Thermal, dynamic and structural properties of drug AT1 antagonist olmesartan in lipid bilayers

It is proposed that AT1 antagonists (ARBs) exert their biological action by inserting into the lipid membrane and then diffuse to the active site of AT1 receptor. Thus, lipid bilayers are expected to be actively involved and play a critical role in drug action. For this reason, the thermal, dynamic and structural effects of olmesartan alone and together with cholesterol were studied using differential scanning calorimetry (DSC), 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR), cross-polarization (CP) MAS NMR, and Raman spectroscopy as well as small- and wide angle X-ray scattering (SAXS and WAXS) on dipalmitoyl-phosphatidylcholine (DPPC) multilamellar vesicles. 13C CP/MAS spectra provided direct evidence for the incorporation of olmesartan and cholesterol in lipid bilayers. Raman and X-ray data revealed how both molecules modify the bilayer's properties. Olmesartan locates itself at the head-group region and upper segment of the lipid bilayers as 13C CP/MAS spectra show that its presence causes significant chemical shift changes mainly in the A ring of the steroidal part of cholesterol. The influence of olmesartan on DPPC/cholesterol bilayers is less pronounced. Although, olmesartan and cholesterol are residing at the same region of the lipid bilayers, due to their different sizes, display distinct impacts on the bilayer's properties. Cholesterol broadens significantly the main transition, abolishes the pre-transition, and decreases the membrane fluidity above the main transition. Olmesartan is the only so far studied ARB that increases the gauche:trans ratio in the liquid crystalline phase. These significant differences of olmesartan may in part explain its distinct pharmacological profile.     

Interactions of the AT1 antagonist valsartan with dipalmitoyl-phosphatidylcholine bilayers

Valsartan is a marketed drug with high affinity to the type 1 angiotensin (AT1) receptor. It has been reported that AT1 antagonists may reach the receptor site by diffusion through the plasma membrane. For this reason we have applied a combination of differential scanning calorimetry (DSC), Raman spectroscopy and small and wide angle X-ray scattering (SAXS and WAXS) to investigate the interactions of valsartan with the model membrane of dipalmitoyl-phosphatidylcholine (DPPC). Hence, the thermal, dynamic and structural effects in bulk as well as local dynamic properties in the bilayers were studied with different valsartan concentrations ranging from 0 to 20 mol%. The DSC experimental results showed that valsartan causes a lowering and broadening of the phase transition. A splitting of the main transition is observed at high drug concentrations. In addition, valsartan causes an increase in enthalpy change of the main transition, which can be related to the induction of interdigitation of the lipid bilayers in the gel phase. Raman spectroscopy revealed distinct interactions between valsartan with the lipid interface localizing it in the polar head group region and in the upper part of the hydrophobic core. This localization of the drug molecule in the lipid bilayers supports the interdigitation view. SAXS measurements confirm a monotonous bilayer thinning in the fluid phase, associated with a steady increase of the root mean square fluctuation of the bilayers as the valsartan concentration is increased. At high drug concentrations these fluctuations are mainly governed by the electrostatic repulsion of neighboring membranes. Finally, valsartans' complex thermal and structural effects on DPPC bilayers are illustrated and discussed on a molecular level.     

Biophysical perturbations induced by ethylazinphos in lipid membranes

Perturbations induced by ethylazinphos on the physical organization of dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol membranes were studied by differential scanning calorimetry (DSC) and fluorescence polarization of 2-, 6-, 12-(9-anthroyloxy) stearic acids and 16-(9-anthroyloxy) palmitic acid. Ethylazinphos (50 and 100 microM) increases the fluorescence polarization of the probes, either in the gel or in the fluid phase of DPPC bilayers, and this concentration dependent effect decreases from the surface to the bilayer core. Additionally, the insecticide displaces the phase transition to a lower temperature range and broadens the transition profile of DPPC. A shifting and broadening of the phase transition is also observed by DSC. Furthermore at insecticide/lipid molar ratios higher than 1/7, DSC thermograms, in addition to the normal transition centered at 41 degrees C, also display a new phase transition centered at 45.5 degrees C. The enthalpy of this new transition increases with insecticide concentration, with a corresponding decrease of the main transition enthalpy. Ethylazinphos in DPPC bilayers with low cholesterol (< or = 20 mol%) perturbs the membrane organization as described above for pure DPPC. However, cholesterol concentrations higher than 20 mol% prevent insecticide interaction, as revealed by fluorescence polarization and DSC data. Apparently, cholesterol significantly modulates insecticide interaction by competition for similar distribution domains in the membrane. The present results strongly support our previous hypothesis that ethylazinphos locates in the cooperativity region, i.e. the region of C1-C9 atoms of the acyl chains, and extends to the lipid-water interface, where it increases lipid packing order sensed across all the thickness of the bilayer. Additionally, and, on the basis of DSC data, a lateral regionalization of ethylazinphos is here tentatively suggested.     

