Preparation and characterization of monolayer and multilayer Langmuir-Blodgett films of a series of 6-O-alkylcelluloses

Monolayer and multilayer Langmuir-Blodgett (LB) films of 6-O-alkylcelluloses with various chain lengths were prepared and studied. The surface pressure (pi)-area (A) isotherms of 6-O-alkylcelluloses exhibited characteristic behaviors depending on the length of the alkyl chain and temperature. 6-O-Stearylcellulose on the subphase formed a homogeneous monolayer at 10 mN m(-1). By transfer ratio, FT-IR, and contact angle measurements, it was proved that the monolayer of 6-O-stearylcellulose on the water surface was transferred successfully onto a substrate by a vertical dipping method to form a Z-type LB film. The transmission and reflection absorption IR spectrum indicated that the hydrocarbon chains had all-trans rotamers and were oriented nearly perpendicular to the surface in the film. AFM section analysis revealed that the thickness per layer was calculated to be 2.35 nm. These results suggested that the hydrocarbon chains were inclined at an angle of about 25.3 degrees to have high packing density in the alkyl region.     

Lipid bilayer ultrastructure. Electron density profiles and chain tilt angles as determined by X-ray diffraction.

High resolution (6A) electron density profiles have been computed on an absolute electron density scale for bilayers composed of both saturated fatty acids and fatty acids associated with the alkaline earth series of divalent cations. Lowangle X-ray diffraction data have been interpreted by an isomorphous replacement technique. The position on the X-ray film of discrete wide-angle reflections has provided direct information on the hydrocarbon chain packing and chain tilt in these bilayers. These results have been correlated to an electron microscopy study of the same bilayers (Waldbilling, R. C., Robertson, J.D. and McIntosh, T. J. (1976) Biochim. Biophys. Acta 448, 1-14) and also to X-ray diffraction studies of fatty acid crystals. A method for forming and structurally analyzing bilayers of well defined chemical asymmetry is also described.     

Lipid bilayer ultrastructure: Electron density profiles and chain tilt angles as determined by X-ray diffraction

High resolution (6 Å) electron density profiles have been computed on an absolute electron density scale for bilayers composed of both saturated fatty acids and fatty acids associated with the alkaline earth series of divalent cations. Low-angle X-ray diffraction data have been interpreted by an isomorphous replacement technique. The position on the X-ray film of discrete wide-angle reflections has provided direct information on the hydrocarbon chain packing and chain tilt in these bilayers. These results have been correlated to an electron microscopy study of the same bilayers (Waldbilling, R. C., Robertson J. D. and McIntosh, T.J. (1976) Biochim. Biophys. Acta 448, 1–14) and also to X-ray diffraction studies of fatty acid crystals. A method for forming and structurally analyzing bilayers of well defined chemical asymmetry is also described.     

Measurement of chain tilt angle in fully hydrated bilayers of gel phase lecithins.

The tilt angle theta tilt of the hydrocarbon chains has been determined for fully hydrated gel phase of a series of saturated lecithins. Oriented samples were prepared on glass substrates and hydrated with supersaturated water vapor. Evidence for full hydration was the same intensity pattern of the low angle lamellar peaks and the same lamellar repeat D as unoriented multilamellar vesicles. Tilting the sample permitted observation of all the wide angle arcs necessary to verify the theoretical diffraction pattern corresponding to tilting of the chains towards nearest neighbors. The length of the scattering unit corresponds to two hydrocarbon chains, requiring each bilayer to scatter coherently rather than each monolayer. For DPPC, theta tilt was determined to be 32.0 +/- 0.5 degrees at 19 degrees C, slightly larger than previous direct determinations and considerably smaller than the value required by recent gravimetric measurements. This new value allows more accurate determinations of a variety of structural parameters, such as area per lipid molecule, A = 47.2 +/- 0.5 A2, and number of water molecules of hydration, nw = 11.8 +/- 0.7. As the chain length n of the lipids was increased from 16 to 20 carbons, the parameters A and nw remained constant, suggesting that the headgroup packing is at its excluded volume limit for this range. However, theta tilt increased by 3 degrees and the chain area Ac decreased by 0.5 A2. This behavior is explained in terms of a competition between a bulk free energy term and a finite or end effect term.     

