Molecular-level pictures of the phase transitions of saturated and unsaturated phospholipid binary mixtures

Abstract

Binary lipid mixtures consisting of saturated and unsaturated lipids are important models for natural cell membranes. However, a detailed molecular picture for the phase transition process of such lipid binary mixtures remains unclear. Herein, by using deuterated dipalmitoylphosphatidylcholine (DPPC-d₆₂) and hydrogenated dioleoylphosphatidic acid (DOPA), we expect to separately analyze the changes of the two lipid components during thermotropic phase transitions by temperature-dependent Fourier transform infrared (FTIR) spectroscopy, and uncover the hidden secrets of a seemingly single endothermic peak observed in differential scanning calorimetry (DSC) experiments. We found that at low DOPA concentrations (10–20 mol%), a gel to fluid conformational transition of DPPC-d₆₂ and a fluidization transition of DOPA by DPPC-d₆₂ were observed. The two lipids were found to have nonsynchronous conformational rearrangements in the tail regions upon thermotropic phase transitions, with the change of DOPA earlier than DPPC-d₆₂. Besides, in the mixed fluid phase at 40 °C, the unsaturated DOPA can be fluidized (loosened) by the saturated DPPC-d₆₂, and such an isothermal fluidization effect is more pronounced at elevated DPPC-d₆₂ concentrations. At higher DOPA contents (30–50 mol%), only DPPC-d₆₂ molecules have conformational transitions upon heating. The present work demonstrates for the first time that the unsaturated lipid component can have significant conformational reorganizations in the phase transition process of a saturated–unsaturated binary lipid mixture.

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1. Introduction

At a given temperature a phospholipid bilayer can exist in either a fluid (liquid crystalline) or a gel phase. All phospholipids have their characteristic transition temperatures at which they change from a gel to a fluid phase. The phase behavior of lipid bilayers is largely determined by the strength of the attractive van der Waals interactions between adjacent lipid molecules. Natural membranes contain a complex mixture of different lipid molecules. If some of the lipid components are in a fluid phase at a given temperature while others are in a gel phase, the two phases can coexist in spatially separated regions (microdomains).¹ This phase separation of the lipid molecules can significantly affect the membrane properties since some membrane proteins prefer to partition into certain lipid phases and thus be locally concentrated or activated.²

The effect of unsaturated lipids on the physical properties of a bilayer is tremendous. The lipids with an unsaturated tail disrupt the packing of those with only saturated tails. The resulting bilayer has more free space and is consequently more permeable to water and other small molecules. In vitro and in vivo, mixtures of saturated and unsaturated lipids can both result in lateral lipid segregation and microdomain formation. The inhomogeneously distributed saturated and unsaturated lipids in the plasma membrane have received enormous attention.^3–14 For example, rafts, rich in saturated lipids and cholesterol, have been implicated in many biological events that are indispensable to cell functions.^15,16 Lipid–lipid interactions are fundamental in determining the organization and physical behavior of biological membranes. Uncovering the molecular picture of the phase structures and phase transition mechanisms is of primary biological importance. Until now, a clear molecular-level understanding of the mixed saturated and unsaturated lipids is still lacking. We have tackled this problem using mixtures of dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidic acid (DOPA). DPPC is the most commonly studied saturated phosphatidylcholine (PC)^17–31 and DOPA is an ideal unsaturated lipid model^32–36 since other negatively charged lipids of biomembranes (such as phosphatidylglycerol, phosphatidylserine and phosphatidylinositol) can all be regarded as PA derivatives. In this work, by using differential scanning calorimetry (DSC) and temperature-dependent Fourier transform infrared (FTIR) spectroscopy, we studied the thermotropic phase behaviors of the DOPA–DPPC binary mixtures. In FTIR experiments, to study the rearrangements of the lipid hydrocarbon chains of the DPPC and DOPA molecules independently, a deuterated lipid, 1,2-dipalmitoyl-d₆₂-sn-glycero-3-phosphocholine (DPPC-d₆₂), was used instead of the common hydrogenated DPPC. To have a direct comparison of the FTIR results with the DSC results, DOPA–DPPC-d₆₂ binary mixtures were also adopted in DSC measurements. The combined use of DSC and FTIR to study the melting process of the individual lipid components of a lipid mixture composed of deuterated and non-deuterated lipids has been reported previously.^37–39 In this work, we will apply such strategy to derive information of the different change paces of the two lipid components during phase transitions.

