Structure and dynamics of primary hydration shell of phosphatidylcholine bilayers at subzero temperatures

A mutation to a fish ice-binding protein synthesized in transgenic Caenorhabditis elegans modulate its cold tolerance

A cryoprotectant known as ice-binding protein (IBP) is thought to facilitate the cold survival of plants, insects, and fungi. Here, we prepared a genetically modified Caenorhabditis elegans strain to synthesize fish-derived IBPs in its body wall muscles and examined whether the antifreeze activity modification of this IBP by point mutation affects the cold tolerance of this worm. We chose a 65-residue IBP identified from notched-fin eelpout, for which the replacement of the 20th alanine residue (A20) modifies its antifreeze activity. These mutant proteins are denoted A20L, A20G, A20T, A20V, and A20I along with the wild-type (WT) protein. We evaluated the survival rate (%) of the transgenic C. elegans that synthesized each IBP mutant following 24 h of preservation at -5, +2, and +5 °C. Significantly, a dramatic improvement in the survival rate was detected for the worms synthesizing the activity-enhanced mutants (A20T and A20I), especially at +2 °C. In contrast, the rate was not improved by the expression of the defective mutants (A20L, A20G, WT and A20V). The survival rate (%) probably correlates with the antifreeze activity of the IBP. These data suggest that IBP protects the cell membrane by employing its ice-binding mechanism, which ultimately improves the cold tolerance of an IBP-containing animal.

Interfacial dynamics in inverted-headgroup lipid membranes

Inverted-headgroup (choline-phosphate) lipids are synthetic lipids that are not found in nature and are used as model systems to understand the role of headgroup dipole orientation. Recently, studies revealed that the net orientation of interfacial water strongly depends on the headgroup electrostatics, i.e., the charges and dipole generated by the phosphate and the choline groups. In order to characterize interfacial H-bond dynamics, we measured two-dimensional infrared spectra of the ester carbonyl band and performed molecular dynamics simulations in fully hydrated 1,2-dioleoyl-sn-glycero-3-phosphocholine and 2-((2,3-bis(oleoyloxy)propyl)-dimethyl-ammonio)ethyl ethyl phosphate (DOCPe) lipid bilayers. The experiments and simulations suggest that the reverse dipole generated by the inverted-headgroup in DOCPe does not affect the carbonyl H-bond populations or the interfacial water H-bond dynamics. However, while phosphate-associated waters in both lipids appear to show a similar H-bond structure, carbonyl-associated waters are characterized by a slightly disrupted H-bond structure in the DOCPe bilayer, especially within the second hydration shell. Our findings show that changes in net water orientation perturb the water H-bonds at the linker region between the headgroup and the lipid tail, although this perturbation is weak.

Subzero Nonfreezing Hypothermia with Insect Antifreeze Protein Dramatically Improves Survival Rate of Mammalian Cells

Cells for therapeutic use are often preserved at +4 °C, and the storage period is generally limited to 2–3 days. Here, we report that the survival rate (%) of mammalian cells is improved to 10–20 days when they are preserved with a subzero supercooled solution containing the antifreeze protein (AFP), for which an ability to stabilize both supercooled water and cell membrane integrity has been postulated. We chose adherent rat insulinoma (RIN-5F) cells as the preservation target, which were immersed into −5 °C-, −2 °C-, or +4 °C-chilled “unfrozen” solution of Euro-Collins or University of Washington (UW) containing the AFP sample obtained from insect or fish. Our results show that the survival rate of the cells preserved with the solution containing insect AFP was always higher than that of the fish AFP solution. A combination of the −5 °C-supercooling and insect AFP gave the best preservation result, namely, UW solution containing insect AFP kept 53% of the cells alive, even after 20 days of preservation at −5 °C. The insect AFP locates highly organized ice-like waters on its molecular surface. Such waters may bind to semiclathrate waters constructing both embryonic ice crystals and a membrane–water interface in the supercooled solution, thereby protecting the cells from damage due to chilling.

