Temperature-Dependent Hydrocarbon Chain Disorder in Phosphatidylcholine Bilayers Studied by Raman Spectroscopy

In-plane and out-of-plane gigahertz sound velocities of saturated and unsaturated phospholipid bilayers from cryogenic to room temperatures

Here, we examined the gigahertz sound velocities of hydrated multibilayers of saturated (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) and unsaturated (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) phospholipids by Brillouin spectroscopy. Out-of-plane and in-plane (lateral) phonons were studied independently of each other. Similar strong temperature dependences of the sound velocities were found for phonons of both types. The sound velocities in the low-temperature limit were two-fold higher than that at physiological temperatures; a significant part of the changes in sound velocity occurs in the solid-like gel phase. The factors that may be involved in the peculiar behavior of sound velocity include changes in the chain conformational state, relaxation susceptibility, changes in the elastic modulus at infinite frequencies, and lateral packing of molecules.

Characterization of conformational states of POPC and DPPCd62 in POPC/DPPCd62/cholesterol mixtures using Raman spectroscopy

Conformational states of phospholipid chains in ternary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), deuterated 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPCd62), and cholesterol (Chol) were studied by Raman spectroscopy. Parameters of Raman peaks sensitive to conformational order have been used to determine chain order for mixtures over a wide range of compositions. A ternary diagram of fractions of phospholipid chains in conformationally ordered and disordered states has been constructed. It was found that the addition of POPC and cholesterol increases the fraction of DPPC chains in disordered conformations. The so-called liquid-ordered phase includes DPPC molecules in both ordered and disordered states in comparable proportions. It was found that POPC chains are partially ordered in mixtures with DPPC and cholesterol, in contrast to the case of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). This maybe the underlying reason why ternary mixtures with POPC have different miscibility behavior compared to DOPC.

A Deeper Insight into the Interfacial Behavior and Structural Properties of Mixed DPPC/POPC Monolayers: Implications for Respiratory Health

1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleyl-sn-glycerol-3-phosphorcholine (POPC) are important components in pulmonary surfactants (PSs), of which the relative content is related to lung compliance. Herein, the phase behavior and thermodynamic structure of mixed DPPC/POPC monolayers were studied to elucidate the intermolecular interaction between DPPC and POPC molecules. Surface pressure-molecular area isotherms demonstrated that POPC significantly affected the phase behavior of the lipid domain structure as a function of its concentration. The compression modulus of the mixed monolayers reduced with the increase in POPC proportion, which can be attributed to the intermolecular repulsion between DPPC and POPC. Brewster angle microscopy analysis showed that the ordered structure of the monolayers trended toward fluidization in the presence of POPC. Raman spectroscopy results revealed that the change in C-C skeleton stretching vibration was the main cause of the decrease in the monolayer packing density. These findings provide new insights into the role of different phospholipid components in the function of PS film at a molecular level, which can help us to understand the synergy effects of the proportional relationship between DPPC and POPC on the formation and progression of lung disease and provide some references for the synthesis of lung surfactants.

Revealed Properties of Various Self-Assemblies in Two Catanionic Surfactant Systems in Relation to Their Polarity and Molecular Packing State

