The transformation of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers from the gel (Lβ\') to the fluid (Lα) phase involves an intermediate ripple (Pβ\') phase forming a few degrees below the main transition temperature (Tm). While the exact cause of bilayer rippling is still debated, the presence of amphiphilic molecules, pH, and lipid bilayer architecture are all known to influence (pre)transition behavior. In particular, fatty acid chains interact with hydrophobic lipid tails, while the carboxylic groups simultaneously participate in proton transfer with interfacial water in the polar lipid region which is controlled by the pH of the surrounding aqueous medium. The molecular-level variations in the DPPC ripple phase in the presence of 2% palmitic acid (PA) were studied at pH levels 4.0, 7.3, and 9.1, where PA is fully protonated, partially protonated, or fully deprotonated. Bilayer thermotropic behavior was investigated by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy which agreed in their characterization of (pre)transition at pH of 9.1, but not at pH 4.0 and especially not at 7.3. Owing to the different insertion depths of protonated and deprotonated PA, along with the ability of protonated PA to undergo flip-flop in the bilayer, these two forms of PA show a different hydration pattern in the interfacial water layer. Finally, these results demonstrated the hitherto undiscovered potential of FTIR spectroscopy in the detection of the events occurring at the surface of lipid bilayers that obscure the low-cooperativity phase transition explored in this work.

Y-shaped polymer brushes represent a special class of binary mixed polymer brushes, in which a combination of different homopolymers leads to unique phase behavior. While most theoretical and simulation studies use monodisperse models, experimental systems are always polydisperse. This discrepancy hampers linking theoretical and experimental results. In this theoretical study, we employed dissipative particle dynamics to study the influence of polydispersity on the phase behavior of Y-shaped brushes grafted to flat surfaces under good solvent conditions. Polydispersity was kept within experimentally achievable values and was modeled via Schulz-Zimm distribution. In total, 10 systems were considered, thus covering the phase behavior of monodisperse, partially polydisperse and fully polydisperse systems. Using such generic representation of real polymers, we observed a rippled structure and aggregates in monodisperse systems. In addition, polydisperse brushes formed a stable perforated layer not observed previously in monodisperse studies, and influenced the stability of the remaining phases. Although the perforated layer was experimentally observed under good solvent conditions and in the melt state, further confirmation of its presence in systems under good solvent conditions required mapping real polymers onto mesoscale models that reflected, for example, different polymer rigidity, and excluded volume effects or direct influence of the surface, just to mention a few parameters. Finally, in this work, we show that mesoscale modeling successfully describes polydisperse models, which opens the way for rapid exploring of complex systems such as polydisperse Y-shaped brushes in selective or bad solvents or under non-equilibrium conditions.

The thermal behavior of unilamellar vesicles has been revisited with differential scanning calorimetry to address the issue of whether it is essential to include interactions between neighboring bilayers in theories and simulations of the ripple phase. The issue focuses on the lower, aka pretransition, and the ripple phase that clearly exists between the lower and main transitions in multilamellar vesicles (MLV). We find anomalous thermal behavior in unilamellar vesicles (ULV) beginning at the same temperature as the lower transition in MLVs, but this feature is considerably broadened and somewhat weaker compared to the lower transition in MLVs. We ascribe this to the difficulty of packing a regular ripple pattern on small spheres. In agreement with a few reports of a ripple phase in direct images of single bilayers, we conclude that interactions between neighboring bilayers are not essential for the ripple phase in lipid bilayers.

Temperature-dependent transmission FT-IR spectroscopy and DSC measurements were conducted on lipid multibilayers constituted from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. Lipid multibilayers made from 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, which do not form a ripple phase, were examined as a reference. Spectra were analyzed using multivariate curve resolution technique with alternating least squares and evolving factor analysis (MCR-ALS with EFA) and lipid phase transition temperatures were determined. Polar parts of lipid molecules exert greater response on a ripple phase formation than non-polar ones. However, vibrational signatures of hydrocarbon chains with intramolecular origins display certain qualitative differences that pave the way for future work oriented on uncoupling the events that drive ripple phase formation.

Membrane scaffold proteins (MSP) nanodiscs have been extensively used in structural study of membrane proteins. In cryo-EM, an incorporation of target proteins into nanodiscs is conducted under a rapid change from cryogenic to ambient temperatures. We present a coarse-grained molecular dynamics (CGMD) study for investigating an effect of temperature on the structural organization of DPPC-nanodisc and POPC-nanodisc. A non-monotonic response of physical quantities (i.e. the lipid order parameter, nanodisc flatness, structural change, solvation property, radius of gyration) with increase in temperature (T = 200-350 K) is found to be associated with the gel-ripple-liquid crystalline phase change within nanodiscs. The reorganization of lipids upon temperature variation induced conformational changes of MSP to minimize hydrophobic exposure of the lipid membrane to an aqueous environment. Structural response to temperature is different to a certain extent between the saturated DPPC and unsaturated POPC.