Mixing of perfluorinated carboxylic acids with dipalmitoylphosphatidylcholine

Perfluorinated acids are emerging as an important class of persistent environmental pollutant, thus raising human health concerns. To understand the behavior of these compounds in biological systems, the mixing behavior of two perfluorinated acids, perfluorododecanoic and perfluorotetradecanoic acid, with dipalmitoylphosphatidylcholine (DPPC) was studied in monolayers at the air-water interface and in fully hydrated DPPC bilayers. The mixing behavior of both acids was indicative of an attractive interaction and partial miscibility with DPPC at the air-water interface. In the bilayer studies, the fluorinated acids cause peak broadening and elimination of the pretransition of DPPC. The onset temperature of the main phase transition remains constant in the presence of the fluorinated acids suggesting immiscibilities in the gel phase. Below X(DPPC)=0.97 significant peak broadening of the main phase transition can be observed. These results suggest strong interaction between the respective acid and DPPC, and that both acids are able to partition into the lipid bilayer. However, their mixing behavior is far from ideal, thus suggesting the presence of domains or lipid aggregates with high acid concentrations which may (adversely) impact the function of biological mono- and bilayers.     

Mixing of perfluorinated carboxylic acids with dipalmitoylphosphatidylcholine

Perfluorinated acids are emerging as an important class of persistent environmental pollutant, thus raising human health concerns. To understand the behavior of these compounds in biological systems, the mixing behavior of two perfluorinated acids, perfluorododecanoic and perfluorotetradecanoic acid, with dipalmitoylphosphatidylcholine (DPPC) was studied in monolayers at the air-water interface and in fully hydrated DPPC bilayers. The mixing behavior of both acids was indicative of an attractive interaction and partial miscibility with DPPC at the air-water interface. In the bilayer studies, the fluorinated acids cause peak broadening and elimination of the pretransition of DPPC. The onset temperature of the main phase transition remains constant in the presence of the fluorinated acids suggesting immiscibilities in the gel phase. Below X(DPPC) = 0.97 significant peak broadening of the main phase transition can be observed. These results suggest strong interaction between the respective acid and DPPC, and that both acids are able to partition into the lipid bilayer. However, their mixing behavior is far from ideal, thus suggesting the presence of domains or lipid aggregates with high acid concentrations which may (adversely) impact the function of biological mono- and bilayers.     

Interactions of the dipeptide paralysin beta-Ala-Tyr and the aminoacid Glu with phospholipid bilayers

Existing evidence points out that the biological activity of beta-Ala-Tyr may in part related to its interactions with the cell membranes. For comparative reasons the effects of Glu were also examined using identical techniques and conditions. In order to examine their thermal and dynamic effects on membrane bilayers a combination of DSC, Raman and solid state NMR spectroscopy on DPPC/water model membranes were applied and the results were compared. DSC data showed that Glu perturbs to a greater degree the model membrane compared to beta-Ala-Tyr. Thus, alteration of the phase transition temperature and half width of the peaks, abolishment of the pretransition and influence on the enthalpy of the phase transition were more pronounced in the Glu loaded bilayers. Raman spectroscopy showed that incorporation of Glu in DPPC/water bilayers increased the order in the bilayers in contrast to the effect of the dipeptide. Several structural and dynamical properties of the DPPC multilamellar bilayers with and without the dipeptide or Glu were compared using high resolution C-13 MAS (Magic Angle Spinning) spectra and spectral simulations of inhomogeneously broadened, stationary P-31 NMR lineshapes measured under CP (Cross-polarization) conditions. These methods revealed that the aminoacid Glu binds in the close realm of the phosphate in the hydrophilic headgroup of DPPC while beta-Ala-Tyr is located more deeply inside the hydrophobic zone of the bilayer. The P-31 NMR simulations indicated restricted fast rotary motion of the phospholipids about their long axes in the organized bilayer structure. Finally, by the applied methodologies it is concluded that the two molecules under study exert dissimilar thermal and dynamic effects on lipid bilayers, the Glu improving significantly the packing of the lipids in contrast to the smaller and opposite effect of the dipeptide.     