A catanionic surfactant system is an aqueous solution or dispersion of cationic and anionic surfactants that spontaneously self-assemble into structures such as micelles, vesicles, and coacervates. Their structural diversity varies depending on the ratios of cationic and anionic surfactants (compositions), the chemical structure of the constituent molecules, etc. Herein, two types of catanionic surfactant systems were systematically characterized: (i) cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), both typical ionic surfactants; and (ii) dodecylmethylimidazolium ammonium bromide ([C₁₂mim]Br) and SDS, where the former is an ionic liquid. By observing the sample appearance, turbidity, and particle size, the phase state of each system was analyzed according to the total concentration of surfactants and the molar ratio of cationic surfactants to the total concentration. Especially, for specific compositions of catanionic surfactant vesicles (cataniosome), the closed structure of the vesicles was confirmed through calcein entrapment and release detected with a fluorescence assay. The polarities of the interface of the prepared self-assemblies were evaluated using a fluorescence probe, Laurdan. The packing state of the molecules in the formed self-assembly structure was estimated using Raman spectroscopy. The results clearly indicate consistent phase-transition behavior for the CTAB-SDS (i) and [C₁₂mim]Br-SDS (ii) systems, depending on the total surfactant concentration and composition, while the membrane properties of the two systems differed. The cataniosome formed in the CTAB-SDS system was in a tightly packed membrane state and more hydrophobic than that formed in the [C₁₂mim]Br-SDS system owing to the difference in the structure of the constituting molecule: [C₁₂mim]Br has a larger head group and shorter acyl chain than CTAB. The self-assembly properties evaluated in this study were compared with those of typical lipid membranes, liposomes (lipid vesicles), to determine a possible application of the catanionic systems with various self-assembly formulations.

Brillouin Spectroscopy of Binary Phospholipid-Cholesterol Bilayers

Multicomponent lipid bilayers are used as models for searching the origin of spatial heterogeneities in biomembranes called lipid rafts, implying the coexistence of domains of different phases and compositions within the lipid bilayer. The spatial organization of multicomponent lipid bilayers on a scale of a hundred nanometers remains unknown. Brillouin spectroscopy providing information about the acoustic phonons with the wavelength of several hundred nanometers has an unexplored potential for this problem. Here, we applied Brillouin spectroscopy for three binary bilayers composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol. The Brillouin experiment for the oriented planar multibilayers was realized for two scattering geometries involving phonons for the lateral and normal directions of the propagation. The DPPC-DOPC mixtures known for the coexistence of the solid-ordered and liquid-disordered phases had bimodal Brillouin peaks, revealing the phase domains with sizes more than a hundred nanometers. Analysis of the Brillouin data for the binary mixtures concluded that the lateral phonons are preferable for testing the lateral homogeneity of the bilayers, while the phonons spreading across the bilayers are sensitive to the layered packing at the mesoscopic scale.

Lipid-driven condensation and interfacial ordering of FUS

Protein condensation into liquid-like structures is critical for cellular compartmentalization, RNA processing, and stress response. Research on protein condensation has primarily focused on membraneless organelles in the absence of lipids. However, the cellular cytoplasm is full of lipid interfaces, yet comparatively little is known about how lipids affect protein condensation. Here, we show that nonspecific interactions between lipids and the disordered fused in sarcoma low-complexity (FUS LC) domain strongly affect protein condensation. In the presence of anionic lipids, FUS LC formed lipid-protein clusters at concentrations more than 30-fold lower than required for pure FUS LC. Lipid-triggered FUS LC clusters showed less dynamic protein organization than canonical, lipid-free FUS LC condensates. Lastly, we found that phosphatidylserine membranes promoted FUS LC condensates having β sheet structures, while phosphatidylglycerol membranes initiated unstructured condensates. Our results show that lipids strongly influence FUS LC condensation, suggesting that protein-lipid interactions modulate condensate formation in cells.

Conformational state diagram of DOPC/DPPCd62/cholesterol mixtures

Raman spectra of aqueous suspensions of vesicles composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), deuterated 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC_d62), and cholesterol (Chol) were studied at room temperature to determine the conformational states of the phospholipid hydrocarbon chains. Deuteration of DPPC_d62 allowed us to characterize the conformational states of DOPC and DPPC_d62 independently. The parameters of Raman peaks, which are sensitive to the conformational order, were studied in a wide range of compositions. It was found that the DOPC molecules are conformationally disordered for all compositions. The conformational state of the DPPC_d62 molecules changes with composition. Their conformational state is influenced by cholesterol-induced partial disordering and DOPC solvation, transforming the DPPC molecules into the disordered state. The conformational state diagram from the Raman experiment was compared with outcomes from the differential scanning calorimetry (DSC) experiment. The Raman spectra also revealed that the DPPC molecules coexist in the disordered and all-trans ordered states for the DOPC/DPPC_d62/Chol mixtures except for the pure liquid-disordered phase.