The membrane-active protein Nogo-66 is found to induce interdigitation in dimyristoylphosphocholine membranes. Extensive molecular dynamics simulations have been employed to probe the interactions of Nogo-66 with these model membranes. This phase change happens when the temperature is close to the main transition temperature of the membrane (Tm) and only in the presence of the protein. No similar interdigitation of the membrane lipids was observed temperatures well above Tm in the presence of the protein. In addition, in protein-free simulations, no interdigitation of the membrane lipids was found both at temperatures near or well above Tm indicating that the observed effect is caused by the interactions of Nogo-66 with the membrane. Analysis of the simulations suggest protein-membrane interactions, even if transient, alter the lifetimes of lipid head defects and can potentially alter the effective Tm and cause interdigitation. This study emphasize the importance of membrane-active proteins and their interactions with membranes leading to phase transitions which would affect other membrane-related processes such as domain formation.

Despite the biological significance of sphingomyelins (SMs), there is far less structural information available for SMs compared to glycerophospholipids. Considerable confusion exists in the literature regarding even the phase behavior of SM bilayers. This work studies both palmitoyl (PSM) and egg sphingomyelin (ESM) in the temperature regime from 3 °C to 55 °C using X-ray diffraction and X-ray diffuse scattering on hydrated, oriented thick bilayer stacks. We observe clear evidence for a ripple phase for ESM in a large temperature range from 3 °C to the main phase transition temperature (TM) of ∼38 °C. This unusual stability of the ripple phase was not observed for PSM, which was in a gel phase at 3 °C, with a gel-to-ripple transition at ∼24 °C and a ripple-to-fluid transition at ∼41 °C. We also report structural results for all phases. In the gel phase at 3 °C, PSM has chains tilted by ∼30° with an area/lipid ∼45 Å2 as determined by wide angle X-ray scattering. The ripple phases for both PSM and ESM have temperature dependent ripple wavelengths that are ∼145 Å near 30 °C. In the fluid phase, our electron density profiles combined with volume measurements allow calculation of area/lipid to be ∼64 Å2 for both PSM and ESM, which is larger than that from most of the previous molecular dynamics simulations and experimental studies. Our study demonstrates that oriented lipid films are particularly well-suited to characterize ripple phases since the scattering pattern is much better resolved than in unoriented samples.

With nanometer lateral and Angstrom vertical resolution, atomic force microscopy (AFM) has contributed unique data improving the understanding of lipid bilayers. Lipid bilayers are found in several different temperature-dependent states, termed phases; the main phases are solid and fluid phases. The transition temperature between solid and fluid phases is lipid composition specific. Under certain conditions some lipid bilayers adopt a so-called ripple phase, a structure where solid and fluid phase domains alternate with constant periodicity. Because of its narrow regime of existence and heterogeneity ripple phase and its transition dynamics remain poorly understood. Here, a temperature control device to high-speed atomic force microscopy (HS-AFM) to observe dynamics of phase transition from ripple phase to fluid phase reversibly in real time is developed and integrated. Based on HS-AFM imaging, the phase transition processes from ripple phase to fluid phase and from ripple phase to metastable ripple phase to fluid phase could be reversibly, phenomenologically, and quantitatively studied. The results here show phase transition hysteresis in fast cooling and heating processes, while both melting and condensation occur at 24.15 °C in quasi-steady state situation. A second metastable ripple phase with larger periodicity is formed at the ripple phase to fluid phase transition when the buffer contains Ca2+ . The presented temperature-controlled HS-AFM is a new unique experimental system to observe dynamics of temperature-sensitive processes at the nanoscopic level.

Organic thin film deposition presents a multiplicity of challenges. Most notably, layer thickness control, homogeneity and subsequent characterization have been not cleared yet. Phospholipid bilayers are frequently used to model cell membranes. Bilayers can be disrupted by changes in mechanical stress, pH and temperature. The strategy presented in this article is based on thermal study of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) through analysis of slight changes in material thickness. The sample was prepared by depositing X- or Y-type DPPC bilayers using Langmuir-Blodgett technique over silicon wafer. Thus, molecular inclination degree, mobility and stability of phases and their respective phase transitions were observed and analyzed through ellipsometric techniques during heating cycles and corroborated by Grazing Incidence X-ray Diffraction and Atomic Force Microcopy measurements. DPPC functional group vibrations were detected by Raman spectra analysis. Scanning Electron Microscope with Field Emission gun (FE-SEM) and conventional SEM micrographs were also used to characterize sample morphology, demonstrating that homogenous bilayer formations coexist with some vesicles or micelles at surface level. Contact angle measurements corroborate DPPC surface wettability, which is mainly related to surface treatment methods of silicon wafer used to create either hydrophilic or hydrophobic nature regarding the substrate surface. Also, shifting and intensity changes of certain functional groups into Raman spectra confirm water presence between DPPC layers. Signal analysis detects certain interdigitation between aliphatic chains. These studies correspond to the base of future biosensors based on proteins or antimicrobial peptides stabilized into phospholipid bilayers over thin hydrogel films as moist scaffold.

A continuum theory for lipid membranes is developed that accounts for mechanical interactions between lipid tilt and membrane shape. For planar membranes, a linear version of the theory is used to predict tilt variations similar to those observed in experiments and molecular dynamics simulations.