Effect of lamellarity and size on calorimetric phase transitions in single component phosphatidylcholine vesicles

Nano-differential scanning calorimetry (nano-DSC) is a powerful tool in the investigation of unilamellar (small unilamellar, SUVs, or large unilamellar, LUVs) vesicles, as well as lipids on supported bilayers, since it measures the main gel-to-liquid phase transition temperature (T_m), enthalpies and entropies. In order to assign these transitions in single component systems, where T_m often occurred as a doublet, nano-DSC, dynamic light scattering and cryo-transmission electron microscopy (cryo-TEM) data were compared. The two T_ms were not attributable to decoupled phase transitions between the two leaflets of the bilayer, i.e. nano-DSC measurements were not able to distinguish between the outer and inner leaflets of the vesicle bilayers. Instead, the two T_ms were attributed to mixtures of oligolamellar and unilamellar vesicles, as confirmed by cryo-TEM images. T_m for the oligolamellar vesicles was assigned to the peak closest to that of the parent multilamellar vesicle (MLV) peak. The other transition was higher than that of the parent MLVs for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and increased in temperature as the vesicle size decreased, while it was lower in temperature than that of the parent MLVs for 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and decreased as the vesicle size decreased. These subtle shifts arose due to small differences in the values of ΔH and ΔS, since T_m is determined by their ratio (ΔH/ΔS). It was not possible to completely eliminate oligolamellar structures for MLVs extruded with the 200 nm pore size filter, even after 120 passes, while these structures were eliminated for MLVs extruded through the 50 nm pore size filter.     

Phase transition behavior of single phosphatidylcholine bilayers on a solid spherical support studied by DSC, NMR and FT-IR

For the first time, the chain melting transition from the gel phase to the liquid crystalline phase of a single DPPC bilayer on a solid, spherical support (silica beads) is observed by differential scanning calorimetry (DSC). This transition is remarkably cooperative, exhibits a transition temperature T_m which is 2°C lower than usually found for DPPC multilamellar vesicles and its excess enthalpy is about 25% less than in DPPC multilayers. 31P- and 2H-NMR data as well as FT-IR data provide evidence that despite the highly asymmetric characteristic of the model system, the whole single bilayer undergoes the transition at T_m, i.e., there is no decoupling of the two monolayer leaflets at the main phase transition. Furthermore, our results show that the formation of the ripple (P_β') phase is inhibited in single bilayers on a solid support. This result confirms a conclusion which we reached previously on the basis of neutron scattering data obtained on planar supported bilayers. The most likely reason for this inhibition as well as for the above mentioned thermodynamic differences between multilamellar vesicles and supported membranes is a significantly higher lateral stress in the latter. Moreover, the exchange of lipids between two populations of spherical supported vesicles (DMPC and chain perdeuterated DMPC) is studied by DSC. It is shown that this exchange process is symmetric and its half-time is a factor of 3-4 higher than observed for small sonicated DMPC vesicles.     

High Temperature Stabilization of DNA in Complexes with Cationic Lipids

The influence on the melting of calf thymus and plasmid DNA of cationic lipids of the type used in gene therapy was studied by ultraviolet spectrophotometry and differential scanning calorimetry. It was found that various membrane-forming cationic lipids are able to protect calf thymus DNA against denaturation at 100°C. After interaction with cationic lipids, the differential scanning calorimetry melting profile of both calf thymus and plasmid DNA revealed two major components, one corresponding to a thermolabile complex with transition temperature, T_m(labile), close to that of free DNA and a second corresponding to a thermostable complex with a transition temperature, T_m(stable), at 105 to 115°C. The parameter T_m(stable) did not depend on the charge ratio, R(±). Instead, the amount of thermostable DNA and the enthalpy ratio ΔH_(stable)/ΔH_(labile) depended upon R(±) and conditions of complex formation. In the case of O-ethyldioleoylphosphatidylcholine, the cationic lipid that was the main subject of the investigation, the maximal stabilization of DNA exceeded 90% between R(±) = 1.5 and 3.0. Several other lipids gave at least 75% protection in the range R(±) = 1.5 to 2.0. Centrifugal separation of the thermostable and thermolabile fractions revealed that almost all the transfection activity was present at the thermostable fraction. Electron microscopy of the thermostable complex demonstrated the presence of multilamellar membranes with a periodicity 6.0 to 6.5 nm. This periodic multilamellar structure was retained at temperatures as high as 130°C. It is concluded that constraint of the DNA molecules between oppositely charged membrane surfaces in the multilamellar complex is responsible for DNA stabilization.     