Raman scattering in protonated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC): Indicators of conformational and lateral orders

The use of deuterocarbons is an effective method in the Raman spectroscopy of multicomponent lipid materials and biological samples. Here, Raman spectra of hydrated multilamellar vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), its deuterated analog 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC_d62), and DPPC-DPPC_d62 mixtures were studied in a wide temperature range to specify the Raman indicators of conformational and lateral orders. The temperature dependence of the 985 cm^-1 line in the deuterated phospholipid unequivocally indicates that this line corresponds to the CC stretching vibrations of deuterated hydrocarbon chains in the all-trans conformation. It was also concluded that the ratio of Raman intensities at the maximum of the peak of the symmetric CD₂ stretching and at a maximum near 2168 cm^-1 reflects the conformational order of the hydrocarbon chain and can be used for an evaluation of the fraction of the all-trans sequences. The frequency of the symmetric CD₂ stretching peak is sensitive to the phase state (gel or fluid) but has a low sensitivity to the partial conformational disordering within the gel phase. The Raman study of DPPC-DPPC_d62 mixtures reveals that the lateral order contributes to the ratio of intensities of the antisymmetric and symmetric CH₂ stretching peaks as a prefactor enhancing the effect of conformational ordering.

Coexistence of lipid phases in multilayer phospholipid films probed by Raman mapping

Raman spectroscopy of stacked phospholipid bilayers reveals the chemical and conformational composition of coexisting domains.

Dynamic response on a nanometer scale of binary phospholipid-cholesterol vesicles: Low-frequency Raman scattering insight

Low-frequency Raman spectroscopy was used to study the dynamic response on a nanometer scale of aqueous suspensions of two-component lipid vesicles. Binary mixtures of saturated phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and cholesterol are interesting for possible coexistence of solidlike and liquid-ordered phases, while the phase coexistence was not reported for unsaturated phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) and cholesterol mixtures. The DOPC-DPPC mixtures represent the well-documented case of coexisting domains of solidlike and liquid-disordered phases. These three series of lipid mixtures are studied here. A broad peak with the maximum in the range of 30-50cm^{-1} and a narrow peak near 10cm^{-1} are observed in the Raman susceptibility of the binary mixtures and attributed to the acousticlike vibrational density of states and layer modes, respectively. Parameters of the broad and narrow peaks are sensitive to lateral and conformational hydrocarbon chain ordering. It was also demonstrated that the low-frequency Raman susceptibility of multicomponent lipid bilayers allows one to determine the phase state of lipid bilayers and distinguish the homogeneous distribution of molecular complexes from coexisting domains with sizes above several nanometers. Thus, the low-frequency Raman spectroscopy provides unique information in studying phase coexistence in lipid bilayers.

Dissimilar action of tamoxifen and 4-hydroxytamoxifen on phosphatidylcholine model membranes

The anticancer drug tamoxifen and its primary metabolite 4-hydroxytamoxifen tend to accumulate in membranes due to its strong hydrophobic character. Thus, in this work we have carried out a systematic study to investigate their effects on model phosphatidylcholine membranes. Tamoxifen and 4-hydroxytamoxifen affect the phase behaviour of phosphatidylcholine model membranes, giving rise to formation of drug/dipalmitoylphosphatidylcholine domains, which is more evident in the case of 4-hydroxytamoxifen. These drugs have differential effects on the polar and apolar regions of the phospholipid supporting a different location of both compounds within the bilayer. Both compounds induce contents leakage in fluid phosphatidylcholine unilamellar liposomes, the effect of 4-hydroxytamoxifen being negligible as compared to that of tamoxifen. Molecular dynamics confirmed the tendency of both drugs to form clusters, tamoxifen locating all along the bilayer, whereas 4-hydroxytamoxifen mostly locates near the lipid/water interface, which can explain the different effects of both drugs in fluid phosphatidylcholine membranes.