Interactions of the dipeptide paralysin β-Ala-Tyr and the aminoacid Glu with phospholipid bilayers

Existing evidence points out that the biological activity of β-Ala-Tyr may in part related to its interactions with the cell membranes. For comparative reasons the effects of Glu were also examined using identical techniques and conditions. In order to examine their thermal and dynamic effects on membrane bilayers a combination of DSC, Raman and solid state NMR spectroscopy on DPPC/water model membranes were applied and the results were compared. DSC data showed that Glu perturbs to a greater degree the model membrane compared to β-Ala-Tyr. Thus, alteration of the phase transition temperature and half width of the peaks, abolishment of the pretransition and influence on the enthalpy of the phase transition were more pronounced in the Glu loaded bilayers. Raman spectroscopy showed that incorporation of Glu in DPPC/water bilayers increased the order in the bilayers in contrast to the effect of the dipeptide. Several structural and dynamical properties of the DPPC multilamellar bilayers with and without the dipeptide or Glu were compared using high resolution C-13 MAS (Magic Angle Spinning) spectra and spectral simulations of inhomogeneously broadened, stationary P-31 NMR lineshapes measured under CP (Cross-polarization) conditions. These methods revealed that the aminoacid Glu binds in the close realm of the phosphate in the hydrophilic headgroup of DPPC while β-Ala-Tyr is located more deeply inside the hydrophobic zone of the bilayer. The P-31 NMR simulations indicated restricted fast rotary motion of the phospholipids about their long axes in the organized bilayer structure. Finally, by the applied methodologies it is concluded that the two molecules under study exert dissimilar thermal and dynamic effects on lipid bilayers, the Glu improving significantly the packing of the lipids in contrast to the smaller and opposite effect of the dipeptide.     

Studies on the interaction of anesthetic steroids with phosphatidylcholine using 2H and 13C solid state NMR

The effects of the anesthetic steroid alphaxalone and its inactive analog delta 16-alphaxalone on model phospholipid membranes were studied using 13C and 2H solid-state nuclear magnetic resonance spectroscopy. Aqueous multilamellar dispersions of dipalmitoylphosphatidylcholine (DPPC) with specific 13C and 2H labels as endogenous probes at the carbonyl and the C-7 methylene groups, respectively, of the sn-2 chain were used to study the conformational and dynamical properties of the bilayer as a function of temperature. There were no significant changes between the 13C and 2H spectra of the DPPC preparation containing the inactive steroid and that of DPPC with no drug. However, the physiologically active steroid produces significant spectral 2H and 13C changes. These changes include a reduction of the main phase transition temperature and a broadening of that transition. Alphaxalone also increases the relative number of gauche conformers in the liquid-crystalline phase of DPPC and increases the rate of axial diffusion in both the gel and liquid-crystalline phase. The thermotropic properties of the above preparations, as monitored by differential scanning calorimetry, were congruent with the spectroscopic data.     

Changes induced by malathion, methylparathion and parathion on membrane lipid physicochemical properties correlate with their toxicity

