Heat Effects of Dehydration of Human Serum Albumin in Hydrophilic Organic Solvents

A thermochemical model for describing the transfer of water from the protein phase to the organic solvent liquid phase and for determining how the solvation ability of organic solvents affects this process was developed. Enthalpy changes on the interaction of dried and hydrated human serum albumin (HSA) with hydrophilic organic solvents (dimethyl sulfoxide, formamide, ethanol, methanol and acetic acid) and water were measured by isothermal calorimetry at 25 °C. The initial hydration level of human serum albumin was varied in the entire water content range from 0–30 % [g water/g HSA]. The dependence of the interaction enthalpies on the initial water content is complex. The interaction enthalpies of the dried HSA with organic solvents are exothermic. At low water contents (less than 0.1 g/g), there is a sharp increase in the interaction enthalpy values. At the highest water contents (more than 0.2 g/g), the interaction enthalpies are endothermic for acetic acid and formamide and exothermic for DMSO, methanol, and ethanol. These thermochemical data were analyzed in conjunction with the results for the water adsorption in organic solvents to calculate the molar enthalpies of dehydration of HSA in organic liquids. It was found that the dehydration enthalpy changes may be endothermic or exothermic depending on the initial water content and the water solvation enthalpy value. From the results obtained, it can be concluded that: (i) only the solvation of water by hydrophilic organic solvent determines the changes in the dehydration enthalpy values, and (ii) the data for the enthalpies of solvation of water by the solvent at infinite dilution reflect this effect.     

Measurement of preferential solvation of some glucans in mixed solvent systems by gel-permeation chromatography

Preferential solvation of the glucans amylose, pullulan, and dextran in binary dimethyl-sulfoxide/water (DMSO/H₂O) solvent mixtures has been measured using gel-permeation chromatography. The preferential solvation behavior of the three glucans in DMSO/H₂O solvent mixtures is indistinguishable in the experiments reported. In solvent mixtures with mol ratio DMSO/H₂O less than 1:2, all three glucans are solvated preferentially by H₂O. The maximum extent of preferential solvation by H₂O is about 2.5 mol H₂O/mol of glucose residues. When the DMSO/H₂O mol ratio exceeds 1:2, DMSO solvates the glucans preferentially to a maximum extent of about 1 mol DMSO/mol of glucose residues. An interpretation of the change in preferential solvation with mixed solvent composition is suggested in terms of the known characteristics of the binary solvent system, and the relationship of preferential solvation, reported here, to the absolute solvation of the glucan chains is discussed.     

Pairwise interaction enthalpies of enantiomers of β-amino alcohols in DMSO + H2O mixtures at 298.15 K

The dilution enthalpies of enantiomers of six β-amino alcohols, namely (R)-(-)-2-amino-1-propanol versus (S)-(+)-2-amino-1-propanol, (R)-(-)-2-amino-1-butanol versus (S)-(+)-2-amino-1-butanol, and (R)-(-)-2-amino-1-pentanol versus (S)-(+)-2-amino-1-pentanol in dimethylsulfoxide (DMSO) + H(2)O mixtures (mass fractions of DMSO w = 0 to 0.3) have been determined respectively using an isothermal titration calorimeter (MicroCal ITC200, Northampton, MA, USA) at 298.15 K. According to the McMillan-Mayer theory, the corresponding homochiral enthalpic pairwise interaction coefficients (h(XX)) of the six amino alcohols have been calculated. It is found that across the whole studied composition range of mixed solvent, values of h(XX) for S-enantiomer are almost universally higher than those of R-enantiomer for each amino alcohol and that the variations of h(XX) depend largely on the composition of mixed solvent. The results were interpreted from the point of view of solute-solute interaction mediated by cosolvent DMSO, as well as competition equilibrium between hydrophobic-hydrophobic, hydrophilic-hydrophilic, and hydrophobic-hydrophilic interactions.     