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

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

Probing the Combined Effect of Flunitrazepam and Lidocaine on the Stability and Organization of Bilayer Lipid Membranes. A Differential Scanning Calorimetry and Dynamic Light Scattering Study

Combined effects of flunitrazepam (FNZ) and lidocaine (LDC) were studied on the thermotropic equilibrium of dipalmitoyl phosphatidylcholine (dpPC) bilayers. This adds a thermodynamic dimension to previously reported geometric analysis in the erythrocyte model. LDC decreased the enthalpy and temperature for dpPC pre- and main-transitions (ΔH _p, ΔH _m, T _p, T _m) and decreased the cooperativity of the main-transition (ΔT _1/2,_m). FNZ decreased ΔH _m and, at least up to 59 μM, also decreased ΔH _p. In conjunction with LDC, FNZ induced a recovery of ?T _1/2,m control values and increased ΔH _m even above the control level. The deconvolution of the main-transition peak at high LDC concentrations revealed three components possibly represented by: a self-segregated fraction of pure dpPC, a dpPC–LDC mixture and a phase with a lipid structure of intermediate stability associated with LDC self-aggregation within the lipid phase. Some LDC effects on thermodynamic parameters were reverted at proper LDC/FNZ molar ratios, suggesting that FNZ restricts the maximal availability of the LDC partitioned into the lipid phase. Thus, beyond its complexity, the lipid–LDC mixture can be rationalized as an equilibrium of coexisting phases which gains homogeneity in the presence of FNZ. This work stresses the relevance of nonspecific drug–membrane binding on LDC–FNZ pharmacological interactions and would have pharmaceutical applications in liposomal multidrug-delivery.     

Experimental studies of thermal denaturation of the Na-DNA system with respect to Manning's model

The sodium and chloride activity coefficients in DNA solution were measured by selective electrodes. These experiments were performed for native and thermally denatured DNA. The ratio of activities in helix and coil states were compared with those given by Manning's model. These results are in good agreement with the theoretical values.We also compare the experimental values of the charge parameter XXX of DNA in the helix (XXX_h) and coil (XXX_c) configurations with the theoretical parameters appearing in Manning's model and which have been adjusted to correspond with the known conformation of the molecule. From this comparison, we deduce the change of enthalpy (ΔH)T_m at the temperature of denaturation (T_m) of DNA.The value of (ΔH)T_m thus calculated is smaller than the theorstical value and comparable with that observed experimentally by Privalov et al.     

Effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles

The effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles at a protein-to-lipid molar ratio of 1:60 have been investigated by employing the techniques of nanosecond emission anisotropy with 1,6-diphenyl-1,3,5-hexatriene as the fluorescent probe, enhancement by polar groups of the weakly allowed 0-0 vibronic transition in the fluorescence spectrum of pyrene, and Raman spectroscopy. The emission anisotropy results, which are found to be consistent with the wobble-in-cone model, show that the protein induces an increase in the order parameter, S, of the acyl chains of unilamellar vesicles below, at, and above their phase transition temperature, T_t, and it decreases strongly the diffusion rate, D_w, only below T_t. On the other hand, for multilamellar vesicles, the protein induces a decrease in S only at T_t and does not affect D_w. These effects are consistent with the observed changes in the degree of enhancement of the 0-0 vibronic transition of pyrene. Moreover, the protein broadens the thermal transition profile of multilamellar vesicles but sharpens dramatically that of unilamellar vesicles and fuses them without changing significantly the T_t in either case. On the other hand, the Raman data detect a decrease in the inter- and intramolecular order of the acyl chains of multilamellar vesicles below T_t and a decrease of only the former above T_t. This disparity between the Raman and the nanosecond emission anisotropy data is discussed in terms of differences in the time scales of the two techniques and in the state of aggregation of the lipid-bound melittin. The data for the enhancement of the 0-0 vibronic transition of pyrene suggest that, for a melittin-to-lipid ratio of 1:60, the size or structure of channels formed in the bilayer by melittin does not allow the penetration of a neutral molecule the size of pyrene deeply into the bilayer.     