Continuous gradient temperature Raman spectroscopy of 1-stearoyl- 2-docosahexaenoyl, 1-stearoyl- 2-arachidonoyl, and 1,2-stearoyl phosphocholines

Mixed chain phospholipids containing a saturated fatty acid at sn1 and a polyunsaturated fatty acid in sn2 are common in the specialized biological membranes prevalent in neural, retinal and organ tissues. Particularly important are mixed lipids containing palmitic or stearic acid and arachidonic or docosahexaenoic acid. Gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements and phase transitions. Herein we utilize GTRS for 1-18:0, 2-20:4n-6 PC; 1-18:0 2-22:6n-3 PC; and 1-18:0, 2-18:0 PC from -80 to 50 °C temperatures. 20 Mb three-dimensional data arrays with 0.2 °C increments and first/second derivatives allowed detailed vibrational mode assignment and analysis. Samples were analyzed neat and with molecular hydration. Previously reported phase transitions for hydrated 18:0-20:4PC and 18:0-22:6PC and numerous spectral differences resulting from hydration and the double bond structure were clearly observed. Molecular models showed that the addition of minimal water molecules results in significant structural differences compared to the neat molecules; 18:0-22:6PC is strikingly compact with water when viewed from the hydrophilic end. This precise Raman data cannot be observed in typically utilized fully hydrated vesicle samples, however the improved GTRS will allow for more precise analysis in fully hydrated vesicles because the underlying modes in the unavoidably broadened spectra can be identified.

Evaluation of the changes in human milk lipid composition and conformational state with Raman spectroscopy during a breastfeed

Human milk fat forms the main energy source for breastfed infants, and is highly variable in terms of concentration and composition. Understanding the changes in human milk lipid composition and conformational state during a breastfeed can provide insight into lipid synthesis and secretion in the mammary gland. Therefore, the aim of this study was to evaluate human milk fatty acid length, degree of unsaturation (lipid composition) and lipid phase (lipid conformational state) at different stages during a single breastfeed (fore-, bulk- and hindmilk). A total of 48 samples from 16 lactating subjects were investigated with confocal Raman spectroscopy. We did not observe any significant changes in lipid composition between fore-, bulk and hindmilk. A new finding from this study is that lipid conformational state at room temperature changed significantly during a breastfeed, from almost crystalline to almost liquid. This observation suggests that lipid synthesis in the mammary gland changes during a single breastfeed.

Alteration of the lipid phase transition during mouse embryos freezing after in vitro culture with linoleic acid

Lipids significantly affect embryo cryopreservation in some mammalian species depending on the cell lipidome quantity and composition. One of the ways to study the relationship between lipid content and cryotolerance of cells is to study the effect of lipidome modification on laboratory mice. The objective of this research was to study how in vitro culture of mouse embryos with linoleic acid (LA) will affect lipid phase transition (LPT) during cooling and subsequent embryo development after cryopreservation. Embryos obtained in vivo at the 2-cell stage were cultured with 200 μM LA for 46 h up to the morula/blastocyst stage. Thereafter, one portion of embryos was slowly frozen to reveal the effect of LA on survival after cryopreservation, another portion was used to characterize the lipid composition and to determine the temperature of the LPT onset. Confocal fluorescence microscopy of Nile Red stained embryos showed a significant increase in lipid content of the LA treated group compared to the controls. Raman measurements showed that the onset of LPT in LA treated embryos is lower than in untreated ones: -5 °C vs +2 °C. However, these changes in the LPT onset did not affect the survival rates of embryos after cryopreservation. In summary, in vitro culture with LA changes the biophysical characteristics of embryos' lipidome and is realized in lower LPT onset, but this does not affect embryo survival after cryopreservation.