Perturbations induced by malathion, methylparathion and parathion on the physicochemical properties of dipalmitoylphosphatidylcholine (DPPC) were studied by fluorescence anisotropy of DPH and DPH-PA and by differential scanning calorimetry (DSC). Methylparathion and parathion (50 microM) increased the fluorescence anisotropy evaluated by DPH-PA and DPH, either in gel or in the fluid phase of DPPC bilayers, but mainly in the fluid phase. Parathion is more effective than methylparathion. On the other hand, malathion had almost no effect. All the three xenobiotics displaced the phase transition midpoint to lower temperature values and broadened the phase transition profile of DPPC, the effectiveness following the sequence: parathion>methylparathion>malathion. A shifting and broadening of the phase transition was also observed by DSC. Furthermore, at methylparathion/lipid molar ratio of 1/2 and at parathion/lipid molar ratio of 1/7, the DSC thermograms displayed a shoulder in the main peak, in the low temperature side, suggesting coexistence of phases. For higher ratios, the phase transition profile becomes sharp as the control transition, but the midpoint is shifted to the previous shoulder position. Conversely to methylparathion and parathion, malathion did not promote phase separation. The overall data from fluorescence anisotropy and calorimetry indicate that the degree of effect of the insecticides on the physicochemical membrane properties correlates with toxicity to mammals. Therefore, the in vivo effects of organophosphorus compounds may be in part related with their ability to perturb the phospholipid bilayer structure, whose integrity is essential for normal cell function.     

Interaction of ceramides with phosphatidylcholine, sphingomyelin and sphingomyelin/cholesterol bilayers

Ceramides (Cers) may exert their biological activity through changes in membrane structure and organization. To understand this mechanism, the effect of Cer on the biophysical properties of phosphatidylcholine, sphingomyelin (SM) and SM/cholesterol bilayers was determined using fluorescence probe techniques. The Cers were bovine brain Cer and synthetic Cers that contained a single acyl chain species. The phospholipids were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glyero-3-phosphocholine (DPPC) and bovine brain, egg yolk and bovine erythrocyte SM. The addition of Cer to POPC and DPPC bilayers that were in the liquid-crystalline phase resulted in a linear increase in acyl chain order and decrease in membrane polarity. The addition of Cer to DPPC and SM bilayers also resulted in a linear increase in the gel to liquid-crystalline phase transition temperature (T(M)). The magnitude of the change was dependent upon Cer lipid composition and was much higher in SM bilayers than DPPC bilayers. The addition of 33 mol% cholesterol essentially eliminated the thermal transition of SM and SM/Cer bilayers. However, there is still a linear increase in acyl chain order induced by the addition of Cer. The results are interpreted as the formation of DPPC/Cer and SM/Cer lipid complexes. SM/Cer lipid complexes have higher T(M)s than the corresponding SM because the addition of Cer reduces the repulsion between the bulky headgroup and allows closer packing of the acyl chains. The biophysical properties of a SM/Cer-rich bilayer are dependent upon the amount of cholesterol present. In a cholesterol-poor membrane, a sphingomyelinase could catalyze the isothermal conversion of a liquid-crystalline SM bilayer to a gel phase SM/Cer complex at physiological temperature.     

The effect of free fatty acids on the thermotropic phase transition of dimyristoyl glycerophosphocholine

The effect of free fatty acids on the phase transition characteristics and fluidity of bilayers of dimyristoyl glycerophosphocholine were studied by pyrene eximer fluorescence and differential scanning calorimetry. High melting saturated fatty acids with chain lengths of 12–18 carbon atoms raise the phase transition temperature and enhance the ability of pyrene to form clusters in the gel state while not affecting the fluidity of the membrane in the liquid crystal state. Low melting unsaturated fatty acids lower the phase transition temperature and decrease the ability of pyrene to form clusters in the gel state while not affecting the fluidity of the membrane in the liquid crystal state. The effects of the very long chain fatty acids, arachidic (C 20) and behenic (C 22) appear to be similar to those of cholesterol in that they cause a broadening of the phase transition with a lowering of the transition enthalpy but have little effect on the temperature at which the phase transition occurs.     

A differential scanning calorimetry study of phosphocholines mixed with paclitaxel and its bromoacylated taxanes