Comparison of the enthalpy state of vesicles of different size by their interaction with α-lactalbumin

The characteristics of small unilamellar, large unilamellar and large multilamellar vesicles of dimyristoylphosphatidylcholine and their interaction with α-lactalbumin are compared at pH 4. (1) By differential scanning calorimetry and from steady-state fluorescence anisotropy data of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene it is shown that the transition characteristics of the phospholipids in the large unilamellar vesicles resemble more those of the multilamellar vesicles than of the small unilamellar vesicles. (2) The size and composition of the lipid-protein complex formed with α-lactalbumin around the transition temperature of the lipid are independent of the vesicle type used. Fluorescence anisotropy data indicate that in this complex the motions of the lipid molecules are strongly restricted in the presence of α-lactalbumin. (3) The previous data and a comparison of the enthalpy changes, $\text{ΔH}$, of the interaction of the three vesicle types with α-lactalbumin allow us to derive that the enthalpy state of the small unilamellar vesicles just below 24°C is about 24 kJ/mol lipid higher than the enthalpy state of both large vesicle types at the same temperature. The abrupt transition from endothermic to exothermic $\text{ΔH}$ values around 24°C for large vesicles approximates the transition enthalpy of the pure phospholipid     

Dynorphin induced magnetic ordering in lipid bilayers as studied by P NMR spectroscopy

Lipid bilayers of dimyristoyl phosphatidylcholine (DMPC) containing opioid peptide dynorphin A(1-17) are found to be spontaneously aligned to the applied magnetic field near at the phase transition temperature between the gel and liquid crystalline states (T_m=24°C), as examined by ³¹P NMR spectroscopy. The specific interaction between the peptide and lipid bilayer leading to this property was also examined by optical microscopy, light scattering, and potassium ion-selective electrode, together with a comparative study on dynorphin A(1-13). A substantial change in the light scattering intensity was noted for DMPC containing dynorphin A(1-17) near at T_m but not for the system containing A(1-13). Besides, reversible change in morphology of bilayer, from small lipid particles to large vesicles, was observed by optical microscope at T_m. These results indicate that lysis and fusion of the lipid bilayers are induced by the presence of dynorphin A(1-17). It turned out that the bilayers are spontaneously aligned to the magnetic field above T_m in parallel with the bilayer surface, because a single ³¹P NMR signal appeared at the perpendicular position of the ³¹P chemical shift tensor. In contrast, no such magnetic ordering was noted for DMPC bilayers containing dynorphin A(1-13). It was proved that DMPC bilayer in the presence of dynorphin A(1-17) forms vesicles above T_m, because leakage of potassium ion from the lipid bilayers was observed by potassium ion-selective electrode after adding Triton X-100. It is concluded that DMPC bilayer consists of elongated vesicles with the long axis parallel to the magnetic field, together with the data of microscopic observation of cylindrical shape of the vesicles. Further, the long axis is found to be at least five times longer than the short axis of the elongated vesicles in view of simulated ³¹P NMR lineshape.     