Accurate determination of local defocus and specimen tilt in electron microscopy

Accurate knowledge of defocus and tilt parameters is essential for the determination of three-dimensional protein structures at high resolution using electron microscopy. We present two computer programs, CTFFIND3 and CTFTILT, which determine defocus parameters from images of untilted specimens, as well as defocus and tilt parameters from images of tilted specimens, respectively. Both programs use a simple algorithm that fits the amplitude modulations visible in a power spectrum with a calculated contrast transfer function (CTF). The background present in the power spectrum is calculated using a low-pass filter. The background is then subtracted from the original power spectrum, allowing the fitting of only the oscillatory component of the CTF. CTFTILT determines specimen tilt parameters by measuring the defocus at a series of locations on the image while constraining them to a single plane. We tested the algorithm on images of two-dimensional crystals by comparing the results with those obtained using crystallographic methods. The images also contained contrast from carbon support film that added to the visibility of the CTF oscillations. The tests suggest that the fitting procedure is able to determine the image defocus with an error of about 10nm, whereas tilt axis and tilt angle are determined with an error of about 2 degrees and 1 degrees, respectively. Further tests were performed on images of single protein particles embedded in ice that were recorded from untilted or slightly tilted specimens. The visibility of the CTF oscillations from these images was reduced due to the lack of a carbon support film. Nevertheless, the test results suggest that the fitting procedure is able to determine image defocus and tilt angle with errors of about 100 nm and 6 degrees, respectively.     

Correlation of topographic surface and volume data from three-dimensional electron microscopy

Three-dimensional(3D) reconstructions from tilt series in an electron microscope show in general an anisotropic resolution due to an instrumentally limited tilt angle. As a consequence, the information in the z direction is blurred, thus making it difficult to detect the boundary of the reconstructed structures. In contrast, high-resolution topography data from microscopic surface techniques provide exactly complementary information. The combination of topographic surface and volume data leads to a better understanding of the 3D structure. The new correlation procedure presented determines both the height scaling of the topographic surface and the relative position of surface and volume data, thus allowing information to be combined. Experimental data for crystalline T4 bacteriophage polyheads were used to test the new method. Three-dimensional volume data were reconstructed from a negatively stained tilt series. Topographic data for both surfaces were obtained by surface relief reconstruction of electron micrographs of freeze-dried and unidirectionally metal-shadowed polyheads. The combined visualization of volume data with the scaled and aligned surface data shows that the correlation technique yields meaningful results. The reported correlation method may be applied to surface data obtained by any microscopic technique yielding topographic data.     

Molecular theory of lipid-protein interaction and the Lalpha-HII transition.

We present a molecular-level theory for lipid-protein interaction and apply it to the study of lipid-mediated interactions between proteins and the protein-induced transition from the planar bilayer (Lalpha) to the inverse-hexagonal (HII) phase. The proteins are treated as rigid, membrane-spanning, hydrophobic inclusions of different size and shape, e.g., "cylinder-like," "barrel-like," or "vase-like." We assume strong hydrophobic coupling between the protein and its neighbor lipids. This means that, if necessary, the flexible lipid chains surrounding the protein will stretch, compress, and/or tilt to bridge the hydrophobic thickness mismatch between the protein and the unperturbed bilayer. The system free energy is expressed as an integral over local molecular contributions, the latter accounting for interheadgroup repulsion, hydrocarbon-water surface energy, and chain stretching-tilting effects. We show that the molecular interaction constants are intimately related to familiar elastic (continuum) characteristics of the membrane, such as the bending rigidity and spontaneous curvature, as well as to the less familiar tilt modulus. The equilibrium configuration of the membrane is determined by minimizing the free energy functional, subject to boundary conditions dictated by the size, shape, and spatial distribution of inclusions. A similar procedure is used to calculate the free energy and structure of peptide-free and peptide-rich hexagonal phases. Two degrees of freedom are involved in the variational minimization procedure: the local length and local tilt angle of the lipid chains. The inclusion of chain tilt is particularly important for studying noncylindrical (for instance, barrel-like) inclusions and analyzing the structure of the HII lipid phase; e.g., we find that chain tilt relaxation implies strong faceting of the lipid monolayers in the hexagonal phase. Consistent with experiment, we find that only short peptides (large negative mismatch) can induce the Lalpha --> HII transition. At the transition, a peptide-poor Lalpha phase coexists with a peptide-rich HII phase.     