Hydration enthalpy characteristics of amino acids in solutions

The enthalpies of solvation of 17 amino acids were evaluated by using the sublimation enthalpies of amino acids and the standard enthalpies of their solution in water. An equation was derived, which relates the volume-specific enthalpy of sublimation (deltaH(subl)/V(w)) to the sum of the common bond lengths in molecules (sigman(i)l(i)) of substances examined. The results obtained are interpreted in terms of the effect of hydrophobic and hydrophilic side chain on the interactions between the zwitterions of amino acids and water molecules.     

The interaction of poly(N5-(3-hydroxypropyl)-L-glutamine) with solvent components in water/dioxane mixtures.

The preferential binding of solvent components with a nonionic homopolypeptide, poly(N⁵-(3-hydroxypropyl)-L -glutamine), ([Gln((CH₂)₃OH)]_n), has been determined in water/dioxane mixtures using differential refractometry. The degree of preferential binding was calculated from the difference between the refractive index increments of [Gln((CH₂)₃OH)]_n obtained from experiments carried out under two conditions: experiments where the molality of dioxane was kept identical in both compartments of the differential cell, and experiments where the chemical potential was kept identical. The polypeptide was preferentially hydrated between 10 and 70 wt % of dioxane; the amount of preferential hydration per gram of the mixed solvent increases monotonically (with a plateau region between 40 and 60 wt %) with the dioxane concentration. A monotonic increase was also observed in the degree of helicity of the polypeptide. The absolute amounts of water and dioxane bound by [Gln((CH₂)₃OH)]_n were investigated in the frozen state by the method of nuclear magnetic resonance. Hydration was measured using a mixed solvent, water/dioxane-d₈; dioxane solvation was measured using a mixed solvent, dioxane/D₂O. The polypeptide binds about 0.35 g of water per g of the polymer in aqueous solution, and hydration decreases gradually with an increase in dioxane concentration. On the other hand, the amount of dioxane solvation increases to 0.04 g per g of the polymer in the dioxane concentration range between 0 and 20 wt %, and then levels off. The rapid increase in solvation is observed before the conformational transition from random coil to α-helix occurs in [Gln((CH₂)₃OH)]_n. The dependence of the preferential and absolute binding of solvent components to [Gln((CH₂)₃OH)]_n on dioxane concentration and the conformational change in the homopolypeptide suggest that addition of dioxane to aqueous solutions induces lowering of water activity and that the helical structure of the polypeptide is enhanced by the formation of intrachain hydrogen bonds. The validity of the frozen method is also discussed.     

Self-association of purine base, 6-methylpurine, in water - organic component mixtures

Thermodynamic quantities of the self-association of 6-methylpurine in water (1)-dimethylsulfoxide (DMSO) (2) mole fraction, x2 less than 0.1) and water (1) - N,N-dimethylformamide (DMF) (2) (x2 less than or equal to 1.0) mixed solvents have been obtained through heat of dilution measurements, at 25 degrees C. In the water-DMSO solvent system, the standard enthalpy and entropy changes, delta Ho and delta So, of the association exhibited an abrupt behavior. They decreased remarkbly with the the mole fraction of DMSO until about x2 = 0.012 and after that, they increased steeply. In the case of water-DMF solvent system, the values of delta Ho and delta So didn't show the abrupt behavior. They decreased steeply until about x2 = 0.1 and, at higher mole fractions, became relatively constant. These behaviors are rationalized on the basis of solvent structural effects and solvation in these association systems.     

Calorimetric study of the interaction between amino acids and sugars in water at 298.15 K

The enthalpies of dissolving glycine and DL-alanine in water solutions of D-glucose, D-maltose, and sucrose at 298.15 K were determined by calorimetry. From the results obtained, the coefficients of enthalpy for pairwise interactions hxy of the amino acids and saccharides in water were calculated. It was found that the hxy values for glycine in solutions of all saccharides studied are negative; in the case of DL-alanine, the hxy values are positive for all saccharides except for sucrose solution. It was shown that the hxy values reflect the sum effect of interactions between the amino acids and saccharides in aqueous solutions and the contribution of hydration of the solutes.     