High sensitivity differential scanning calorimetry (DSC) was used to investigate the thermotropic phase properties of binary mixtures of disaturated phosphocholines (PCs) and alpha-bromoacyl taxane derivatives. The alpha-bromoacyl taxanes were synthesized as hydrolyzable hydrophobic prodrugs of paclitaxel. The PCs used were 1, 2-dimyristoyl-sn-glycero-3-phosphatidyl-choline (DMPC), 1, 2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1, 2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC). The bromoacyl chain lengths of the taxane prodrugs were varied from 6 to 12 or 16 carbons. For comparison, paclitaxel and PC mixtures were also examined. DSC data from DPPC and bromoacyl taxane mixtures showed a complete abolition of the pretransition and significant broadening of the main phase transition with increasing amounts of bromoacyl taxane prodrugs. The effects were more pronounced with the long-chain compared to the short-chain prodrugs. Under equivalent DSC conditions, the short-chain DMPC showed greater changes in thermotropic phase behavior than with DPPC on taxane addition, suggesting an enhanced degree of association with the fluid-type bilayers. Under similar conditions, the long-chain DSPC bilayers showed a far less significant change in phase behavior on taxane addition than DPPC. These changes were also chain length-dependent for both the PCs and the taxane prodrugs. In contrast, PC and paclitaxel (lacking the acyl chain) mixtures under similar conditions showed insignificant changes in the endotherms, suggesting only slight insertion of the molecule into the PC bilayers. From the DSC data it is apparent that taxane prodrugs solvated in DMPC bilayers more than in DPPC and DSPC bilayers, and taxane prodrugs with longer acyl chains were able to associate with PCs better than those with shorter chain prodrugs. DSC data also suggest that paclitaxel was poorly associated with any of the PCs. In general, the amount of taxane association with bilayers decreased in order: DMPC > DPPC > DSPC. In contrast, the transition enthalpy (DeltaH) of DMPC, DPPC, and DSPC mixtures with paclitaxel showed significantly lower enthalpies than with taxane prodrugs. Taken together, the DSC data suggest that the acyl chains of paclitaxel prodrugs have some access into the bilayers via alignment with the acyl chain of the PC component.     

Effect of acylation on the interaction of the N-Terminal segment of pulmonary surfactant protein SP-C with phospholipid membranes

SP-C, the smallest pulmonary surfactant protein, is required for the formation and stability of surface-active films at the air-liquid interface in the lung. The protein consists of a hydrophobic transmembrane alpha-helix and a cationic N-terminal segment containing palmitoylated cysteines. Recent evidence suggests that the N-terminal segment is of critical importance for SP-C function. In the present work, the role of palmitoylation in modulating the lipid-protein interactions of the N-terminal segment of SP-C has been studied by analyzing the effect of palmitoylated and non-palmitoylated synthetic peptides designed to mimic the N-terminal segment on the dynamic properties of phospholipid bilayers, recorded by spin-label electron spin resonance (ESR) spectroscopy. Both palmitoylated and non-palmitoylated peptides decrease the mobility of phosphatidylcholine (5-PCSL) and phosphatidylglycerol (5-PGSL) spin probes in dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG) bilayers. In zwitterionic DPPC membranes, both peptides have a greater effect at temperatures below than above the main gel-to-liquid-crystalline phase transition, the palmitoylated peptide inducing greater immobilisation of the lipid than does the non-palmitoylated form. In anionic DPPG membranes, both palmitoylated and non-palmitoylated peptides have similar immobilizing effects, probably dominated by electrostatic interactions. Both palmitoylated and non-palmitoylated peptides have effects comparable to whole native SP-C, as regards improving the gel phase solubility of phospholipid spin probes and increasing the polarity of the bilayer surface monitored by pK shifts of fatty acid spin probes. This indicates that a significant part of the perturbing properties of SP-C in phospholipid bilayers is mediated by interactions of the N-terminal segment. The effect of SP-C N-terminal peptides on the chain flexibility gradient of DPPC and DPPG bilayers is consistent with the existence of a peptide-promoted interdigitated phase at temperatures below the main gel-to-liquid-crystalline phase transition. The palmitoylated peptide, but not the non-palmitoylated version, is able to stably segregate interdigitated and non-interdigitated populations of phospholipids in DPPC bilayers. This feature suggests that the palmitoylated N-terminal segment stabilizes ordered domains such as those containing interdigitated lipids. We propose that palmitoylation may be important to promote and facilitate association of SP-C and SP-C-containing membranes with ordered lipid structures such as those potentially existing in highly compressed states of the interfacial surfactant film.     