Dramatic in situ conformational dynamics of the transmembrane protein bacteriorhodopsin.

The conformational dynamic capabilities of the in situ bacteriorhodopsin (bR) can be studied by determination of the changes of the bR net helical segmental tilt angle (the angle between the polypeptide segments and the membrane normal) induced by various perturbations of the purple membrane (PM). The analysis of the far-UV oriented circular dichroism (CD) of the PM provides one means of achieving this. Previous CD studies have indicated that the tilt angle can change from approximately 10 degrees to 39 degrees depending on the perturbants used with no changes in the secondary structure of the bR. A recent study has indicated that the bleaching-induced tilt angle can be enhanced from approximately 24 degrees to 39 degrees by cross-linkage and papain-digestion perturbations which by themselves do not alter the tilt angle. To add further credence, this study has been repeated using midinfrared (IR) linear dichroic spectral analysis. In contrast to the CD method, analysis by the IR method depends on the orientation of the amide plane of the helix assumed. Excellent consistency is achieved between the two methods only when it is assumed that the structural characteristics of the alpha-helices of the bR are equally alpha I and alpha II in nature. Furthermore, the analysis of the IR data becomes essentially independent of the three amide transitions utilized. The net tilt angle of segments completely randomized relative to the incident light must be 54.736 in view of helix symmetry. A value of 54.735 degrees +/- 0.001 degree was achieved by the IR method for the ethanol-treated PM film, establishing this kind of film as an ideal random state standard and demonstrating the accuracy potential of the IR method.     

Fatty-acid chain tilt angles and directions in dipalmitoyl phosphatidylcholine bilayers.

X-ray diffraction has been applied to determine the various tilt angles and directions (if any) which can be assumed by oriented gel phase multilayers of dipalmitoyl phosphatidylcholine (DPPC) as a function of hydration. We report for the first time that oriented DPPC multilayers with a repeat spacing (d-spacing) of 55.2A at 25 degrees C and 0% relative humidity (RH) have hydrocarbon chains tilted at an angle theta of 21.5 degrees with respect to the bilayer normal. In addition, the chains are tilted along one of the bisectors (omega = 0 degrees) of the hexagonal lattice (8 wide-angle maxima, 2 unique), a phase not previously reported in DPPC studies. At 100% RH, the chain tilt angle and d-spacing increased to approximately 29.0 degrees and 58.9A, respectively. Since at 100% RH only 4 wide-angle maxima are observed, we analyze the data on the assumption that the hydrocarbon chains may rotate independently of the hexagonal lattice (omega = 0-30 degrees), at a fixed chain tilt angle theta (Stamatoff, J.B., et al. 1979. Biophys. J. 25:253-262). The largest observed angle phi made by the wide-angle maxima with the equator is 29.5 degrees corresponding to a theta of approximately 32.6 degrees (omega avg. = 24 degrees) and the sample having a d-spacing of 64.0 A (excess water condition). Finally, theta remains relatively constant (approximately 21.5 degrees) up to a RH of approximately 45% and a d-spacing of 57.8A, after which, with increases in RH, theta increases to a maximum of 32.6 degrees.     

