∶ 1 stoichiometry of the silver(I) tetra-N-benzylcyclam complex in this solvent mixture.

A Computer Simulation of Functional Group Contributions to Free Energy in Water and a DPPC Lipid Bilayer

A series of all-atom molecular dynamics simulations has been performed to evaluate the contributions of various functional groups to the free energy of solvation in water and a dipalmitoylphospatidylcholine lipid bilayer membrane and to the free energies of solute transfer (Δ(Δ G°) X) from water into the ordered-chain interior of the bilayer. Free energies for mutations of the α-H atom in p-toluic acid to six different substituents ( CH 3, Cl, OCH 3, CN, OH, COOH) were calculated by a combined thermodynamic integration and perturbation method and compared to literature results from vapor pressure measurements, partition coefficients, and membrane transport experiments. Convergence of the calculated free energies was indicated by substantial declines in standard deviations for the calculated free energies with increased simulation length, by the independence of the ensemble-averaged Boltzmann factors to simulation length, and the weak dependence of hysteresis effects on simulation length over two different simulation lengths and starting from different initial configurations. Calculated values of Δ(Δ G°) X correlate linearly with corresponding values obtained from lipid bilayer transport experiments with a slope of 1.1 and from measurements of partition coefficients between water and hexadecane or decadiene, with slopes of 1.1 and 0.9, respectively. Van der Waals interactions between the functional group of interest and the acyl chains in the ordered chain region account for more than 95% of the overall potential energy of interaction. These results support the view that the ordered chain region within the bilayer interior is the barrier domain for transport and that solvation interactions within this region resemble those occurring in a nonpolar hydrocarbon.