2. Experimental section

2.1. Sample preparation

DOPA and DPPC-d₆₂ (their molecular structures are shown in Fig. S1†) were purchased from Avanti Polar Lipids Inc. (Birmingham, AL). The weighed amounts of DPPC-d₆₂ and DOPA were dissolved in chloroform/methanol. The molar percentage of DOPA (X_DOPA, %) was calculated using the equation: X_DOPA (%) = (m_DOPA/M_DOPA)/(m_DOPA/M_DOPA + m_DPPC-d62/M_DPPC-d62) × 100%, where m_DOPA and m_DPPC-d62 represent the mass weight of DOPA and DPPC-d₆₂, respectively; while M_DOPA and M_DPPC-d62 are the molecular weight of DOPA and DPPC-d₆₂, respectively. The solvent was evaporated under a stream of dry nitrogen and was then placed in a vacuum for more than 12 h to remove the residual solvent. Double deionized H₂O with a resistivity of 18.2 MΩ cm was then added to the DOPA–DPPC-d₆₂ mixtures to obtain a lipid/water ratio of 1/3 (wt/wt). Homogeneous lipid dispersions were prepared by vortexing the samples at 60 °C for 5 min, and were then subject to repeated thermal cycles between −20 and 60 °C.

2.2. DSC

DSC data were obtained with a differential scanning calorimeter DSC821^e equipped with the high-sensitivity sensor HSS7 (Mettler-Toledo Co., Switzerland). The scan rate was 0.5 °C min⁻¹.

2.3. Synchrotron wide angle X-ray scattering

Synchrotron wide angle X-ray scattering (WAXS) experiments were performed at the beam line 1W2A of the Beijing Synchrotron Radiation Facility (BSRF) (λ = 1.54 Å). The experimental setup is similar to that described in the ref. 40. A standard silver behenate sample was used for the calibration of diffraction spacings. X-ray scattering intensity patterns were recorded during 120 s exposure of the samples to the synchrotron beam. To obtain the SAXS and WAXS data, we fixed the sample to detector distance at 400 mm. A Linkam thermal stage (Linkam Scientific Instruments, the United Kingdom) was used for temperature control (±0.1 °C). The X-ray powder diffraction intensity data were analyzed using the program Fit2D. The precision of the calculated scattering vector (the q value) is estimated to be ±0.1 nm⁻¹.

2.4. FTIR spectroscopy

FTIR spectra were recorded using a Nicolet 5700 Fourier transform infrared spectrometer with a DTGS detector in the range of 4000–900 cm⁻¹ with a spectral resolution of 2 cm⁻¹ and a zero filling factor of 2. Samples were coated onto the inner surfaces of a pair of CaF₂ windows, which were mounted on a Linkam heating–cooling stage for temperature control (±0.1 °C). Spectra were recorded every ∼30 s and each spectrum consists of 16 scans. The heating rate was 0.5 °C min⁻¹.

3. Results and discussion

The general thermotropic phase behavior of DOPA–DPPC-d₆₂ binary mixtures was studied by DSC. Shown in Fig. 1A are the thermograms upon heating at a rate of 0.5 °C min⁻¹. The neat DPPC-d₆₂ sample shows two endothermic peaks at T_peak = 31.4 °C and 37.9 °C, which correspond to lamellar gel (L_β) to rippled gel (P_β) phase transition (pretransition) and P_β to lamellar liquid crystalline (L_α) phase transition (main transition), respectively.⁴¹ This is very similar to hydrogenated DPPC, where the two peaks only shift to higher temperatures at T_peak = ∼36 °C and ∼42 °C.^21,22,42,43 The pretransition temperature decreases with increasing DOPA content and the pretransition peak finally disappears at 10% DOPA, leaving only a broad peak at T_peak = ∼33 °C. Upon further increasing the DOPA content, the seemingly single broad peak at 10% DOPA changes to a two-peak pattern at 14% DOPA and 20% DOPA samples, and the overall phase transition temperature decreases as the DOPA content increases to 50%. Based on the DSC data of both heating and cooling scans, and also the following synchrotron wide angle X-ray scattering (WAXS) results, a partial phase diagram was constructed (shown in Fig. 1B). This partial phase diagram was used only to aid the explanations of the IR results. From this phase diagram, we can see that below 10% DOPA, the initial L_β phase first converts to a P_β phase (via a very narrow L_β and P_β coexistence state) and then to an L_α phase (via a broad P_β and L_α coexistence state) upon heating. When the DOPA content is above 10%, the P_β phase disappears, the initial gel phase changes to L_α phase via a broad gel and L_α coexistence state. Our following FTIR studies will focus on the samples with the DOPA content above 10%.