Molecular dynamics simulation study of a pulmonary surfactant film interacting with a carbonaceous nanoparticle

This article reports an all-atom molecular dynamics simulation to study a model pulmonary surfactant film interacting with a carbonaceous nanoparticle. The pulmonary surfactant is modeled as a dipalmitoylphosphatidylcholine monolayer with a peptide consisting of the first 25 residues from surfactant protein B. The nanoparticle model with a chemical formula C₁₈₈H₅₃ was generated using a computational code for combustion conditions. The nanoparticle has a carbon cage structure reminiscent of the buckyballs with open ends. A series of molecular-scale structural and dynamical properties of the surfactant film in the absence and presence of nanoparticle are analyzed, including radial distribution functions, mean-square displacements of lipids and nanoparticle, chain tilt angle, and the surfactant protein B peptide helix tilt angle. The results show that the nanoparticle affects the structure and packing of the lipids and peptide in the film, and it appears that the nanoparticle and peptide repel each other. The ability of the nanoparticle to translocate the surfactant film is one of the most important predictions of this study. The potential of mean force for dragging the particle through the film provides such information. The reported potential of mean force suggests that the nanoparticle can easily penetrate the monolayer but further translocation to the water phase is energetically prohibitive. The implication is that nanoparticles can interact with the lung surfactant, as supported by recent experimental data by Bakshi et al.     

Scanning near-field fluorescence resonance energy transfer microscopy

A new microscopic technique is demonstrated that combines attributes from both near-field scanning optical microscopy (NSOM) and fluorescence resonance energy transfer (FRET). The method relies on attaching the acceptor dye of a FRET pair to the end of a near-field fiber optic probe. Light exiting the NSOM probe, which is nonresonant with the acceptor dye, excites the donor dye introduced into a sample. As the tip approaches the sample containing the donor dye, energy transfer from the excited donor to the tip-bound acceptor produces a red-shifted fluorescence. By monitoring this red-shifted acceptor emission, a dramatic reduction in the sample volume probed by the uncoated NSOM tip is observed. This technique is demonstrated by imaging the fluorescence from a multilayer film created using the Langmuir-Blodgett (LB) technique. The film consists of L-alpha-dipalmitoylphosphatidylcholine (DPPC) monolayers containing the donor dye, fluorescein, separated by a spacer group of three arachidic acid layers. A DPPC monolayer containing the acceptor dye, rhodamine, was also transferred onto an NSOM tip using the LB technique. Using this modified probe, fluorescence images of the multilayer film reveal distinct differences between images collected monitoring either the donor or acceptor emission. The latter results from energy transfer from the sample to the NSOM probe. This method is shown to provide enhanced depth sensitivity in fluorescence measurements, which may be particularly informative in studies on thick specimens such as cells. The technique also provides a mechanism for obtaining high spatial resolution without the need for a metal coating around the NSOM probe and should work equally well with nonwaveguide probes such as atomic force microscopy tips. This may lead to dramatically improved spatial resolution in fluorescence imaging.     

Male and female rats express similar blood pressure responses to "push-pull" gravitational stress.

Brief exposure to -G(z) ("push") reduces eye-level blood pressure (elbp) during subsequent exposure to +G(z) ("pull"). This is called the "push-pull effect." To evaluate the influence of gender and the axis of rotation (pitch vs. roll) on the push-pull effect, 10 isoflurane-anesthetized male and 10 female Sprague-Dawley rats were restrained supine on a heated tilt board. Rats were subjected to two G profiles: a control profile consisting of rotation from 0 G(z) to 90 degrees head-up tilt (+1 G(z)) for 10 s and a push-pull profile consisting of rotation from 0 G(z) to 90 degrees head-down tilt (-1 G(z)) for 2 s immediately preceding 10 s of +1 G(z) stress. A total of 16 tilts consisting of equal numbers of control and push-pull trials and equal numbers of pitch and roll rotations were imposed by using a counterbalanced design. Gender exerted a significant effect on baseline (0 G(z)) ELBP (pressure was approximately 4 mmHg higher in females). In males and females, ELBP rose to a similar extent ( approximately 8 mmHg) during push, fell to a similar extent (approximately 18 mmHg) during control +G(z) stress, and fell to a similar extent (approximately 22 mmHg) during push-pull +G(z) stress. Altering the axis of rotation between the x-axis (roll) and the y-axis (pitch) did not influence the results. Thus males and females exhibit a push-pull effect; however, gender and axis of rotation do not appear to influence the push-pull effect in anesthetized rats subjected to tilting.     