Membrane lateral phase separation induced by proteins of the prothrombinase complex

Blood coagulation factors X and V, as well as prothrombin fragment 1 caused changes in the observed transition temperature (T_m) of appropriately constituted phospholipid vesicles upon binding to the membrane surface. Factor X- and prothrombin fragment 1-induced T_m shifts were calcium-dependent, while factor V changed the T_m in a calcium-independent manner. The results were consistent with clustering of the acidic phospholipid molecules due to protein binding. In all cases, protein binding to acidic phospholipid-containing vesicles caused the observed T_m to approach that for the neutral phospholipid. This resulted in a T_m increase for phospholipid mixtures containing bovine brain phosphatidylserine (PS) plus dipalmitoylphosphatidylcholine (DPPC) and a T_m decrease for mixtures of dipalmitoylphosphatidic acid (DPPA) and dimyristoylphosphatidylcholine (DMPC). Maximum T_m shifts induced in PS-DPPC (10:90) vesicles were very similar for all the prothrombinase proteins and the extent of the change was proportional to the actual amount of membrane-bound protein as determined by light-scattering techniques. For the vitamin K-dependent proteins, T_m changes were greater in the presence of protein plus calcium than in the presence of calcium alone, indicating that lateral phase separation occurs subsequent to initial protein-membrane contact. Lateral phase separation of acidic phospholipids appears to be an important process in the formation of the prothrombinase complex.     

Calcium-induced fusion of proteoliposomes and protein-free liposomes Effect of their phosphatidylethanolamine content on the structure of fused vesicles

The acidic phospholipid cardiolipin was shown to be very efficient in promoting calcium-induced fusion of proteoliposomes. The degree of fusion was dependent on the phosphatidylethanolamine content of the vesicles. Addition of CaCl₂ to proteoliposomes containing phosphatidylcholine and cardiolipin but without phosphatidylethanolamine did not induce fusion. Fusion of cytochrome oxidase vesicles, containing less than 50 mol% phosphatidylethanolamine resulted in monolamellar vesicles with a diameter of about 200 nm. The vesicles could be induced to fuse further by establishing an osmotic pressure across their membranes. When proteoliposomes containing more than 50 mol% phosphatidylethanolamine were fused, large vesicles with a diameter exceeding 1 μm were formed. They appeared in the electron microscope as a mixture of multilamellar and monolamellar vesicles. Fusion of corresponding liposomes resulted in formation of even larger structures appearing as dense multilamellar bodies and paracrystalline honeycomb-like lattices.     

The lipidic particle as an intermediate structure in membrane fusion processes and bilayer to hexagonal HII transitions

Small unilamellar vesicles comprised of a mixture of phosphatidylethanolamine/phosphatidylcholine/cholesterol (3 : 1 : 2) fuse to form large multilamellar vesicles on increasing the temperature from 0 to 50°C. This event is associated with the appearance of lipidic particles at the fusion sites, consistent with a role as intermediary structures during the fusion process. Further, for phosphatidylcholine/cardiolipin (1 : 1) liposomes in the presence of Mn²⁺ a direct relationship between lipidic particles and the hexagonal (H_II) phase is demonstrated which suggests that lipidic particles can also occur as intermediaries between bilayer and hexagonal (H_II) structures.     

Enhanced fluctuations in small phospholipid bilayer vesicles containing cholesterol

Ultrasonic and calorimetric studies of small homogenously-sized DMPC unilamellar vesicles showed two thermal transitions at temperatures T _c1 and T _c2 (T _c2 T _c1 ); T _c2 is close to the phase transition temperature, T _c , of large vesicles. The process at T _c2 is not a fusion of vesicles and is interpreted as characterizing an order-disorder transition essentially similar to that of large vesicles. The temperatures T _c1 and T _c2 become increasingly similar as the cholesterol content is increased, while the clusters at T _c2 (85 lipid molecules in pure DMPC) increase in size up to approximately 180 lipid molecules at 12 mol% cholesterol. Incorporation of cholesterol thus brings about enhanced fluctuations in this model system of a membrane.Abbreviations DMPC dimyristoylphosphatidylcholine - SUV small unilamellar vesicles - LUV large unilamellar vesicles - MLV multilamellar vesicles     

Extracellular polysaccharide of Nostoc commune (Cyanobacteria) inhibits fusion of membrane vesicles during desiccation