Influence of Substrate Hydrophilicity on Structural Properties of Supported Lipid Systems on Graphene, Graphene Oxides, and Silica

Pristine graphene, a range of graphene oxides, and silica substrates were used to investigate the effect of surface hydrophilicity on supported lipid bilayers by means of all-atom molecular dynamics simulations. Supported 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayers were found in close-contact conformations with hydrophilic substrates with as low as 5% oxidation level, while self-assembled monolayers occur on pure hydrophobic graphene only. Lipids and water at the surface undergo large redistribution to maintain the stability of the supported bilayers. Deposition of bicelles on increasingly hydrophilic substrates shows the continuous process of reshaping of the supported system and makes intermediate stages between self-assembled monolayers and supported bilayers. The bilayer thickness changes with hydrophilicity in a complex manner, while the number of water molecules per lipid in the hydration layer increases together with hydrophilicity.

Lipid carbonyl groups terminate the hydrogen bond network of membrane-bound water

We present a combined experimental sum-frequency generation (SFG) spectroscopy and ab initio molecular dynamics simulations study to clarify the structure and orientation of water at zwitterionic phosphatidylcholine (PC) lipid and amine N-oxide (AO) surfactant monolayers. Simulated O-H stretch SFG spectra of water show good agreement with the experimental data. The SFG response at the PC interface exhibits positive peaks, whereas both negative and positive bands are present for the similar zwitterionic AO interface. The positive peaks at the water/PC interface are attributed to water interacting with the lipid carbonyl groups, which act as efficient hydrogen bond acceptors. This allows the water hydrogen bond network to reach, with its (up-oriented) O-H groups, into the headgroup of the lipid, a mechanism not available for water underneath the AO surfactant. This highlights the role of the lipid carbonyl group in the interfacial water structure at the membrane interface, namely, stabilizing the water hydrogen bond network.

Interactions of Graphene Oxide with Model Cell Membranes: Probing Nanoparticle Attachment and Lipid Bilayer Disruption

With the rapid growth in the application of graphene oxide (GO) in diverse fields, the toxicity of GO toward bacterial and mammalian cells has recently attracted extensive research attention. While several mechanisms have been proposed for the cytotoxicity of GO, the attachment of GO to cell membranes is expected to be the key initial process that precedes these mechanisms. In this study, we investigate the propensity for GO to attach to and disrupt model cell membranes using supported lipid bilayers (SLBs) and supported vesicular layers (SVLs) that are composed of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The deposition kinetics of GO on SLBs were determined using quartz crystal microbalance with dissipation monitoring and were observed to increase with increasing electrolyte (NaCl and CaCl2) concentrations, indicating that GO attachment to SLBs was controlled by electrostatic interactions. The GO deposition kinetics measured at elevated electrolyte concentrations were lower than mass-transfer-limited kinetics, likely due to the presence of hydration forces between GO and SLBs. Upon the attachment of GO to supported vesicles that were encapsulated with a fluorescent dye, dye leakage was detected, thus indicating that the lipid vesicles were disrupted. When the exposure of the SVL to the GO suspension was terminated, the leakage of dye decreased significantly, demonstrating that the pores on the lipid bilayers have a self-healing ability.

α-Tocopherol's Location in Membranes Is Not Affected by Their Composition

To this day, α-tocopherol's (aToc) role in humans is not well known. In previous studies, we have tried to connect aToc's biological function with its location in a lipid bilayer. In the present study, we have determined, by means of small-angle neutron diffraction, that not only is aToc's hydroxyl group located high in the membrane but its tail also resides far from the center of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers. In addition, we located aToc's hydroxyl group above the lipid backbone in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), and sphingomyelin bilayers, suggesting that aToc's location near the lipid-water interface may be a universal property of vitamin E. In light of these data, how aToc efficiently terminates lipid hydroperoxy radicals at the membrane center remains an open question.