Correlation between trochlear groove depth and patellar position during open and closed kinetic chain exercises in subjects with anterior knee pain

The purpose of this study was to correlate the trochlear shape and patellar tilt angle and lateral patellar displacement at rest and maximal voluntary isometric contraction (MVIC) exercises during open (OKC) and closed kinetic chain (CKC) in subjects with and without anterior knee pain. Subjects were all women, 20 who were clinically healthy and 19 diagnosed with anterior knee pain. All subjects were evaluated and subjected to magnetic resonance exams during OKC and CKC exercise with the knee placed at 15, 30, and 45 degrees of flexion. The parameters evaluated were sulcus angle, patellar tilt angle and patellar displacement using bisect offset. Pearson's r coefficient was used, with p < .05. Our results revealed in knee pain group during CKC and OKC at 15 degrees that the increase in the sulcus angle is associated with a tilt increase and patellar lateral displacement. Comparing sulcus angle, patellar tilt angle and bisect offset values between MVIC in OKC and CKC in the knee pain group, it was observed that patellar tilt angle increased in OKC only with the knee flexed at 30 degrees. Based on our results, we conclude that reduced trochlear depth is correlated with increased lateral patellar tilt and displacement during OKC and CKC at 15 degrees of flexion in people with anterior knee pain. By contrast, 30 degrees of knee flexion in CKC is more recommended in rehabilitation protocols because the patella was more stable than in other positions.     

Modifying dipalmitoylphosphatidylcholine monolayers by n-hexadecanol and dipalmitoylglycerol

The monolayer structure of pure dipalmitoylphosphatidylcholine (DPPC) and equimolar mixtures of DPPC/n-hexadecanol (C(16)OH) and DPPC/dipalmitoylglycerol (DPG) are studied by the film balance technique and grazing incidence X-ray diffraction measurements. At 20 degrees C, the binary systems exhibit complete miscibility. In contrast to pure DPPC monolayers, a condensing effect is observed in the presence of both non-phospholipid additives; but the phase transition behavior differs. The tilt angle of the hydrocarbon chains in the DPPC/C(16)OH mixture is significantly smaller than in pure DPPC monolayers. The tilt of the chains is even further reduced in the mixed monolayer of DPPC/DPG. A comparison of the three systems reveals distinct structural features such as phase state, chain tilt, and molecular area over a wide range of surface pressures. Therefore, these monolayers provide a highly suitable model to investigate the influence of structural parameters on biological processes occurring at the membrane surface, e.g. enzymatic reactions and adsorption events.     

Structural properties of a monobrominated analog of 1, 2-dipalmitoyl-sn-glycero-3-phosphorylcholine

Hydrated multibilayers of 1-palmitoyl-2-monobromopalmitoyl-sn-glycero-3-phosphorylcholine (BrDPPC), where the 2-chain is brominated at either the C-9 or C-10 position, have been studied by low and wide angle X-ray diffraction methods. Oriented and unoriented samples were investigated. The long spacing was observed over the temperature interval -15 degrees C to 80 degrees C. A monotonic increase from approx. 50 A to approx. 62 A (28 wt. % H2O) occurred with decreasing temperature. The BrDPPC showed no evidence of a sharp gel-to-liquid crystal phase transition. Wide angle scattering showed a diffuse peak corresponding to (4.5 A)-1. Differential scanning calorimetry measurements for hydrated liposomes (50 wt. % H2O) also showed no evidence for a phase transition (-40 less than or equal to T less than or equal to 60 degrees C). These results suggest a low temperature amorphous (glass) state for the acyl side chains of BrDPPC. Monolayer film properties of monobrominated stearic acid also reflect a chain disordering effect occurring upon midchain substitution.     