Cells of the cyanobacterium Nostoc commune secrete a complex, high molecular weight, extracellular polysaccharide (EPS) which accumulates to more than 60% of the dry weight of colonies. The EPS was purified from the clonal isolate N. commune DRH1. The midpoint of the membrane phase transition (T_m) of desiccated cells of N. commune CHEN was low (T_{m dry} = 8 °C) and was comparable to the T_m of rehydrated cells((T_m)_H20 = 6 °C). The EPS was not responsible for the depression of T_m. However, the EPS, at low concentrations, inhibited specifically the fusion of phosphatidylcholine membrane vesicles when they were dried in vitro at0% relative humidity (−400 MPa). Low concentrations of a trehalose:sucrose mixture, in a molar ratio which corresponded with that present in cells in vivo, together with small amounts of the EPS, were efficient in preventing leakage of carboxyfloroscein (CF) from membrane vesicles. Freeze-fracture electron microscopy resolved complex changes in the structure of the EPS and the outer membrane in response to rehydration of desiccated cells. The capacity of the EPS to prevent membrane fusion, the maintenance of a low T_{m dry} in desiccated cells, and the changes in rheological properties of the EPS in response to water availability, constitute what are likely important mechanisms for desiccation tolerance in this cyanobacterium. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of aminoglycoside antibiotics on the thermodynamic properties of liposomal vesicles

Liposomes are ideal drug-delivery systems because they can alter the pharmacokinetic characteristics and biodistribution profile of the incorporated bioactive molecule. The effect of the aminoglycoside antibiotics, gentamicin (GN), tobramycin (TOB), and amikacin (AMI), on the thermodynamic properties of multilamellar vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied by using differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR), and ³¹P nuclear magnetic resonance (NMR) spectroscopy. The relationship between the structure of aminoglycoside antibiotics and their effect on the physical properties of the liposomal bilayers was investigated. The incorporation of the drugs was achieved and an osmotic gradient created by controlling the mole ratio of the drug inside to that outside of the DPPC vesicles so that [drug_{inside DPPC}]/[drug_{outside DPPC}] was 1:0, 1:0.2, 1:1, or 1:2.5. Incorporation of the drugs into liposomes caused the T_m to shift to a higher temperature and the δH_m and δT_1/2 values to decrease. The 2A_max and the order parameter (S), obtained from the EPR spectra, indicated that the fluidity of the liposomal membrane was affected by the type of drug and by the concentration used; GN and TOB decreased the fluidity and disturbed chain packing at mole ratios of [drug_{inside DPPC}]/[drug_{outside DPPC}] ranging from 1:0 to 1:0.2, while AMI increased the fluidity and disrupted chain packing at an osmotic gradient of 1:2.5. In conclusion, the molecular organization and thermotropic properties of the multilamellar DPPC vesicles were dependent on the osmotic gradient and structure of the aminoglycoside.     

Lipid-polyethylene glycol interactions: II. Formation of defects in bilayers

Summary Polyethylene glycol, a known cell fusogen, is found to induce the formation of structural defects in egg phosphatidylcholine multilamellar vesicles, as shown by freeze-fracture microscopy.³¹P NMR spectra of these vesicles reveal the existence of a nonbilayer (isotropic) phase. The observed disruption in the bilayers is believed to be associated with an intermediate stage of membrane fusion.Abbreviations PEG Polyethylene glycol - IMP Intramembranous particle - PC Phosphatidylcholine - PS Phosphatidylserine - SUV Small unilamellar vesicles - MLV Multilamellar vesicles - DPPC Dipalmitoyl phosphatidylcholine - DSC Differential scanning calorimetry - DMPC Dimyristoylphosphatidylcholine - T _c Phase transition temperature     

Liquid-liquid phase transition temperatures increase when lipid bilayers are supported on glass

Membranes made from certain ternary mixtures of lipids can display coexisting liquid phases. In giant unilamellar vesicles, these phases appear as liquid domains which diffuse and coalesce after the vesicle is cooled below its miscibility transition temperature (T_m). Converting vesicles to supported lipid bilayers alters the mobility of the lipids and domains in the bilayer. At the same time, the miscibility transition temperature of the lipid mixture is altered. Here we compare T_m in vesicles and in supported bilayers formed by rupturing the same vesicles onto glass. We determine transition temperatures using fluorescence microscopy, and identify an increase in T_m when it is measured in identical membranes in solution and on a glass surface. We systematically alter the lipid composition of our membranes in order to observe the correlation between membrane composition and variation in T_m.     