Estimation of Differently Bound Water Molecules for the Gel Phase of Dimyristoylphosphatidylethanolamine-Water System as Studied by DSC and (2)H-NMR Spectroscopy

A measurement of (2)H spin-lattice relaxation time, T(1), forD(2)O was performed with a high resolution liquid NMR apparatus fortwo samples of dimyristoylphosphatidylethanolamine (DMPE)-D(2)Osystem in a full hydration at varying temperatures of -20, -10, and 5 (°)C, and both components and compositions of differently boundfreezable water molecules were estimated from a best-fitted curve toexperimental inversion recovery data. A choice of the best-fitted curve wasbased on a distribution of weighted residuals for the experimental data. Asingle component was found for a temperature of -20 (°)C. At 5 (°)C, where all the freezable water exists in the liquid state, threecomponents were observed to be characterized by T(1) values ofapproximately 20, 100, and 200 ms, respectively. By comparingcompositions of these individual components with those obtained in ourprevious DSC study, it was revealed that the first and secondarycomponents are members of freezable interlamellar water and the last oneis comparable to bulk water.

Vibrational spectroscopy of water in hydrated lipid multi-bilayers. I. Infrared spectra and ultrafast pump-probe observables

The vibrational spectroscopy of hydration water in dilauroylphosphatidylcholine lipid multi-bilayers is investigated using molecular dynamics simulations and a mixed quantum/classical model for the OD stretch spectroscopy of dilute HDO in H(2)O. FTIR absorption spectra, and isotropic and anisotropic pump-probe decay curves have been measured experimentally as a function of the hydration level of the lipid multi-bilayer, and our goal is to make connection with these experiments. To this end, we use third-order response functions, which allow us to include non-Gaussian frequency fluctuations, non-Condon effects, molecular rotations, and a fluctuating vibrational lifetime, all of which we believe are important for this system. We calculate the response functions using existing transition frequency and dipole maps. From the experiments it appears that there are two distinct vibrational lifetimes corresponding to HDO molecules in different molecular environments. In order to obtain these lifetimes, we consider a simple two-population model for hydration water hydrogen bonds. Assuming a different lifetime for each population, we then calculate the isotropic pump-probe decay, fitting to experiment to obtain the two lifetimes for each hydration level. With these lifetimes in hand, we then calculate FTIR spectra and pump-probe anisotropy decay as a function of hydration. This approach, therefore, permits a consistent calculation of all observables within a unified computational scheme. Our theoretical results are all in qualitative agreement with experiment. The vibrational lifetime of lipid-associated OD groups is found to be systematically shorter than that of the water-associated population, and the lifetimes of each population increase with decreasing hydration, in agreement with previous analysis. Our theoretical FTIR absorption spectra successfully reproduce the experimentally observed red-shift with decreasing lipid hydration, and we confirm a previous interpretation that this shift results from the hydrogen bonding of water to the lipid phosphate group. From the pump-probe anisotropy decay, we confirm that the reorientational motions of water molecules slow significantly as hydration decreases, with water bound in the lipid carbonyl region undergoing the slowest rotations.

Dielectric relaxation dynamics of water in model membranes probed by terahertz spectroscopy

We study hydrated model membranes, consisting of stacked bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine lipids, using terahertz time-domain spectroscopy and infrared spectroscopy. Terahertz spectroscopy enables the investigation of water dynamics, owing to its sensitivity to dielectric relaxation processes associated with water reorientation. By controlling the number of water molecules per lipid molecule in the system, we elucidate how the interplay between the model membrane and water molecules results in different water dynamics. For decreasing hydration levels, we observe the appearance of new types of water dynamics: the collective bulklike dynamics become less pronounced, whereas an increased amount of both very slowly reorienting (i.e., irrotational) and very rapidly reorienting (i.e., fast) water molecules appear. Temperature-dependent measurements reveal the interconversion between the three distinct types of water present in the system.

In vivo assessment of cold adaptation in insect larvae by magnetic resonance imaging and magnetic resonance spectroscopy

BACKGROUND

Temperatures below the freezing point of water and the ensuing ice crystal formation pose serious challenges to cell structure and function. Consequently, species living in seasonally cold environments have evolved a multitude of strategies to reorganize their cellular architecture and metabolism, and the underlying mechanisms are crucial to our understanding of life. In multicellular organisms, and poikilotherm animals in particular, our knowledge about these processes is almost exclusively due to invasive studies, thereby limiting the range of conclusions that can be drawn about intact living systems.