Raman Spectroscopic Study of Phase Coexistence in Binary Phospholipid Bilayers

Binary phospholipid bilayers composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) were studied by Raman spectroscopy and differential scanning calorimetry (DSC). We examined features in Raman scattering spectra that are sensitive to the lipid phase and, therefore, could indicate the phase coexistence. It was found that the low-frequency half-width of half-maximum (LHWHM) of the 2850 cm^-1 Raman line, corresponding to the symmetric CH₂ stretching vibrations, unequivocally reveals the coexisting phospholipids in ordered and disordered conformational states, which correspond to ordered and disordered phases coexistence, in the DPPC mole concentration range from 0.4 to 0.9. The phase coexistence in this concentration range was supported by the particular concentration behavior of the ratio between the intensities of the 2880 cm^-1 antisymmetric CH₂ vibration line and the 2850 cm^-1 symmetric one. It was also shown that the spectral shape of the 1300 cm^-1 Raman line, corresponding to the CH₂ twisting vibrations, is a good indicator for the phase state and phase coexistence in the phospholipid bilayers. Comparison with the DSC curves confirmed that in the DPPC mole concentration range from 0.4 to 0.9, the two phase transition peaks are observed in DSC curve, those positions are independent of the DPPC concentration. The outcome of the study is the robust label-free contactless approach for the detection of the lipid phase separation, which can be realized with the micrometer resolution.

Raman spectroscopy and DSC assay of the phase coexistence in binary DMPC/cholesterol multilamellar vesicles

The phospholipid/cholesterol binary model systems are an example of simple models whose structure has caused controversy and genuine interest over many decades. The cornerstone underlying the description of such models is the answer to the question of whether these membranes are separated into coexisting phases or domains. Here, we apply label-free Raman spectroscopy and differential scanning calorimetry (DSC) to verify the phase coexistence in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol binary model. Raman spectra demonstrate the peculiarity at 30% molar fraction of cholesterol. Above this concentration, Raman data demonstrate similar characteristics at T = 291, 298, 303 K. At lower molar fractions, at 303 K, we found the agreement of Raman spectra with the predictions of the lever rule of cholesterol. Taken together, low cooperativity of the transition at 30 mol% and the fulfillment of the lever rule suggest the existence of nanoclusters composed of approximately 4 DMPC and 2 cholesterol molecules. At 298 K, the compliance of the lever rule was found in the range from 0 to 20 mol% of cholesterol. At 291 K, the addition of 5% cholesterol leads to the abrupt change of Raman spectra parameters and their continuous evolution with the further increase of cholesterol molar fraction. It seems that cholesterol plays a twofold role in binary mixtures; it reduces the intermolecular cooperativity and forms clusters whose size and DMPC-to-cholesterol ratio depend on cholesterol concentration and temperature.

Effect of the Hydrocarbon Chain Disorder in Phosphatidylcholine Bilayers on Gigahertz Sound Velocity

Suspensions of multilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and hydrated and dehydrated aligned multilamellar samples of DMPC were studied by Brillouin spectroscopy in the temperature range from 90 to 333 K. The sound velocity of the longitudinal acoustic wave was evaluated from the Brillouin spectra. It was found that phase transition, hydration state, and planar or vesicular form of bilayers affect the gigahertz sound velocity. Usually, the temperature dependence of the sound velocity is weak in solid substances. Amazingly, the sound velocity of hydrated DMPC samples showed significant temperature-induced changes of up to 1.8 times, even within the solid-like gel phase. We explained this effect by temperature-induced excitations of the disordered conformational states of the hydrocarbon chains as well as anharmonic effects. In addition, the relevance of the gigahertz sound velocity to the description of subterahertz Raman features was demonstrated.

Membrane-Sugar Interactions Probed by Low-Frequency Raman Spectroscopy: The Monolayer Adsorption Model