Fig. 1 (A) DSC heating results of DOPA–DPPC-d₆₂ binary mixtures. The DOPA content in the binary mixture was indicated on the right of the figure. (B) A partial phase diagram of DOPA–DPPC-d₆₂ binary mixtures constructed by using the DSC data and the WAXS results in Fig. 2. This partial phase diagram was constructed only to aid the explanations of the IR results. The phase state above 30% DOPA changes from a mixed gel and fluid phase to a final fluid phase, via an intermediate gel + fluid mixed state in which the gel phase content gradually decreases upon heating.

The identification of the phase states of the lipid binary mixtures were achieved with the help of the WAXS results (Fig. 2), which can reflect the packing state of the lipid hydrocarbon tails. The results for the 0% DOPA sample (Fig. 2A) show that at 20 °C, the L_β phase has a single sharp peak (the peak of the scattering vector) at q_m = 14.9 nm⁻¹. The quantity relates to the repeat distance (d) through the equation d = 2π/q_m = 0.422 nm, which can reflect the interchain packing state for L_β phase.^14,21,22 For the P_β phase at 35 °C, the q_m resides at 14.7 nm⁻¹, corresponding to a d value of 0.427 nm. For 2.3% and 5% DOPA samples (Fig. 2B and C), the scattering peaks reside at 15.1 and 14.8 nm⁻¹ for L_β and P_β phases, respectively. Thus, we can conclude that the L_β phase has a scattering peak at 14.9–15.1 nm⁻¹, while the P_β phase has a scattering peak at 14.7–14.8 nm⁻¹. For higher DOPA content samples (10–30%, see Fig. 2D), the low temperature phase has a scattering peak at 15.0–15.1 nm⁻¹, indicating that the samples are in L_β phase. Such a conclusion is supported by the fact that the formation of a ripple phase is unfavorable with the presence of an increasing content of the non-ripple forming lipid, DOPA. However, to confirm whether the ripple structure is present or not, more rigorous characterizations such as freeze-fracture electron microscopy (FFEM) technique should be carried out. Herein, because the exact identification of the gel phase is not important, and also to avoid mistakes, we have denoted the low temperature phase of the higher DOPA content samples (>10%) as gel phase instead of specifying them as P_β or L_β phase. For the samples with the DOPA content above 30% (e.g., 50%, see Fig. 2E), some additional DOPA molecules were excluded to form a DOPA-rich fluid phase, existing together with the DPPC-d₆₂–DOPA mixed gel phase. Upon heating to above 8 °C, the gel phase in the 50% DOPA sample begins to melt, and thus the content of the fluid phase increases. Thus for high DOPA content samples (>30%), they change from a mixed gel and fluid phase to a final fluid phase, via an intermediate gel + fluid mixed state in which the fluid phase content gradually increases upon heating.

Fig. 2 Synchrotron wide angle X-ray scattering results of (A) 0% DOPA, (B) 2.3% DOPA, (C) 5% DOPA, (D) 10%, 20% and 30% DOPA and (E) 50% DOPA samples.