METHODOLOGY

Given that non-destructive techniques like (1)H Magnetic Resonance (MR) imaging and spectroscopy have proven useful for in vivo investigations of a wide range of biological systems, we aimed at evaluating their potential to observe cold adaptations in living insect larvae. Specifically, we chose two cold-hardy insect species that frequently serve as cryobiological model systems--the freeze-avoiding gall moth Epiblema scudderiana and the freeze-tolerant gall fly Eurosta solidaginis.

RESULTS

In vivo MR images were acquired from autumn-collected larvae at temperatures between 0 degrees C and about -70 degrees C and at spatial resolutions down to 27 microm. These images revealed three-dimensional (3D) larval anatomy at a level of detail currently not in reach of other in vivo techniques. Furthermore, they allowed visualization of the 3D distribution of the remaining liquid water and of the endogenous cryoprotectants at subzero temperatures, and temperature-weighted images of these distributions could be derived. Finally, individual fat body cells and their nuclei could be identified in intact frozen Eurosta larvae.

CONCLUSIONS

These findings suggest that high resolution MR techniques provide for interesting methodological options in comparative cryobiological investigations, especially in vivo.

Water at the surfaces of aligned phospholipid multibilayer model membranes probed with ultrafast vibrational spectroscopy

The dynamics of water at the surface of artificial membranes composed of aligned multibilayers of the phospholipid dilauroyl phosphatidylcholine (DLPC) are probed with ultrafast polarization selective vibrational pump-probe spectroscopy. The experiments are performed at various hydration levels, x = 2 - 16 water molecules per lipid at 37 degrees C. The water molecules are approximately 1 nm above or below the membrane surface. The experiments are conducted on the OD stretching mode of dilute HOD in H 2O to eliminate vibrational excitation transfer. The FT-IR absorption spectra of the OD stretch in the DLPC bilayer system at low hydration levels shows a red-shift in frequency relative to bulk water, which is in contrast to the blue-shift often observed in systems such as water nanopools in reverse micelles. The spectra for x = 4 - 16 can be reproduced by a superposition of the spectra for x = 2 and bulk water. IR Pump-probe measurements reveal that the vibrational population decays (lifetimes) become longer as the hydration level is decreased. The population decays are fit well by biexponential functions. The population decays, measured as a function of the OD stretch frequency, suggest the existence of two major types of water molecules in the interfacial region of the lipid bilayers. One component may be a clathrate-like water cluster near the hydrophobic choline group and the other may be related to the hydration water molecules mainly associated with the phosphate group. As the hydration level increases, the vibrational lifetimes of these two components decrease, suggesting a continuous evolution of the hydration structures in the two components associated with the swelling of the bilayers. The agreement of the magnitudes of the two components obtained from IR spectra with those from vibrational lifetime measurements further supports the two-component model. The vibrational population decay fitting also gives an estimation of the number of phosphate-associated water molecules and choline-associated water molecules, which range from 1 to 4 and 1 to 12, respectively, as x increases from 2 to 16. Time-dependent anisotropy measurements yield the rate of orientational relaxation as a function of x. The anisotropy decay is biexponential. The fast component is almost independent of x, and is interpreted as small orientational fluctuations that occur without hydrogen-bond rearrangement. The slower component becomes very long as the hydration level decreases. This component is a measure of the rate of complete orientational randomization, which requires H-bond rearrangement and is discussed in terms of a jump reorientation model.

Effect of adsorbed layer surface roughness on the QCM-D response: focus on trapped water

The effect of surface roughness on the quartz crystal microbalance with dissipation monitoring (QCM-D) response was investigated with emphasis on determining the amount of trapped water. Surfaces with different nanoroughnesses were prepared on silica by self-assembly of cationic surfactants with different packing parameters. We used surfactants with quaternary ammonium bromide headgroups: the double-chained didodecyltrimethylammonium bromide (C12)2DAB (DDAB), the single-chained hexadecyltrimethylammonium bromide C16TAB (CTAB), and dodecyltrimethyl-ammonium bromide C12TAB (DTAB). The amount of trapped water was obtained from the difference between the mass sensed by QCM-D and the adsorbed amount detected by optical reflectometry. The amount of water, which is sensed by QCM-D, was found to increase with the nanoroughness of the adsorbed layer. The water sensed by QCM-D cannot be assigned primarily to hydration water, because it differs substantially for adsorbed surfactant layers with similar headgroups but with different nanoscale topographies.