The heat of transfer of lipid and surfactant from vesicles into micelles in mixtures of phospholipid and surfactant.

We study the heat associated with the transformation of vesicles into micelles in mixtures of bilayer-forming phospholipids and micelle-forming surfactants. We subdivide the total heat evolution deltaQ(coex) within the range of coexistence of vesicles and micelles into three contributions related to the transition of dN(D)m-b molecules of surfactant and dN(L)m-b molecules of lipid from micelles to vesicles and to the extraction of dN(D)m-w molecules of surfactant from micelles to the aqueous solution, so that deltaQ(coex) = deltaH(D)m-w x dN(D)m-w + deltaH(D)m-b x dN(D)m-b + deltaH(L)m-b x dN(L)m-b where deltaH(D)m-w, deltaH(L)m-b, and deltaH(D)m-b are the respective molar "transfer" enthalpies. We design a method for the evaluation of all three molar enthalpies, from isothermal calorimetric titrations conducted according to two different protocols of titration of lipid-surfactant mixtures. In the first protocol the mixture is titrated with an aqueous solution of pure lipid vesicles, and in the second the mixture is titrated with an aqueous solution of pure surfactant. Titration of the mixed systems by a buffer solution serves to verify the results obtained under these protocols. In addition to the values of molar enthalpies, our method yields the cmc value of the pure surfactant. We apply our method to investigating the heat evolution in mixtures of egg yolk phosphatidylcholine and the nonionic surfactant octylglucoside in a phosphate-buffered saline solution at 28 degrees C. These studies gave the following values: deltaH(D)m-w = -1732 cal/mol, deltaH(L)m-b = -592 cal/mol, deltaH(D)m-b = 645 cal/mol, and cmc = 23.5 mM. We discuss the possible physical insight of these values and the perspectives of applications of the proposed method.     

Amylose optical rotation in some mixed solvent systems

This paper extends a previous study in which a discontinuity in the specific rotation of open chain α-l,4-linked glucopyranosides in the water–dimethyl sulfoxide (H₂O–DMSO) system was attributed to a symmetry change about a polymer chain segment. Optical rotation of amylose, cyclohexamylose, methyl β-maltoside, and dextran was measured in the following mixed solvent systems: formamide–dimethyl sulfoxide (F-DMSO), ethylenediamine–dimethyl sulfoxide (E–DMSO), and hexamethylphosphoramide–dimethyl sulfoxide (HMPA–DMSO). Refractive index measurements were used in an attempt to detect hydrogen bonding between solvent components. The specific rotation of amylose corrected for variation in refractive index (CSR), as a function of solvent composition, showed a discontinuity at solvent compositions corresponding to about 1 mole F to 2 moles DMSO and to 1 mole E to at least 8 moles DMSO. A discontinuity in the CSR function of amylose in the H₂O-DMSO mixed solvent that occurs at 25°C is not observed at 70°C. The CSR function of methyl-β-maltoside exhibits a discontinuity in solvent composition corresponding to mole ratios between 2F–DMSO and 3F–DMSO. Present results indicate that an amylose chain segment may undergo a symmetry change in solvent compositions corresponding to mole ratios between F–DMSO and F–2DMSO. Our CSR measurements of amylose and model compounds in E–DMSO and HMPA–DMSO do not permit us to distinguish between possible changes in amylose chain segment symmetry and solvent interactions that could affect symmetry properties of the glucopyranose ring.     