Temperature-dependent FTIR spectroscopy was used to unveil the molecular-level details of the phase transitions occurring in the 10–50% DOPA samples. The peak positions of the symmetric stretching vibrations of CH₂ (ν_sCH₂) and CD₂ (ν_sCD₂) in the tail regions of hydrogenated DOPA and deuterated DPPC-d₆₂ can sensitively reflect the conformational changes of the lipid tails, respectively. Their shifts towards high frequencies upon heating (Fig. 3) indicate that the CH₂ chains of DOPA and the CD₂ chains of DPPC-d₆₂ change from a more ordered state to a less ordered state with increased gauche conformers.^44–50 Based on their wavenumber values, the initial state of DPPC-d₆₂ is rich in all-trans conformers (ν_sCD₂ at ∼2089 cm⁻¹ before phase transition); while for DOPA, its initial state is already rich in gauche conformers (ν_sCH₂ at ∼2853 cm⁻¹ before heating-induced transition). The data in Fig. 3 show that at relatively low DOPA concentrations (10%, 14% and 20%), the two lipids in the binary mixtures have nonsynchronous acyl tail conformational changes upon heating. Table 1 summarizes all the transition temperatures determined by sigmoidal fitting of the ν_sCH₂ in DOPA and ν_sCD₂ in DPPC-d₆₂ (we have also shown a sigmoidal fitting example for 10% DOPA sample in Fig. S2 in the ESI†). In the 10% DOPA sample (Fig. 3A), for example, the transitional change (not the “thermal fluctuation-induced” wavenumber change) of ν_sCH₂ in DOPA occurs at ∼26 °C, and it ends at ∼35 °C (with a phase transition temperature T_m of 30.0 ± 0.2 °C). While ν_sCD₂ in DPPC-d₆₂ begins to have a significant change at ∼31 °C (the onset temperature of the transition) and ends at ∼36 °C (T_m = 33.8 ± 0.1 °C). After the completions of the transitions of the two lipids, the upward and linear shift in wavenumber is due to further fluidization (loosening) of the lipid tails upon heating (the thermal fluctuation induced loosening). Such a thermal fluctuation induced wavenumber shift is gradual and linear, and a similar case can be seen in neat DOPA sample upon heating. As shown in the ESI (Fig. S3†), DOPA has a phase transition temperature at T_onset = −6.5 °C (or T_peak = −3.5 °C), which means that above 0 °C, DOPA is in fluid phase. However, temperature-dependent FTIR data (Fig. S4†) reveal that the ν_sCH₂ peak in DOPA still has a linear wavenumber increase of ∼0.7 cm⁻¹ as the temperature changes from 0 to 50 °C. The further fluidization (loosening) of the fluid-phase of DOPA upon heating is similar to the case observed in Fig. 3. The significant temperature delay (ΔT_m = 3.8 °C) of the change of ν_sCD₂ in DPPC-d₆₂ as compared with the change of ν_sCH₂ in DOPA indicates that the seemingly single peak observed in the DSC curve in Fig. 1 for the 10% DOPA is not as simple as it seems. The molecular picture of the thermal event recorded by the broad DSC peak can now be uncovered: the nonsynchronous change of the lipid tail regions of DOPA and DPPC-d₆₂ in the binary mixtures contributes to a total transition temperature range from ∼26 °C to ∼36 °C, the same as the transition temperature range (∼26 °C to ∼36 °C) observed in DSC. The broad DSC peak may contain two overlapped peaks, corresponding to two thermal events originating from the conformational rearrangements of the two lipids.

Fig. 3 Dependencies of FTIR peak positions of ν_sCH₂ in DOPA and ν_sCD₂ in DPPC-d₆₂ on temperature in DOPA–DPPC-d₆₂ mixtures. (A) 10% DOPA + 90% DPPC-d₆₂, (B) 14% DOPA + 86% DPPC-d₆₂, (C) 20% DOPA + 80% DPPC-d₆₂, (D) 30% DOPA + 70% DPPC-d₆₂, (E) 40% DOPA + 60% DPPC-d₆₂, (F) 50% DOPA + 50% DPPC-d₆₂. The red short dash lines are guidelines to show the changing trend of the data.

Table 1 Phase transition temperatures (T_m) determined by sigmoidal fitting of the temperature-dependent IR wavenumber changes of the v_sCH₂ and v_sCD₂

Sample (DOPA%)	Tm from vsCH2 (°C)	Tm from vsCD2 (°C)
10	30.0 ± 0.2	33.8 ± 0.1
14	28.5 ± 0.3	34.0 ± 0.1
20	22.6 ± 0.5	29.0 ± 0.1
30	—	28.1 ± 0.2
40	—	24.0 ± 0.2
50	—	21.3 ± 0.2

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In the 14% DOPA sample (Fig. 3B), the change of ν_sCH₂ in DOPA occurs at ∼20 °C, and it ends at ∼37 °C (T_m = 28.5 ± 0.3 °C). While ν_sCD₂ in DPPC-d₆₂ begins to change at ∼23 °C, and ends at ∼36 °C (T_m = 34.0 ± 0.1 °C). The significant temperature delay (ΔT_m = 5.5 °C) of the change of ν_sCD₂ in DPPC-d₆₂ as compared with that of ν_sCH₂ in DOPA indicates that the double-peak pattern observed in the DSC curve in Fig. 2 for the 14% DOPA has its molecular origins: there are two thermal events corresponding to the nonsynchronous conformational changes of the two lipid components. Besides, it can also be seen that the phase transition temperature range revealed by FTIR (∼20 °C to ∼37 °C) agrees well with that revealed by DSC (∼20 °C to ∼37 °C).