Components and fractions for differently bound water molecules of dipalmitoylphosphatidylcholine–water system as studied by DSC and 2H-NMR spectroscopy

Differently bound water molecules of dipalmitoylphosphatidylcholine (DPPC)-H2O system were investigated with differential scanning calorimetry (DSC). According to a method previously reported by us, the ice-melting DSC curves of the DPPC-H2O samples of varying water contents were deconvoluted into multiple components, and the ice-melting enthalpies for the individual deconvoluted components were used to estimate average molar ice-melting enthalpies for freezable interlamellar and bulk waters, respectively. With these average molar ice-melting enthalpies, the numbers of differently bound water molecules of the DPPC-H2O system were calculated at varying water contents and were used to construct a water distribution diagram of this system. Furthermore, to evaluate the reliability of the present DSC deconvolution method, 2H-NMR T1 measurements of DPPC-2H2O system were carried out at 5 degrees C of the gel phase temperature, and components and fractions for differently bound water (2H2O) molecules were estimated from the analysis of nonexponential magnetization recovery curves.

The interaction of water with the phospholipid head group and its relationship to the lipid electrical conductivity

We have studied the interaction of water with the lipid head group by gravimetrically measuring the lipid water adsorption and the lateral dc electrical conductivity increase resulting from this hydration. We have done this for dimyristoyl phosphatidylcholine (DMPC) having protonated or deuterated hydrocarbon chains. These studies were also done for two cationic lipids having rather different polar head groups. All three lipids behave as strong water adsorbers and all three display a steep, logarithmic increase in the conductivity as the first 1-3 waters per lipid are adsorbed. This increase is usually 5-6 orders of magnitude. After the initial 1-3 waters are adsorbed, the conductivity increases much more gradually, upon additional water adsorption. This electrical behavior is also found for weak water adsorbers and appears to be independent of the head group composition. The conductivity behavior suggests two types of water interacting with the head group. Our studies also indicate that a choline-like component is responsible for the strong water binding nature of the lipids, although, both phosphate and choline make significant contributions to the total amount of adsorbed water. The conductivity behavior, however, does not depend on the presence of both these head group components.

New insights into water–phospholipid model membrane interactions

Modulating the relative humidity (RH) of the ambient gas phase of a phospholipid/water sample for modifying the activity of phospholipid-sorbed water [humidity-controlled osmotic stress methods, J. Chem. Phys. 92 (1990) 4519 and J. Phys. Chem. 96 (1992) 446] has opened a new field of research of paramount importance. New types of phase transitions, occurring at specific values of this activity, have been then disclosed. Hence, it is become recognized that this activity, like the temperature T, is an intensive parameter of the thermodynamical state of these samples. This state can be therefore changed (phase transition) either, by modulating T at a given water activity (a given hydration level), or, by modulating the water activity, at a given T. The underlying mechanisms of these two types of transition differ, especially when they appear as disorderings of fatty chains. In lyotropic transitions, this disordering follows from two thermodynamical laws. First, acting on the activity (the chemical potential) of water external to a phospholipid/water sample, a transbilayer gradient of water chemical potential is created, leading to a transbilayer flux of water (Fick's law). Second, water molecules present within the hydrocarbon region of this phospholipid bilayer interact with phospholipid molecules through their chemical potential (Gibbs-Duhem relation): the conformational state of fatty chains (the thermodynamical state of the phospholipid molecules) changes. This process is slow, as revealed by osmotic stress time-resolved experiments. In thermal chain-melting transitions, the first rapid step is the disordering of fatty chains of a fraction of phospholipid molecules. It occurs a few degrees before the main transition temperature, T(m), during the pretransition and the sub-main transition. The second step, less rapid, is the redistribution of water molecules between the different parts of the sample, as revealed by T-jump time-resolved experiments. Finally, in lyotropic and thermal transitions, hydration and conformation are linked but the order of anteriority of their change, in each case, is probably not the same. In this review, first, the interactions of phospholipid submolecular fragments and water molecules, in the interfacial and hydrocarbon regions of phospholipid/water multibilayer stacks, will be described. Second, the coupling of the conformational states of phospholipid and water molecules, during thermal and lyotropic transitions, will be demonstrated through examples.