Molecular dynamics and quantum chemical studies of solvent effects on cyclo glycylglycine and glycylalanine dipeptides

Hydrogen bond (H-bond) interactions between the two cyclo dipeptides, cyclo(glycyl-glycine) (CGG) and cyclo(glycyl-alanine) (CGA), and water have been studied using molecular dynamics (MD) and quantum chemical methods. The MD studies have been carried out on CGG and CGA in water using fixed charge force field (AMBER ff03) for over 10 ns with a MD time step of 2 fs. The results of this study show that the solvation pattern influences the conformations of the cyclo dipeptides. Following molecular simulations, post Hartree–Fock and density functional theory methods have been used to explore the molecular properties of the cyclo dipeptides in gaseous and aqueous phase environments. The self-consistent reaction field theory has been used to optimise the cyclopeptides in diethyl ether (? = 4.3) and water (? = 78.5), and the solvent effects have been analysed. A cluster of eight water molecules leads to the formation of first solvation shell of CGG and CGA and the strong H-bonding mainly contributes to the interaction energies. The H-bond interactions have been analysed by the calculation of electron density ρ(r) and its Laplacian ▽²ρ(r) at bond critical points using atoms in molecules theory. The natural bond orbital analysis was carried out to reveal the nature of H-bond interactions. In the solvated complexes, the keto carbons registered the maximum NMR chemical shifts.     

α‐chymotrypsin in water–acetone and water–dimethyl sulfoxide mixtures: Effect of preferential solvation and hydration

We investigated water/organic solvent sorption and residual enzyme activity to simultaneously monitor preferential solvation/hydration of protein macromolecules in the entire range of water content at 25°C. We applied this approach to estimate protein destabilization/stabilization due to the preferential interactions of bovine pancreatic α‐chymotrypsin with water‐acetone (moderate‐strength H‐bond acceptor) and water‐DMSO (strong H‐bond acceptor) mixtures. There are three concentration regimes for the dried α‐chymotrypsin. α‐Chymotrypsin is preferentially hydrated at high water content. The residual enzyme activity values are close to 100%. At intermediate water content, the dehydrated α‐chymotrypsin has a higher affinity for acetone/DMSO than for water. Residual enzyme activity is minimal in this concentration range. The acetone/DMSO molecules are preferentially excluded from the protein surface at the lowest water content, resulting in preferential hydration. The residual catalytic activity in the water‐poor acetone is ～80%, compared with that observed after incubation in pure water. This effect is very small for the water‐poor DMSO. Two different schemes are operative for the hydrated enzyme. At high and intermediate water content, α‐chymotrypsin exhibits preferential hydration. However, at intermediate water content, in contrast to the dried enzyme, the initially hydrated α‐chymotrypsin possesses increased preferential hydration parameters. At low water content, no residual enzyme activity was observed. Preferential binding of DMSO/acetone to α‐chymotrypsin was detected. Our data clearly demonstrate that the hydrogen bond accepting ability of organic solvents and the protein hydration level constitute key factors in determining the stability of protein–water–organic solvent systems.     

Effects of solvent composition and ionic strength on the interaction of quinoline antimalarials with ferriprotoporphyrin IX

Enthalpy-entropy compensation in the interaction of quinoline antimalarials with ferriprotoporphyrin IX (Fe(III)PPIX) in 40% aqueous dimethyl sulfoxide (DMSO) has been compared with that in pure aqueous solution. The data indicate that the degree of desolvation and loss of conformational freedom is virtually identical in both systems. Taken together with previous findings showing that the molar free energies of association of these drugs with Fe(III)PPIX in both solvent systems are very similar, this suggests that the recognition site on the metalloporphyrin is comparable in both cases. This is despite the fact that Fe(III)PPIX exists as a dimer in aqueous solution, but is monomeric in 40% DMSO. Free energies of association of chloroquine, quinine and quinidine with Fe(III)PPIX are largely insensitive to the concentration of sodium perchlorate in 40% DMSO. This demonstrates that electrostatic interactions play only a minor role in the overall stability of these complexes under these conditions. Increasing DMSO concentration greatly weakens the interactions of chloroquine, amodiaquine, quinine, quinidine and 9-epiquinine with Fe(III)PPIX. This suggests that hydrophobic interaction plays a major role in the stability of these complexes. Further investigation of chloroquine has revealed that the free energy of association with Fe(III)PPIX also weakens as a function of decreasing solvent polarity in pure organic solvents. However, the free energies of association are weaker in the mixed aqueous solvent than in pure organic solvents. This indicates that dispersion and electrostatic interactions are relatively strong in the non-aqueous environment. The results demonstrate that any successful model of antimalarial drug-Fe(III)PPIX interactions will need to take both solvation and electrostatic factors into account.     