Effects of proteins on the thermotropic phase transitions of phospholipid membranes

A variety of proteins have been studied for their ability to interact and alter the thermotropic properties of phospholipid bilayer membranes as detected by differential scanning calorimeter. The proteins studied included: basic myelin protein (A1 protein), cytochrome c, major apoprotein of myelin proteolipid (N-2 apoprotein), gramicidin A, polylysine, ribonuclease and hemoglobin. The lipids used for the interactions were dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol. The interactions were grouped in three categories each having very different effects on the phospholipid phase transition from solid to liquid crystalline. The calorimetric studies were also correlated with data from vesicle permeability and monolayer expansion.Ribonuclease and polylysine which exemplify group 1 interactions, show strong dependence on electrostatic binding. Their effects on lipid bilayers include an increase in the enthalpy of transition (ΔH) accompanied by either an increase or no change in the temperature of transition (T_c). In addition, they show minimal effects on vesicle permeability and monolayer expansion. It was concluded that these interactions represent simple surface binding of the protein on the lipid bilayer without penetration into the hydrocarbon region.Cytochrome c and Al protein, which exemplify group 2 interactions, also show a strong dependence on the presence of net negative charges on the lipid bilayers for their binding. In contrast to the first group, however, they induce a drastic decrease in both T_c and ΔH of the lipid phase transition. Furthermore, they induce a large increase in the permeability of vesicles and a substantial expansion in area of closely packed monolayers at the air-water interface. It was concluded that group 2 interactions represent surface binding followed by partial penetration and/or deformation of the bilayer.Group 3 interactions, shown by proteolipid apoprotein and gramicidin A, were primarily non-polar in character, not requiring electrostatic charges and not inhibited by salt and pH changes. They had no appreciable effect on the T_c but did induce a linear decrease in the magnitude of the ΔH, proportional to the percentage of protein by weight. Membranes containing 50% proteolipid protein still exhibited a thermotropic transition with a ΔH one half that of the pure lipid, and only a small diminution of the size of the cooperative unit. It was concluded that in this case the protein was embedded within the bilayer, associating with a limited number of molecules via non-polar interactions, while the rest of the bilayer was largely unperturbed.     

Influence of DPPE surface undulations on melting temperature determination: UV/Vis spectroscopic and MD study

One of the most distinguished quantities that describes lipid main phase transition, i.e. the transition from the gel (L_β(′)) to the fluid (L_α) phase, is its melting temperature (T_m). Because melting is accompanied by a large change in enthalpy the, L_β(′) → L_α transition can be monitored by various calorimetric, structural and spectroscopic techniques and T_m should be the same regardless of the metric monitored or the technique employed. However, in the case of DPPE multilamellar aggregates there is a small but systematic deviation of T_m values determined by DSC and FTIR spectroscopy. The aim of this paper is to explain this discrepancy by combined UV/Vis spectroscopic and MD computational approach. Multivariate analysis performed on temperature-dependent UV/Vis spectra of DPPE suspensions demonstrated that at 55 ± 1 °C certain phenomenon causes a small but detectable change in suspension turbidity, whereas a dominant change in the latter is registered at 63.2 ± 0.4 °C that coincides with T_m value determined from DSC curve. If this effect should be ignored, the overall data give T_m value the same as FTIR spectra data (61.0 ± 0.4 °C). As the classical MD simulations suggest that about 10° below T_m certain undulations appear at the surface of DPPE bilayers, we concluded that certain discontinuities in curvature fluctuations arise at reported temperature which are to some extent coupled with lipid melting. Ultimately, such events and the associated changes in curvature affect T_m value measured by different techniques.