A (2)H NMR study of macroscopically aligned bilayer membranes containing interfacial hydroxyl residues

The polar interface of membranes containing phosphatidylglycerol or cholesterol was studied by (2)H nuclear magnetic resonance (NMR) as a function of membrane hydration. The membranes were macroscopically aligned and hydrated with deuterium oxide. Water uptake and membrane annealing was achieved under NMR control, using a novel hydration technique. Well-resolved (2)H quadrupolar doublets were obtained from individual hydroxyl residues and from the interlamellar water. The response of the phosphatidylglycerol headgroup and of the cholesterol molecule to the spontaneous evaporation of interlamellar water could be thus monitored continuously. It is shown that the phosphatidylglycerol headgroup undergoes changes of conformation and average orientation with respect to the membrane surface and that the off-axis motion of the cholesterol molecule decreases. The deuteron exchange between hydroxyl residues and surface-associated D(2)O was determined by an inversion transfer technique. The exchange rates of the hydroxyl residues in the phosphatidylglycerol headgroup were different and depended strongly on the total hydration of the membrane. Significantly lower and almost hydration-independent rates were obtained for cholesterol. These results will be discussed with reference to earlier reports on the headgroup dynamics of phosphatidylglycerol and on the interaction of cholesterol with the membrane-water interface.

Hydration of phospholipid bilayers in the presence and absence of cholesterol

Differential scanning calorimetry (DSC) was used for determining the number of unfreezable water molecules per molecule of phosphatidylserine from bovine spinal cord (PS) or dimyristoyl phosphatidylserine (DMPS) and dimyristoyl phosphatidylcholine (DMPC), alone or in mixtures with cholesterol. It was assumed that the unfreezable water molecules are tightly bound to the phospholipid. It was found that when the phospholipids are in the gel state and in the absence of cholesterol, PS binds 2.5 water molecules, DMPS 3.8 water molecules and DMPC 3.5 water molecules. In the presence of cholesterol the number of water molecules bound increases in the region where phase separation of cholesterol takes place [D. Bach, Chem. Phys. Lipids 35 (1984) 385-392; E.J. Wachtel, N. Borochov, D. Bach, Biochim. Biophys. Acta 1066 (1991) 63-69; D. Bach, N. Borochov, E. Wacktel, Chem. Phys. Lipids, submitted].

Membrane packing geometry of diphytanoylphosphatidylcholine is highly sensitive to hydration: phospholipid polymorphism induced by molecular rearrangement in the headgroup region

Diphytanoylphosphatidylcholine (DPhPC) has often been used in the study of protein-lipid interaction and membrane channel activity, because of the general belief that it has high bilayer stability, low ion leakage, and fatty acyl packing comparable to that of phospholipid bilayers in the liquid-crystalline state. In this solid-state 31P and 2H NMR study, we find that the membrane packing geometry and headgroup orientation of DPhPC are highly sensitive to the temperature studied and its water content. The phosphocholine headgroup of DPhPC starts to change its orientation at a water content as high as approximately 16 water molecules per lipid, as evidenced by hydration-dependent 2H NMR study at room temperature. In addition, a temperature-induced structural transition in the headgroup orientation is detected in the temperature range of approximately 20-60 degrees C for lipids with approximately 8-11 water molecules per DPhPC. Dehydration of the lipid by one more water molecule leads to a nonlamellar, presumably cubic, phase formation. The lipid packing becomes a hexagonal phase at approximately 6 water molecules per lipid. A phase diagram of DPhPC in the temperature range of -40 degrees C to 80 degrees C is thus constructed on the basis of NMR results. The newly observed hydration-dependent DPhPC lipid polymorphism emphasizes the importance of molecular packing in the headgroup region in modulating membrane structure and protein-induced pore formation of the DPhPC bilayer.