Studies on Escherichia coli enzymes involved in the synthesis of uridine diphosphate-N-acetyl-muramyl-pentapeptide

The specific activities of l-alanine:d-alanine racemase, d-alanine:d-alanine ligase, and the l-alanine, d-glutamic acid, meso-diaminopimelic acid, and d-alanyl-d-alanine adding enzymes were followed during growth of Escherichia coli. The specific activities were nearly independent of the growth phase. d-Alanine:d-alanine ligase was inhibited by d-alanyl-d-alanine, d-cycloserine, glycine, and glycyl-glycine. l-Alanine:d-alanine racemase was found to be sensitive to d-cycloserine, glycine, and glycyl-glycine. The l-alanine adding enzyme was inhibited by glycine and glycyl-glycine.     

Solvent effects on coupling yields during rapid solid-phase synthesis of CGRP(8-37) employing in situ neutralization.

The success of solid-phase peptide synthesis is often dependent upon solvation of the resin and the growing resin-bound peptide chain. We investigated the relationship between solvent properties and solvation of the resin and peptide-resin in order to obtain satisfactory coupling yields for the rapid solid-phase peptide synthesis, using butyloxycarbonyl-(Boc)-amino acid derivatives, of human-alpha-calcitonin gene-related peptide(8-37) (CGRP(8-37)). Solvation of (p-methylbenzhydrylamine)copoly(styrene-1% divinylbenzene (DVB) (resin) and resin covalently bound to the fully protected amino acid sequence of CGRP(8-37) (peptide-resin) was correlated to solvent Hildebrand solubility (delta) and hydrogen-bonding (delta(h)) parameters. Contour solvation plots of delta(h) vs. delta revealed maximum solvation regions of resin and peptide-resin. Maximum resin solvation occurred with N-methylpyrrolidinone (NMP), NMP : dimethylsulfoxide (DMSO) (8 : 2) and DMSO. Inefficient solvation of the peptide-resin occurred with these solvents and resulted in poor syntheses with average coupling yields of 78.1, 88.9 and 91.8%, respectively. Superior peptide-resin solvation was obtained using dimethylacetamide (DMA) and dimethylformamide (DMF), resulting in significantly higher average coupling yields of 98.0 and 99.5%, respectively. Thus, the region of maximum peptide-resin solvation shifts to solvents with higher delta(h) values. DMF provided the most effective peptide-resin solvation and was the only solvent from which CGRP(8-37) was obtained as a single major product in the crude cleaved material.     

Temperature dependence of histidine ionization constants in myoglobin

The standard enthalpy of ionization of six titratable histidines in horse metaquomyoglobin was determined by repeating proton NMR titrations as a function of temperature and using the van't Hoff relationship. It was found that deltaH degrees varies between 16 and 37 kJ mol(-1) in the protein, compared with a value of 29 kJ mol(-1) in free histidine. The standard entropy change was evaluated by combining the enthalpy and free energy changes derived from the pKa values. Although the entropy change could not be precisely and accurately obtained by this method, it could be established that it spans a wide range, from -60 to 0 J K(-1) mol(-1), about the value of -23 J K(-1) mol(-1) for the free histidine. The entropy change was used within the framework of enthalpy-entropy compensation to partition the solvation component from the standard thermodynamic quantities for each of the titrating residues. It was shown that the partitioning of the values in the protein is not readily understood in terms of solvent accessibility or electrostatic interactions. The contribution of solvation effects to the temperature response appeared to be significant only in the case of His-119 and His-48. The standard quantities were also used to explore the energetics of proton binding in the native state at temperatures below the onset of thermal denaturation.     