Small sugars are known to stabilize biological membranes under extreme conditions of freezing and desiccation. The proposed mechanisms of stabilization suggest membrane-sugar interactions to be either attractive or repulsive. To obtain new insight into the problem, we use a recently developed low-frequency Raman scattering approach which allows detecting membrane mechanical vibrations. For model membranes of palmitoyl-oleoyl-glycero-phosphocholine (POPC) hydrated in aqueous sucrose and trehalose solutions, we studied the Raman peak between 12 and 15 cm^-1 that is attributed to an eigenmode of the normal mechanical vibrations of a lipid monolayer. For both sugars, similar results were obtained. With an increase in sugar concentration in solution, the frequency position of the peak was found to decrease by ∼13% which was interpreted as a consequence of the membrane thickening due sugar monolayer adsorption on the membrane surface. The concentration dependence of the peak frequency position was satisfactorily described by a Langmuir monolayer adsorption model. It is concluded that, at small sugar concentrations (less than 0.2 M), the membrane-sugar interactions are attractive, while at higher concentrations (more than 0.4 M) the attraction disappears. The data obtained show that one sugar molecule on the surface interacts with approximately 3-4 polar lipid heads.

Raman Spectroscopy Study of Structurally Uniform Hydrogenated Oligomers of α-Olefins

The expansion of the range of physico-chemical methods in the study of industrially significant α-olefin oligomers and polymers is of particular interest. In our article, we present a comparative Raman study of structurally uniform hydrogenated dimers, trimers, tetramers, and pentamers of 1-hexene and 1-octene, that are attractive as bases for freeze-resistant engine oils and lubricants. We found out that the joint monitoring of the disorder longitudinal acoustic mode (D-LAM) and symmetric C–C stretching modes allows the quantitative characterization of the number and length of alkyl chains (i.e., two structural characteristics), upon which the pour point and viscosity of the hydrocarbons depend, and to distinguish these compounds from both each other and linear alkanes. We demonstrated that the ratio of the contents of CH₂ and CH₃ groups in these hydrocarbons can be determined by using the intensities of the bands in the spectra, related to the asymmetric stretching vibrations of these groups. The density functional theory (DFT) calculations were applied to reveal the relations between the wavenumber and bandshape of the symmetric C–C stretching mode and a conformation arrangement of the 1-hexene and 1-octene dimers. We found that the branched double-chain conformation results in the splitting of the C–C mode into two components with the wavenumbers, which can be used as a measure of the length of branches. This conformation is preferable to the extended-chain conformation for hydrogenated 1-hexene and 1-octene dimers.

Raman spectroscopy evidence of lipid separation in domestic cat oocytes during freezing

Although lipid droplets are believed to play an important role in cryopreservation of mammalian embryos and oocytes, the effect of low temperatures on lipid droplets and related mechanisms of cryodamage are still obscure. Here, we provide Raman spectroscopy evidence of lipid separation inside the lipid droplets in domestic cat oocytes during slow freezing. It was shown that at -25 °C lipids coexist in two separated phase states inside lipid droplets. The scale of detected domains was a few micrometers size. We also found that under certain conditions these areas have a specific spatial distribution. Lipids with high melting temperatures are distributed near the surface of lipid droplets while fusible lipids are located deep inside. Raman spectroscopy was found to be a prospective approach to study inhomogeneity of lipid phase transition in cells and to reveal effects of this inhomogeneity on cryopreservation of biological cells.

Deciphering the orientation of lipid molecules by principal component analysis of Raman mapping data

Raman spectroscopy reveals the orientational ordering of dry and hydrated phospholipids.

Raman Spectroscopy Study of Curvature-Mediated Lipid Packing and Sorting in Single Lipid Vesicles

Cellular plasma membrane deformability and stability is important in a range of biological processes. Changes in local curvature of the membrane affect the lateral movement of lipids, affecting the biophysical properties of the membrane. An integrated holographic optical tweezers and Raman microscope was used to investigate the effect of curvature gradients induced by optically stretching individual giant unilamellar vesicles (GUVs) on lipid packing and lateral segregation of cholesterol in the bilayer. The spatially resolved Raman analysis enabled detection of induced phase separation and changes in lipid ordering in individual GUVs. Using deuterated cholesterol, the changes in lipid ordering and phase separation were linked to lateral sorting of cholesterol in the stretched GUVs. Stretching the GUVs in the range of elongation factors 1-1.3 led to an overall decrease in cholesterol concentration at the edges compared to the center of stretched GUVs. The Raman spectroscopy results were consistent with a model of the bilayer accounting for cholesterol sorting in both bilayer leaflets, with a compositional asymmetry of 0.63 ± 0.04 in favor of the outer leaflet. The results demonstrate the potential of the integrated holographic optical tweezers-Raman technique to induce deformations to individual lipid vesicles and to simultaneously provide quantitative and spatially resolved molecular information. Future studies can extend to include more realistic models of cell membranes and potentially live cells.