Estimation of the muscle fibre semi-length under varying joint positions on the basis of non-invasively extracted motor unit action potentials.

Changes in muscle fibre length and surface electrode position with respect to the muscle fibres affect the amplitude and frequency characteristics of surface electromyography (SEMG) in different ways. Knowledge of changes in muscle fibre length would help towards a better interpretation of the signals. The possibility of estimating the length through SEMG during voluntary contractions was checked in this study. The fibres' semi-length was estimated from the product of the conduction velocity and conduction time during which the wave of excitation propagated from the end-plate region to the ends of the fibres. Short (10 s), moderate (30% of maximum voluntary contraction) isometric contractions were performed by 10 subjects at different elbow joint angles (80-140 degrees in steps of 20 degrees ). Monopolar signals were detected non-invasively, using a two-dimensional electrode array. High spatial resolution EMG and a decomposition technique were utilised to extract single motor unit activities for triggered averaging and to estimate conduction velocity. A significant increase with joint angle was found in conduction time and estimated fibre semi-length. Changes in conduction velocity with joint angle were found to be not significant. The methodology described allows the relative changes in fibres' semi-length to be estimated from SEMG data.     

Tilt angles of transmembrane model peptides in oriented and non-oriented lipid bilayers as determined by 2H solid-state NMR

Solid-state NMR methods employing (2)H NMR and geometric analysis of labeled alanines (GALA) were used to study the structure and orientation of the transmembrane alpha-helical peptide acetyl-GWW(LA)(8)LWWA-amide (WALP23) in phosphatidylcholine (PC) bilayers of varying thickness. In all lipids the peptide was found to adopt a transmembrane alpha-helical conformation. A small tilt angle of 4.5 degrees was observed in di-18:1-PC, which has a hydrophobic bilayer thickness that approximately matches the hydrophobic length of the peptide. This tilt angle increased slightly but systematically with increasing positive mismatch to 8.2 degrees in di-C12:0-PC, the shortest lipid used. This small increase in tilt angle is insufficient to significantly change the effective hydrophobic length of the peptide and thereby to compensate for the increasing hydrophobic mismatch, suggesting that tilt of these peptides in a lipid bilayer is energetically unfavorable. The tilt and also the orientation around the peptide axis were found to be very similar to the values previously reported for a shorter WALP19 peptide (GWW(LA)(6)LWWA). As also observed in this previous study, the peptide rotates rapidly around the bilayer normal, but not around its helix axis. Here we show that these properties allow application of the GALA method not only to macroscopically aligned samples but also to randomly oriented samples, which has important practical advantages. A minimum of four labeled alanine residues in the hydrophobic transmembrane sequence was found to be required to obtain accurate tilt values using the GALA method.     

On two-dimensional passive random walk in lipid bilayers and fluid pathways in biomembranes