Effect of acylation on the interaction of the N-Terminal segment of pulmonary surfactant protein SP-C with phospholipid membranes

SP-C, the smallest pulmonary surfactant protein, is required for the formation and stability of surface-active films at the air-liquid interface in the lung. The protein consists of a hydrophobic transmembrane α-helix and a cationic N-terminal segment containing palmitoylated cysteines. Recent evidence suggests that the N-terminal segment is of critical importance for SP-C function. In the present work, the role of palmitoylation in modulating the lipid-protein interactions of the N-terminal segment of SP-C has been studied by analyzing the effect of palmitoylated and non-palmitoylated synthetic peptides designed to mimic the N-terminal segment on the dynamic properties of phospholipid bilayers, recorded by spin-label electron spin resonance (ESR) spectroscopy. Both palmitoylated and non-palmitoylated peptides decrease the mobility of phosphatidylcholine (5-PCSL) and phosphatidylglycerol (5-PGSL) spin probes in dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG) bilayers. In zwitterionic DPPC membranes, both peptides have a greater effect at temperatures below than above the main gel-to-liquid-crystalline phase transition, the palmitoylated peptide inducing greater immobilisation of the lipid than does the non-palmitoylated form. In anionic DPPG membranes, both palmitoylated and non-palmitoylated peptides have similar immobilizing effects, probably dominated by electrostatic interactions. Both palmitoylated and non-palmitoylated peptides have effects comparable to whole native SP-C, as regards improving the gel phase solubility of phospholipid spin probes and increasing the polarity of the bilayer surface monitored by pK shifts of fatty acid spin probes. This indicates that a significant part of the perturbing properties of SP-C in phospholipid bilayers is mediated by interactions of the N-terminal segment. The effect of SP-C N-terminal peptides on the chain flexibility gradient of DPPC and DPPG bilayers is consistent with the existence of a peptide-promoted interdigitated phase at temperatures below the main gel-to-liquid-crystalline phase transition. The palmitoylated peptide, but not the non-palmitoylated version, is able to stably segregate interdigitated and non-interdigitated populations of phospholipids in DPPC bilayers. This feature suggests that the palmitoylated N-terminal segment stabilizes ordered domains such as those containing interdigitated lipids. We propose that palmitoylation may be important to promote and facilitate association of SP-C and SP-C-containing membranes with ordered lipid structures such as those potentially existing in highly compressed states of the interfacial surfactant film.     

A molecular basis explanation of the dynamic and thermal effects of vinblastine sulfate upon dipalmitoylphosphatidylcholine bilayer membranes

Differential scanning calorimetry has been employed to study the thermal effects of vinblastine sulfate upon aqueous, single and multiple bilayer dispersions of 1,2-dipalmitoyl-3-sn-phosphatidylcholine (DPPC). The calorimetric results summarized to an increase in the gel to liquid-crystalline phase transition enthalpy and the abolishment of the L(beta)' (gel phase) to P(beta)' (ripple phase) pretransition for the uni- and multilamellar dispersions, as well as an increase in the transition temperature T(m) and the transition cooperativity for single bilayer DPPC/vinblastine mixed vesicles, are consistent with an induced, partially interdigitated, gel phase. Computational analysis has been successfully applied to clarify the intermolecular effects and verify the feasibility of the proposed interdigitation for the vinblastine sulfate molecules and also for the ursodeoxycholic acid (UDCAH) and bromocylated taxanes, which have been shown to induce an interdigitated gel phase in DPPC bilayers.     

Losartan's molecular basis of interaction with membranes and AT1 receptor

Physicochemical methods were used to study the thermal and dynamic changes caused by losartan in the membrane bilayers. In addition, molecular modeling was implemented to explore its topography both in membranes and AT(1) receptor. Its incorporation resulted in the modification of thermal profile of dipalmitoyl phosphatidylcholine (DPPC) bilayers in a concentration dependent way up to 20mol% as it is depicted from the combination of differential scanning calorimetry (DSC) and MAS data. In particular, the presence of losartan caused lowering of the phase transition temperature and abolishment of the pretransition. T(1) experiments revealed the location of the drug into the membrane bilayers. The use of a combination of biophysical methods along with docking experiments brought out a possible two-step mechanism which involves incorporation of losartan at the interface of membrane bilayers and diffusion in the upper parts of AT(1) receptor helices IV-VII.     