In the 20% DOPA sample (Fig. 3C), the transition of DOPA is not so evident as those of the 10% and 14% samples. However, we can still discern that the transition of ν_sCH₂ in DOPA occurs at ∼19 °C and ends at ∼29 °C (T_m = 22.6 ± 0.5 °C). While ν_sCD₂ in DPPC-d₆₂ begins to change at ∼21 °C, and ends at ∼31 °C (T_m = 29.0 ± 0.1 °C). A temperature delay (ΔT_m) of 6.4 °C was observed for the change of ν_sCD₂ in DPPC-d₆₂ and the change of ν_sCH₂ in DOPA. Besides, the phase transition temperature range revealed by FTIR is ∼19 °C to ∼31 °C, which is also in good agreement with that revealed by DSC (∼18 °C to ∼33 °C).

In the samples with higher DOPA contents (30, 40 and 50%), DOPA molecules do not have a discernable conformational transition, and only linear upward-shift lines can be observed, suggesting that DOPA molecules remain in fluid state throughout the entire temperature range. While DPPC-d₆₂ molecules still have evident phase transitions, and the phase transition temperature ranges revealed by FTIR data completely match with those observed in DSC curves.

Fig. 4 shows the changes of ν_sCH₂ in DOPA with the mole percentage of DOPA (X_DOPA) at 0 °C and 40 °C. The data at 40 °C show that the ν_sCH₂ of the peak position increases significantly for ∼2 cm⁻¹ as X_DOPA changes from 100% to 10%. This is a very large wavenumber shift since a typical gel to fluid transition is accompanied by a wavenumber shift of at most 3–5 cm⁻¹ in ν_sCH₂.^22,46 Since DOPA is in fluid state at 40 °C, such a significant increase of the ν_sCH₂ peak position upon increasing DPPC-d₆₂ content indicates that the unsaturated DOPA molecules are further fluidized (loosened) by the saturated DPPC-d₆₂ molecules in the mixed fluid phase composed of DOPA and DPPC-d₆₂, and more DPPC-d₆₂ molecules lead to a more fluidized state of DOPA molecules. However, at 0 °C, such a fluidization effect was not observed and the ν_sCH₂ peak position of DOPA only has a small fluctuation at around 2853 cm⁻¹, with a maximum change amplitude of only ∼0.5 cm⁻¹ with the change of DOPA content. Thus from the figure, we can see that for the 10% DOPA sample, it has a largest upward wavenumber shift (3 cm⁻¹) as the temperature changes from 0 to 40 °C. Besides, the higher the content of DOPA, the smaller the shift in ν_sCH₂ of DOPA as the temperature changes from 0 to 40 °C. The wavenumber ∼2856 cm⁻¹ of ν_sCH₂ at 40 °C in 10% DOPA sample is significantly greater than those of usual fluid phases reported, where they typically reside at 2853–2854 cm⁻¹.^22,45–47 This further suggests that there is a fluidization transition of DOPA by DPPC-d₆₂.

Fig. 4 Dependence of ν_sCH₂ in DOPA on DOPA content at 40 °C and 0 °C in DOPA–DPPC-d₆₂ mixtures.

It is interesting to find that the unsaturated lipid component DOPA can have a significant conformational reorganization (from a less loosely packed to a more loosely packed state) in the tail region in low DOPA content (10–20%) samples, while no discernable conformational transitions of DOPA molecules can be observed in high DOPA content (30–50%) samples. We can easily exclude the possibility of the formation of pure DPPC-d₆₂ domain in these high DOPA content samples since both the DSC and FTIR data show much lower phase transition temperatures as compared with that of the neat DPPC samples. The observed endothermic peaks in DSC curves for the 30%, 40% and 50% samples mainly correspond to the conformational changes of DPPC-d₆₂ in the lipid mixture. As X_DOPA changes from 30% to 40%, and finally to 50%, there are more unsaturated lipids to fluidize the saturated DPPC-d₆₂ molecules in the gel phase before heating, and thus a more loosely packed gel phase was formed at elevated DOPA contents, which results in a decreased gel-to-fluid transition temperature of the mixture upon heating. While the observed endothermic DSC peaks for the 10%, 14% and 20% samples come from conformational changes of both DPPC-d₆₂ and DOPA. As revealed by FTIR data, the DPPC-d₆₂ molecules undergo a gel–fluid transition upon heating, while the DOPA molecules experience a fluidization transition by DPPC-d₆₂ at elevated temperatures. The reason why there is a conformational transition of the unsaturated DOPA sample at the low DOPA content sample is the fluidization of DOPA by the saturated DPPC-d₆₂ lipids at increasing temperatures. The extent of such conformational transition of DOPA upon heating decreases as the content of DOPA increases from 10% to 14%, and finally to 20% (Fig. 4), which may be due to the fact that there are less surrounding saturated DPPC-d₆₂ molecules to mix and affect the packing state of the unsaturated DOPA molecules as the DOPA content increases in the mixture. At DOPA content above 20%, the surrounding saturated DPPC-d₆₂ molecules were not enough to induce a disrupting effect on the packing of the unsaturated DOPA molecules within the lipid mixture at the high temperatures, and thus no conformational transition was observed in these high DOPA content samples (30%, 40% and 50%).

