Effect of amphiphilic surfactant LDAO on the solubilization of DOPC vesicles and on the activity of Ca(2+)-ATPase reconstituted in DOPC vesicles.

Solubilization of large unilamellar 1,2-dioleoylphosphatidylcholine (DOPC) vesicles by N-dodecyl-N,N-dimethylamine-N-oxide (LDAO) was studied using turbidimetry. From turbidity data, the LDAO partition coefficient between the aqueous phase and DOPC bilayers was obtained. Using this partition coefficient, the LDAO:DOPC molar ratio in the bilayer was calculated and effects of LDAO on the bilayer stability, bilayer thickness and on the phosphohydrolase activity of sarcoplasmic reticulum Ca(2+) transporting ATPase (SERCA) reconstituted into DOPC were compared at the same LDAO:DOPC molar ratios in the bilayer. The sequence "bilayers in vesicles - bilayer fragments (flat mixed micelles) - tubular mixed micelles - globular mixed micelles" was suggested for the solubilization mechanism of DOPC vesicles from the combined turbidimetric and small-angle neutron scattering (SANS) results. The effective molecular packing parameter delta = 0.5, corresponding to the mixed bilayer - mixed tubular micelle transition, was calculated from fragmental DOPC and LDAO volumes at the molar ratio LDAO:DOPC = 2.00 in bilayers, in the middle of transition region observed earlier experimentally by small-angle neutron scattering (SANS). The bilayer thickness decrease induced by LDAO in DOPC observed by SANS did not result in the SERCA phosphohydrolase activity decrease and this indicates that some other factors compensated this bilayer effect of LDAO. The ATPase activity decrease at higher LDAO concentrations was caused by the bilayer deformation. This deformation resulted in the formation of non-bilayer aggregates in LDAO+DOPC system.

Influence of N-dodecyl-N,N-dimethylamine N-oxide on the activity of sarcoplasmic reticulum Ca(2+)-transporting ATPase reconstituted into diacylphosphatidylcholine vesicles: efects of bilayer physical parameters.

Sarcoplasmic reticulum Ca-transporting ATPase (EC 3.6.1.38) was isolated from rabbit white muscle, purified and reconstituted into vesicles of synthetic diacylphosphatidylcholines with monounsaturated acyl chains using the cholate dilution method. In fluid bilayers at 37 degrees C, the specific activity of ATPase displays a maximum (31.5+/-0.8 IU/mg) for dioleoylphosphatidylcholine (diC18:1PC) and decreases progressively for both shorter and longer acyl chain lengths. Besides the hydrophobic mismatch between protein and lipid bilayer, changes in the bilayer hydration and lateral interactions detected by small angle neutron scattering (SANS) can contribute to this acyl chain length dependence. When reconstituted into dierucoylphosphatidylcholine (diC22:1PC), the zwitterionic surfactant N-dodecyl-N,N-dimethylamine N-oxide (C12NO) stimulates the ATPase activity from 14.2+/-0.6 to 32.5+/-0.8 IU/mg in the range of molar ratios C12NO:diC22:1PC=0/1.2. In dilauroylphosphatidylcholines (diC12:0PC) and diC18:1PC, the effect of C12NO is twofold-the ATPase activity is stimulated at low and inhibited at high C12NO concentrations. In diC18:1PC, it is observed an increase of activity induced by C12NO in the range of molar ratios C12NO:diC18:1PC< or =1.3 in bilayers, where the bilayer thickness estimated by SANS decreases by 0.4+/-0.1 nm. In this range, the 31P-NMR chemical shift anisotropy increases indicating an effect of C12NO on the orientation of the phosphatidylcholine dipole N(+)-P- accompanied by a variation of the local membrane dipole potential. A decrease of the ATPase activity is observed in the range of molar ratios C12NO:diC18:1PC=1.3/2.5, where mixed tubular micelles are detected by SANS in C12NO+diC18:1PC mixtures. It is concluded that besides hydrophobic thickness changes, the changes in dipole potential and curvature frustration of the bilayer could contribute as well to C12NO effects on Ca(2+)-ATPase activity.

Effects of N-alkyl-N,N-dimethylamine-N-oxides on the activity of purified sarcoplasmic reticulum Ca(2+)-transporting ATPase.