1-[2-Hydroxy-3-octadecan-1'-oate]propyl-2'',2'',5'',5''-tetramethyl pyrolidine-N-oxyl-3''-carboxylate as a potential spin probe for membrane structure studies

The synthesis of a new minimum steric perturbing proxyl nitroxide, which is a derivative of glycerol and contains a stearic acid moiety, has been carried out. Its localization in model membrane L-alpha-dipalmitoyl phosphatidyl choline (DPPC) was ascertained with the help of ESR, DSC, 1H and 31P NMR techniques. The nitroxide was used for detecting the changes in the phase transition temperature of the model membranes in the presence and absence of drugs. The permeation of the vasodilating drug epinephrine has also been studied using this spin label. The results prove the potential applicability of the new spin probe in the spin labeling of biomembranes.     

Deuterium order parameters in relation to thermodynamic properties of a phospholiped bilayer. A statistical mechanical interpretation.

The physical properties of bilayers of dipalmitoyl-3-sn-phosphatidylcholine are analyzed in terms of a statistical model proposed by Marcelja (S. Marcelja (1974), Biochim. Biophys. Acta 367, 165). The model is used to calculate the segmental order parameters of the hydrocarbon chains, the transition temperature of the crystalline leads to liquid crystalline phase transition, the entropy change of the transition, the bilayer thickness, and the linear thermal expansion coefficient. The theoretical predictions are in excellent agreement with experimental results obtained by deuterium magnetic resonance, differential scanning calorimetry, and X-ray diffraction. The model yields the probabilities of trans and gauche conformations and also those of more specific conformational defects like kinks or jogs.     

Effect of fatty acids on phase behavior of hydrated dipalmitoylphosphatidylcholine bilayer: saturated versus unsaturated fatty acids

The effect of some fatty acids on the phase behavior of hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer was investigated with special interest in possible difference between saturated and unsaturated fatty acids. The phase behavior of hydrated DPPC bilayer was followed by a differential scanning calorimetry and a Fourier transform infrared spectroscopy. The addition of palmitic acid (PA) increased the bilayer phase transition temperature with the increase of the PA content in the mixture. In addition, DPPC molecules in gel phase bilayer became more rigid in the presence of PA compared with those in the absence of PA. This effect of PA on the phase behavior of hydrated DPPC bilayer is common to other saturated fatty acids, stearic acid, myristic acid, and also to unsaturated fatty acid with trans double bond, elaidic acid. Contrary to these fatty acids, oleic acid (OA), the unsaturated fatty acid with cis double bond in the acyl chain, exhibited quite different behavior. The effect of OA on the bilayer phase transition temperature was rather small, although a slight decrease in the temperature was appreciable. Furthermore, the IR spectral results demonstrated that the perturbing effect of OA on the gel phase bilayer of DPPC was quite small. These results mean that OA does not disturb the hydrated DPPC bilayer significantly.     

A comparative calorimetric study of the effects of cholesterol and the plant sterols campesterol and brassicasterol on the thermotropic phase behavior of dipalmitoylphosphatidylcholine bilayer membranes

We present a comparative differential scanning calorimetric study of the effects of the animal sterol cholesterol (Chol) and the plant sterols campesterol (Camp) and brassicasterol (Bras) on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers. Camp and Bras differ from Chol in having a C24 methyl group and, additionally for Bras, a C22 trans-double bond. Camp and especially Bras decrease the temperature, cooperativity and enthalpy of the DPPC pretransition more than Chol, although these effects are attenuated at higher sterol levels. This indicates that they destabilize gel-state DPPC bilayers to a greater extent, but are less soluble, than Chol. Not surprisingly, all three sterols have similar effects on the sterol-poor sharp component of the DPPC main phase transition. However, Camp and especially Bras less effectively increase the temperature and decrease the cooperativity and enthalpy of the broad component of the main transition than Chol. This indicates that at higher sterol concentrations, Camp and Bras are less miscible and less effective than Chol at ordering the hydrocarbon chains of the sterol-enriched fluid DPPC bilayers. Overall, these alkyl side chain modifications generally reduce the ability of Chol to produce its characteristic effects on DPPC bilayer physical properties. These differences are likely due to the less extended and more bent conformations of the alkyl side chains of Camp and Bras, producing sterols with a greater effective cross-sectional area and reduced length than Chol. Hence, the structure of Chol is likely optimized for maximum solubility in, as opposed to maximum ordering of, phospholipid bilayers.