Lipid chain mobility and packing in DOPC bilayers at cryogenic temperatures

Low-temperature molecular mobility and packing in biological tissues are important for their survival upon cryopreservation. Electron paramagnetic resonance (EPR) in its pulsed version of electron spin echo (ESE) allows studying stochastic librations of spin-labeled molecules, the type of motion which dominates at low temperatures. These librations are characterized by the parameter <α²>τ_c where <α²> is the mean squared angular amplitude and τ_c is the correlation time for the motion. This parameter is known to be larger for higher temperature and for looser intermolecular structure. In this work, ESE data for the bilayers comprised of doubly-unsaturated DOPC (dioleoyl-glycero-phosphocholine) lipids and mono-unsaturated POPC (palmitoyl-oleoyl-glycero-phosphocholine) lipids with spin-labeled stearic acids added were obtained in the temperature range between 80 and 210 K; the results were compared also with the previously obtained data for fully-saturated DPPC (dipalmitoyl-glycero-phosphocholine) lipid bilayers [J. Phys. Chem. B2014, 118, 12,478-12,485; Appl. Magn. Reson. 2018, 49, 1369-1383]. It turned out that for DOPC bilayers the <α²>τ_c values are of intermediate magnitude between those for POPC and DPPC bilayers, which implies an intermediate density of lipid packing. A possible explanation of this result could be rearrangement at cryogenic temperatures of the DOPC lipid tails, with their terminal segments folding cooperatively. This interpretation is also in agreement with the known thermodynamic properties of gel-fluid transition for DOPC bilayer.

Examination of the association states of dehydroergosterol towards understanding the association structures of sterols in a membrane

Cholesterol plays a number of roles in cell membranes, and dehydroergosterol (DHE) is a fluorescent derivative of cholesterol, which is used to investigate the association structure of cholesterol. Although the fluorescent property of DHE depends on its association state, it is insufficient to distinguish the association state of DHE only by its fluorescence. Circular dichroism (CD) spectroscopy is an effective way to investigate the molecular geometry of DHE. In the present study, therefore we investigated the association structure of DHE by CD and fluorescence spectroscopy in solution and in a lipid membrane. DHE was shown to exist as three states (monomeric, microcrystalline, and micelle states) in methanol-water mixtures depending on solvent hydrophobicity. The CD spectrum of DHE in a liposome was similar to that of the micelle state, indicating that the association state of DHE in a liposome exhibits a parallel arrangement similar to that in the micelle state. This result is difficult to be obtained only from the measurement of the fluorescence spectra. The combination of CD and fluorescence spectroscopic techniques is necessary to investigate the association of DHE.

The lipid profile of three Malassezia species assessed by Raman spectroscopy and discriminant analysis

Malassezia yeasts constitute the major eukaryotic cutaneous flora of homoeothermic vertebrates. These lipophilic yeasts are able to cause, trigger, or aggravate common skin diseases under favorable conditions. Species identification and subspecies differentiation is currently based on morphological characteristics, lipid assimilation profile, and molecular tests. Mass spectrometry has been also reported as a reliable, yet costly and labor-intensive, method to classify Malassezia yeasts. Here, we introduce Raman spectroscopy as a new molecular technique able to differentiate three phylogenetically close Malassezia species (M.globosa, M.pachydermatis, and M.sympodialis) by examining their lipid metabolic profile. Using Raman spectroscopy, lipid fingerprints of Malassezia cultures on Leeming-Notman agar, were analyzed by spectral bands assignment and partial least squares discriminant analysis. Our results demonstrate differential utilization of lipid supplements among these three species and the ability of Raman spectroscopy to rapidly and accurately discriminate them by predictive modelling.