N-alkyl-N,N-dimethylamine-N-oxides (CnNO, n = 10-20 is the number of alkyl carbon atoms) stimulate the skeletal sarcoplasmic reticulum (SR) Ca(2+)-transporting ATPase activity at low concentrations and inhibit it at high concentrations. The minimum concentration (cmin), at which CnNO inhibits the ATPase, continuously decreases up to n = 16-18 and then increases. The values of Cmin are smaller than the CnNO critical micelle concentration (cmc) for C10NO-C14NO homologs, but larger than cmc for C18NO-C20NO homologs. The ATPase inhibition is caused by the CnNO-induced lipid bilayer structural perturbation in the ATPase annular region, modulated by the partition equilibria of the CnNO molecules between the bilayer and aqueous phase for short alkyl chain (n = 10-16) CnNO homologs, and between the bilayer, micelles and aqueous phase for long alkyl chain (n = 18-20) CnNO homologs.

[Solubilization of unilamellar egg yolk phosphatidylcholine liposomes by N-alkyl-N,N-dimethylamine N-oxides]. / Solubilizácia unilamelárnych lipozomómov z vajecného fosfatidylcholínu N-alkyl-N,N-dimetylamín N-oxidmi.

The solubilization of extruded (100 nm) unilamellar egg yolk phosphatidylcholine (EYPC) liposomes by a series of N-alkyl-N,N-dimethylamine N-oxides (CnNO, n = 10-14 carbon atoms in the alkyl substituent) was studied using turbidimetry. The solubilizing concentration of CnNO (cS) was estimated as the CnNO concentration causing the half-maximum decrease in turbidance. From the linear cS dependence on EYPC concentration, the lipid--aqueous phase molar partition coefficient (Kp) and the CnNO:EYPC molar ratio in the CnNO + EYPC aggregates (nL:nEYPC) at cS were obtained: Kp = 82 +/- 25 and nL:nEYPC = 0.70 +/- 0.20 for C10NO, Kp = 507 +/- 215 and nL:nEYPC = 0.60 +/- 0.16 for C12NO, and Kp = 12357 +/- 93 and nL:nEYPC = 1.13 +/- 0.01 for C14NO. The value of Gibbs free energy of CnNO alkyl methylene group transfer from the aqueous to the lipid phase calculated from the Kp dependence on n is -1.2 +/- 0.2 RT (R = gas constant, T = absolute temperature), within the experimental error being the same as -1.026 +/- 0.006 RT obtained from the critical micellar concentrations of CnNO. The increased value of nL:nEYPC for C14NO is caused by the decreased hydrophobic mismatch of CnNO and EYPC hydrocarbon chain lengths. This mismatch results in a structural defect in the bilayer hydrophobic core, the primary cause of bilayer destabilization and solubilization.

X-ray diffraction and neutron scattering studies of amphiphile-lipid bilayer organization.

The lipid bilayer thickness d(L), the transbilayer distance of lipid phosphate groups d(pp/inf> and the lipid surface area A(L) of fluid hydrated bilayers of lamellar phases of egg phosphatidylcholine or dipalmitoylphosphatidylcholine containing N-alkyl-N,N-dimethylamine N-oxides (CnNO), 1,4-butanedi-ammonium-N,N'-dialkyl-N,N,N',N'-tetramethyl dibromides (GSn) or mono-hydrochlorides of [2-(alkyloxy)phenyl]-2-(1-piperidinyl)ethylesters of carbamic acid (CnA) were obtained by X-ray diffraction, and the bilayer thickness in extruded unilamellar dioleoylphosphatidylcholine vesicles containing C12NO was obtained by the neutron scattering. The values of d(L), d(pp/inf> and A(L) change linearly up to the 1:1 amphiphile:lipid molar ratio. The slopes of these dependencies increase for d(L) and d(pp/inf> and decrease for AL) with an increasing number of carbons n in the amphiphile long hydrocarbon substituent (18> or =n> or =8 for CnNO, 16> or =n> or =9 for GSn, 12> or =n> or =5 for CnA), while the opposite trends are observed for the short substituent (8> or =n>/=6 for CnNO, 9> or =n> or =7 for GSn, 5> or =n> or =3 for CnA). In case of long substituents, the effects on dL), dpp/inf> and AL) are caused by the decrease in the difference between the lipid and amphiphile hydrocarbon chain lengths and by the increase in their van der Waals attraction. The short substituent amphiphiles are mobile and exchange between multiple binding sites in the bilayer, minimizing the bilayer surface area.