Compensational phenomena in reactivation of dimethyl- and diethylphosphoryl butyrylcholinesterases.

The thermodynamic and kinetic parameters for spontaneous and oxime reactivation of dimethyl- and diethylphosphoryl butyrylcholinesterases (acylcholine acyl-hydrolase, EC 3.1.1.8) are reported. The enthalpy and entropy changes in both the binding (deltaH0 and deltaS0) and the dephosphorylation steps (deltaH* and deltaS*) were found to be coupled, resulting in a minor variation in free energy changes (deltaG0 and deltaG*). While neither enthalpies nor entropies alone bore any relationship with the kinetic parameters KD and kR, the changes of free energies (deltaG0 and deltaG*) correlated linearly with the logarithmic values of the dissociation constants (KD) and bimolecular rate constants (kR/KD), respectively. Compensation plots of entropies versus enthalpies gave straight lines with compensation temperatures of 275 K for the binding 260 K for the dephosphorylation. Spontaneous reactivation of dimethyl phosphoryl butyrylcholinesterase was investigated at various pH values and three temperatures. It implicated two catalytic sites with values of pKi of 9.4 and 7.5, and heats of ionisation of 5.3 and 9.6 kcal - mol-1, respectively. Possible conformational alteration of the inhibited enzyme arising from the binding of oximes is discussed.     

Insights into the dynamics of DMSO in phosphatidylcholine bilayers.

The solvation effects of dimethyl sulfoxide (DMSO) on the phase stability of dimyristoylphosphatidylcholine (DMPC) have been fully characterized using differential scanning calorimetry (DSC) and fluorescence spectroscopy with 1,6-diphenyl-1,3,5-hexatriene (DPH). The temperatures of the sub-, pre-, and main transitions of DMPC were found to increase linearly with increasing mole fraction of DMSO up to mole fraction X=0.13 DMSO/H(2)O. Beyond X=0.13, the pre-transition peak started to merge with the peak representing the main transition. Simultaneously, the subtransition peak began to disappear as its transition temperature also decreased. At X=0.18, with both the subtransition and pre-transition absent, the main transition between the planar gel and the liquid-crystalline phase was observed at 30.3 degrees C. Transition enthalpy values indicated that the subgel, planar gel and rippled gel phases are most stable at X=0.11, 0.16 and 0.20 DMSO/H(2)O, respectively. This demonstrates that DMSO exerts distinct effects on each respective phase and corresponding transition. Temperature-dependent fluorescence emission scans show an increase in hydration as the system proceeds from the subgel phase all the way to the liquid-crystalline phase and correlated well with the effects of DMSO on the transition temperatures of DMPC observed in our calorimetry data. Initial observations for the sub- and main transition are further confirmed by fluorescence anisotropy using DPH as a probe. The results illustrate the differences in the microviscosity of each phase and how DMSO affects the phase transitions. Ultimately, our results suggest the most likely mechanism governing the biological actions of DMSO may involve the regulation of the solvation effects of water on the phospholipid bilayer.     

Powerful solvent systems useful for synthesis of sparingly-soluble peptides in solution.