Summary The lateral mobility of pyrene, pyrene decanoic acid, and 1-palmitoyl-2-pyrene decanoyl-phosphatidyl choline (pyrene lecithin) in lipid bilayers is determined by the excimer formation technique. This method is applied to vesicles of lecithins differing in chain length and in the degree of saturation of the hydrocarbon chains. These values are compared with results in cephalins of different chain length and in dipalmitoyl phosphatidic acid at variable pH. The influence of cholesterol is investigated. The results are analyzed in terms of the Montroll model of two-dimensional random walk. The jump frequency of the probe molecule within the lipid lattice is obtained. The advantage of this measure of transport in lipid layers is that it does not involve lipid lattice parameters.The main results of the present work are: (i) The lateral mobility of a given solute molecule in lamellae of saturated lecithins is independent of hydrocarbon chain length and rather a universal function of temperature. (ii) In unsaturated dioleyl lecithin the amphiphatic molecules have lateral mobilities of the same size as in saturated lipids. The jump frequency of pyrene, however, is by a factor of two larger in the unsaturated lecithin. (iii) The jump frequencies in phosphatidyl ethanolamines are about equal to those in lecithins. (iv) In phosphatidic acid layers the hopping frequencies depend on the chargers of the head groups of both the lipids and the probes. (v) Cholesterol strongly reduces the jump frequency in fluid layers. (vi) The lateral mobility in biological membranes is comparable to that in artificial lipid bilayers.The experimental results are discussed in terms of the free volume model of diffusion in fluids. Good agreement with the predictions made from this model is found. A striking result is the observation of a tilt in dioleyl-lecithin bilayer membranes from the hopping frequencies of pyrene and pyrene lecithin. A tilt angle of -17° is estimated.     

Tilt angle of a trans-membrane helix is determined by hydrophobic mismatch

In order to investigate the compensation mechanism of a trans-membrane helix in response to hydrophobic mismatch, the tilt and rotation angles of the trans-membrane helix of Vpu aligned in lipid bilayers of various thickness were determined using orientation-dependent frequencies obtained from solid-state NMR experiments of aligned samples. A tilt angle of 18 degrees was observed in 18:1-O-PC/DOPG (9:1) lipid bilayers, which have a hydrophobic thickness that approximately matches the hydrophobic length of the trans-membrane helix of Vpu. Upon decreasing the hydrophobic thickness of lipid bilayers, no significant change in rotation angle was observed. However, the tilt angle increased systematically with increasing positive mismatch to 27 degrees in 14:0-O-PC/DMPG (9:1), 35 degrees in 12:0-O-PC/DLPG (9:1), and 51 degrees in 10:0 PC/10:0 PG (9:1) lipid bilayers, indicating that the change in tilt angle of the trans-membrane helix is a principal compensation mechanism for hydrophobic mismatch. In addition, the distinctive kink in the middle of the helix observed in 18:1 bilayers disappears in thinner bilayers. Although the opposite of what might be expected, this finding suggests that a helix kink may also be a part of the hydrophobic matching mechanism for trans-membrane helices.     

The behaviour of beta-carotene in the phosphatidylcholine bilayer as revealed by a molecular simulation study

A molecular dynamics (MD) simulation of the fully hydrated bilayer made of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and containing beta-carotene (beta-Car) molecules was carried out as a complementary approach to experimental techniques to investigate the orientation of beta-Car in the lipid membrane as well as its influence on the bilayer properties. The bilayer reached thermal equilibrium after 1200 ps of MD simulation and the productive run was carried out for 2800 ps. The results indicate that the carotene rings are located in the region occupied by the carbonyl groups of the POPC gamma-chain with no trace of penetration towards the centre of the bilayer. Carotene exhibits an ordering effect on both the beta- and the gamma-chain. While the fully saturated gamma-chain is affected evenly along, the order of the mono-unsaturated beta-chain is modified mainly below the double bond. In general, a high value of the order parameter and the chain tilt in the range from 11.4 degrees to 26.7 degrees were observed for the beta-Car molecules. However, for chain segment adjacent to methyl groups the value of the order parameter is low and the tilt angle is close to 75 degrees . Moreover, the probability of trans conformation being generally close to 1.0 along the beta-Car chain is reduced for these segments. Our MD simulation study suggests two pools of the preferential orientation of beta-Car: a slightly bent structure corresponding to a small chain tilt angle and a rather stretched structure that corresponds to a higher chain tilt. The results are discussed in the light of experimental findings.