Since DOPA has a relatively small headgroup as compared with PE or PC lipids, the membrane curvature of the mixed lipids will be altered with the presence of DOPA due to the introduction of a negative membrane curvature. Besides, the protonation state of the DOPA molecules may also change depending on the type of lipids and the environment (i.e., pH and ionic strength).^51–53 However, other negatively charged lipids such as phosphatidylglycerol, phosphatidylserine and phosphatidylinositol also have similar problems – they have a larger headgroup that leads to a large positive membrane curvature,⁵⁴ along with their different protonation states at different solutions, which can also introduce complexity into the binary mixed systems. Thus, a further investigation on the effects of the membrane curvature and the protonation state of lipids will be helpful for a deeper understanding of the phase transition mechanism of the neutral-charged lipid mixtures, which will be performed in the future.

The previous work has concentrated on the phase transition mechanisms of a saturated–unsaturated lipid mixture composed of neutral lipids.^7,55,56 For example, Schmidt et al. have constructed a partial phase diagram for DOPC and DPPC-d₆₂, and have found the presence of a subgel phase below 8–12 °C. In this work, we did not observe the formation of subgel phase possibly because of the different lipid molecules used and the techniques we used are not sensitive enough to detect the formation of the subgel phase. However, a detailed investigation of the subgel phase is beyond our scope, since our main aim of the present work is to reveal a molecular level picture of the gel to fluid phase transition mechanism of the binary lipid mixture upon heating.

The present work provides a new physical insight that not only the unsaturated lipid components can loosen the packing of the saturated lipids, but also the saturated lipids can loosen the packing of the unsaturated lipids. This finding deepens our understanding on the phase transition process of saturated–unsaturated lipid binary mixtures.

4. Conclusion

In this work, we investigated the phase transition process of DOPA–DPPC-d₆₂ binary mixtures by using DSC and temperature-dependent FTIR techniques. The most important finding is that we observed a heating-induced fluidization transition of the unsaturated acyl tails of DOPA for the low DOPA content samples (10%, 14% and 20%), and the conformational transitions of the acyl tails of DOPA and DPPC-d₆₂ are nonsynchronous upon heating, with DOPA molecules changing earlier than DPPC-d₆₂. While for high DOPA content samples (30%, 40% and 50%), only DPPC-d₆₂ molecules have conformational transitions in the tail region, and no conformational transition was observed for unsaturated DOPA molecules over the entire temperature range. Besides, an isothermal (40 °C) fluidization effect exerted by saturated DPPC-d₆₂ molecules on unsaturated DOPA molecules was observed in the mixed fluid phase, with more DPPC-d₆₂ molecules leading to a more fluidized state of DOPA molecules. The present work gives us for the first time the individual change paces of the binary lipid mixtures composed of saturated and unsaturated lipids, and the method can be extended to uncover the molecular mystery of phase transition processes occurring in various other lipid mixtures (such as lipid rafts) or surfactant mixtures.

Acknowledgements

This work was supported by grants from the Natural Science Foundation of China (NSFC: Grant no. 21133009, 21273130 and 21303017) and the Natural Science Foundation of Jiangsu Province (KB20130601). The WAXS data were collected at the beamline 1W2A of the Beijing Synchrotron Radiation Facility (BSRF) with the assistances of Prof. Zhonghua Wu (Institute of High Energy Physics) and the station scientists.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Molecular structures of DOPA and DPPC-d₆₂, determination of the phase transition temperature by the sigmoidal fitting method, DSC curve for a pure DOPA aqueous dispersion and the dependence of FTIR peak positions of ν_sCH₂ in DOPA on temperature in a pure DOPA aqueous dispersion. See DOI: 10.1039/c4ra07569b

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