Our maximum protection strategy for the synthesis of human parathyroid hormone(1-84) indicates that fully protected peptide segments in the form of Boc-peptide phenacyl (Pac) ester are relatively soluble in ordinary organic solvents such as DMF, NMP or DMSO, which are suitable for coupling segments. However, about 1% of such segments synthesized were found to be insoluble even in the most polar solvent, DMSO. Thus, a more powerful solvent which can be used for their peptide synthesis was pursued. Among the solvent systems tested, a mixture of trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) and trichloromethane (TCM) or dichloromethane (DCM) was found to be most powerful for dissolving such sparingly-soluble protected peptides. These solvent systems were confirmed to be useful for the removal reaction of the carboxy-terminal Pac esters from the sparingly-soluble segments. They were then tested for the coupling reactions of fully protected Boc-peptides with other sparingly-soluble peptide esters. The TFE/TCM or TFE/DCM system was extremely useful for coupling segments without danger of racemization and of trifluoroester formation, if WSCI was used as the coupling reagent in the presence of 3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine (HOOBt).     

Proton affinity of para-substituted acetophenones in gas phase and in solution: a theoretical study

The gas phase proton affinities PA and basicities GB for a series of para-substituted acetophenones weak bases (B) p.X???C₆H₄CO*CH₃ with X?=?H, F, Cl, Br, I, Me, CF₃, CN, NO₂, OCH₃, NH₂, CH₂OH, N(CH₃)₂, OH, $ {\text{NH}}_3^{+} $ , … have been calculated at 298.15 K at the density functional theory DFT/B3LYP level with a 6-311++G (2d,2p) basis set. Conformational results lead to only one stable planar conformer for both unprotonated compounds and their O*-protonated forms. Satisfactory accuracy and computational efficiency could be reached if the computed PAs are scaled by a factor 0.983. Protonation at more than one site is discussed and the carbonyl oxygen atom is found to be the preferential protonated site rather than the substituent X. The calculated gas phase PAs show a good agreement with the experimental available data. The electron-donating/electron-withdrawing nature of the substituents has an enormous influence upon the thermochemical and structural properties. The influence of environment on the proton affinity has been studied by means of SCRF solvent effect computations using PCM solvation model for two solvents: water and SO₂CI₂. Confrontation between computed and experimental pK(B) values exhibits better agreement in aqueous solution than in organic solvent.     

The mechanism of the stabilization of the hexagonal II (HII) phase in phosphatidylethanolamine membranes in the presence of low concentrations of dimethyl sulfoxide

Dimethyl sulfoxide (DMSO), a water-miscible organic solvent, has been used as a cryoprotectant for cells. It is known that DMSO stabilizes the HII phase of phosphatidylethanolamine (PE) membranes rather than the Lalpha phase, while most other water-miscible organic solvents such as acetone and ethanol destabilize the HII phase. To elucidate the mechanism for this stabilizing effect of DMSO on the HII phase, we have investigated its effects on the structures and physical properties of PE membranes. X-ray diffraction data indicated that dipalmitoleoylphosphatidylethanolamine (DPOPE) membranes in H2O at 20 degrees C were in the Lalpha phase and that an Lalpha to HII phase transition occurred at X=0.060 (mole fraction of DMSO) in water/DMSO mixtures. As the DMSO concentration increased, the basis vector length of the dioleoylphosphatidylethanolamine (DOPE)/ 16 wt% tetradecane membrane and also of the DPOPE/ 16 wt% tetradecane membrane in the HII phase decreased, suggesting that the spontaneous curvature of these membranes increased. We have also investigated the effects of DMSO on the physical properties of the PE membranes, and compared them with those of acetone. As the DMSO concentration increased, the excimer to monomer fluorescence intensities of pyrene-phosphatidylcholine in the PE membranes decreased, indicating that the membrane fluidity decreased, and also the generalized polarization value of the Laurdan fluorescent probe in the DPOPE membrane increased, indicating that the polarity of the membrane interface decreased. On the other hand, acetone had the opposite effects to DMSO. The interaction free energy between the membrane surface segments and solvent increased with an increase in DMSO concentration. It decreased the amount of solvent in the membrane interface, inducing an increase in the spontaneous curvature. This can reasonably explain the effects of DMSO on the phase stability and the physical properties of the membranes.