Lipid Droplet Phase Transition in Freezing Cat Embryos and Oocytes Probed by Raman Spectroscopy

Embryo and oocyte cryopreservation is a widely used technology for cryopreservation of genetic resources. One limitation of cryopreservation is the low tolerance to freezing observed for oocytes and embryos rich in lipid droplets. We apply Raman spectroscopy to investigate freezing of lipid droplets inside cumulus-oocyte complexes, mature oocytes, and early embryos of a domestic cat. Raman spectroscopy allows one to characterize the degree of lipid unsaturation, the lipid phase transition from the liquid-like disordered to solid-like ordered state, and the triglyceride polymorphic state. For all cells examined, the average degree of lipid unsaturation is estimated as ∼1.3 (with ±20% deviation) double bonds per acyl chain. The onset of the lipid phase transition occurs in a temperature range from -10 to +4°C and does not depend on the cell type. Lipid droplets in cumulus-oocyte complexes are found to undergo abrupt lipid crystallization shifted in temperature from the ordering of the lipid conformational state. In the case of mature oocytes and early embryos obtained in vitro, the lipid crystallization is broadened. In the frozen state, lipid droplets inside cumulus-oocyte complexes have a higher content of triglyceride polymorphic β and β' phases than estimated for mature oocytes and early embryos. For the first time, to our knowledge, the temperature evolution of the phase state of lipid droplets is examined. Raman spectroscopy is proved to be a promising tool for in situ monitoring of the lipid phase state in a single embryo/oocyte during its freezing.

Light Scattering By Optically-Trapped Vesicles Affords Unprecedented Temporal Resolution Of Lipid-Raft Dynamics

A spectroscopic technique is presented that is able to identify rapid changes in the bending modulus and fluidity of vesicle lipid bilayers on the micrometer scale, and distinguish between the presence and absence of heterogeneities in lipid-packing order. Individual unilamellar vesicles have been isolated using laser tweezers and, by measuring the intensity modulation of elastic back-scattered light, changes in the biophysical properties of lipid bilayers were revealed. Our approach offers unprecedented temporal resolution and, uniquely, physical transformations of lipid bilayers can be monitored on a length scale of micrometers. As an example, the deformation of a membrane bilayer following the gel-to-fluid phase transition in a pure phospholipid vesicle was observed to take place across an interval of 54 ± 5 ms corresponding to an estimated full-width of only ~1 m°C. Dynamic heterogeneities in packing order were detected in mixed-lipid bilayers. Using a ternary mixture of lipids, the modulated-intensity profile of elastic back-scattered light from an optically-trapped vesicle revealed an abrupt change in the bending modulus of the bilayer which could be associated with the dissolution of ordered microdomains (i.e., lipid rafts). This occurred across an interval of 30 ± 5 ms (equivalent to ~1 m°C).

Normal vibrational modes of phospholipid bilayers observed by low-frequency Raman scattering

Low-frequency Raman spectra of multilamellar vesicles made either of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) have been studied in a wide temperature range. Below 0^{∘}C two peaks are found at frequencies around 8-9 and 14-17cm^{-1} and attributed to the normal vibrational modes of the phospholipid bilayer, which are determined by the bilayer thickness and stiffness (elastic modulus). The spectral positions of the peaks depend on the temperature and the bilayer composition. It is suggested that the ratio of the intensities of the first and second peaks can serve as a measure of the interleaflet elastic coupling. The addition of cholesterol to the phospholipid bilayer leads to peak shift and broadening, which may be assigned to the composition heterogeneities commonly attributed to the lipid raft formation.

Pressure-induced ionic liquid crystal in 1-dodecyl-3-methylimidazolium tetrafluoroborate

The phase behaviors of 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C₁₂MIM][BF₄]) had been investigated by means of Raman spectroscopy and polarized optical microscopy under pressure values up to 2.0 GPa at the temperature of 80.0 °C.