Yicong Ma

Long-range interlayer alignment of intralayer domains in stacked lipid bilayers.

Liquid-crystalline phases of stacked lipid bilayers represent a pervasive motif in biomolecular assemblies. Here we report that, in addition to the usual smectic order, multicomponent multilayer membranes can exhibit columnar order arising from the coupling of two-dimensional intralayer phase separation and interlayer smectic ordering. This coupling propagates across hundreds of membrane lamellae, producing long-range alignment of phase-separated domains. Quantitative analysis of real-time dynamical experiments reveals that there is an interplay between intralayer domain growth and interlayer coupling, suggesting the existence of cooperative multilayer epitaxy. We postulate that such long-range epitaxy is solvent-assisted, and that it originates from the surface tension associated with differences in the network of hydrogen-bonded water molecules at the hydrated interfaces between the domains and the surrounding phase. Our findings might inspire the development of self-assembly-based strategies for the long-range alignment of functional lipid domains.

Demonstration of Feasibility of X-Ray Free Electron Laser Studies of Dynamics of Nanoparticles in Entangled Polymer Melts

The recent advent of hard x-ray free electron lasers (XFELs) opens new areas of science due to their exceptional brightness, coherence, and time structure. In principle, such sources enable studies of dynamics of condensed matter systems over times ranging from femtoseconds to seconds. However, the studies of “slow” dynamics in polymeric materials still remain in question due to the characteristics of the XFEL beam and concerns about sample damage. Here we demonstrate the feasibility of measuring the relaxation dynamics of gold nanoparticles suspended in polymer melts using X-ray photon correlation spectroscopy (XPCS), while also monitoring eventual X-ray induced damage. In spite of inherently large pulse-to-pulse intensity and position variations of the XFEL beam, measurements can be realized at slow time scales. The X-ray induced damage and heating are less than initially expected for soft matter materials.

Cholesterol Partition and Condensing Effect in Phase-Separated Ternary Mixture Lipid Multilayers

The cholesterol partitioning and condensing effect in the liquid-ordered (Lo) and liquid-disordered (Ld) phases were systematically investigated for ternary mixture lipid multilayers consisting of 1:1 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phosphocholine with varying concentrations of cholesterol. X-ray lamellar diffraction was used to deduce the electron density profiles of each phase. The cholesterol concentration in each phase was quantified by fitting of the electron density profiles with a newly invented basic lipid profile scaling method that minimizes the number of fitting parameters. The obtained cholesterol concentration in each phase versus total cholesterol concentration in the sample increases linearly for both phases. The condensing effect of cholesterol in ternary lipid mixtures was evaluated in terms of phosphate-to-phosphate distances, which together with the estimated cholesterol concentration in each phase was converted into an average area per molecule. In addition, the cholesterol position was determined to a precision of (±0.7Å) and an increase of disorder in the lipid packing in the Lo phase was observed for total cholesterol concentration of 20∼30%.

Accurate calibration and control of relative humidity close to 100% by X-raying a DOPC multilayer.

In this study, we have designed a compact sample chamber that can achieve accurate and continuous control of the relative humidity (RH) in the vicinity of 100%. A 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) multilayer can be used as a humidity sensor by measuring its inter-layer repeat distance (d-spacing) via X-ray diffraction. We convert from DOPC d-spacing to RH according to a theory given in the literature and previously measured data of DOPC multilamellar vesicles in polyvinylpyrrolidone (PVP) solutions. This curve can be used for calibration of RH close to 100%, a regime where conventional sensors do not have sufficient accuracy. We demonstrate that this control method can provide RH accuracies of 0.1 to 0.01%, which is a factor of 10-100 improvement compared to existing methods of humidity control. Our method provides fine tuning capability of RH continuously for a single sample, whereas the PVP solution method requires new samples to be made for each PVP concentration. The use of this cell also potentially removes the need for an X-ray or neutron beam to pass through bulk water if one wishes to work close to biologically relevant conditions of nearly 100% RH.

Localized surface plasmon assisted contrast microscopy for ultrathin transparent specimens

We demonstrate a high contrast imaging technique, termed localized surface plasmon assisted contrast microscopy, by combining localized surface plasmon resonances (LSPR) and dark-field microscopy technique. Due to the sensitive response of LSPR to the refractive index of the surrounding media, this technique is capable of converting a small refractive index difference to a change in scattering intensity, resulting in a high-contrast, diffraction limited image of a thin unstained specimen with small, gradual refractive-index variation.

Phase Separation in Model Membranes Controlled by Hybrid Lipids

Biological membranes are currently considered as a heterogeneous and highly dynamical organization of lipids and proteins. Some of these components phase separate in finite-size domains (known as ‘rafts’) to play a vital role in cellular signaling and transport process. Recent theoretical approaches have suggested that the hybrid lipids (lipids with one saturated and one unsaturated tail) stabilize nano-sized domains by controlling the line tension between ‘gel’ and ‘fluid’ phases [1,2]. Our x-ray scattering experiments on highly oriented multilamellar samples prepared on a hydrophilic surface have exhibited a strong effect of such hybrid lipids on the phase separation behavior in mixtures of saturated and unsaturated lipids. The phase transition temperature is found to decrease dramatically with the added mole percent of the hybrid lipids. Further, the bilayer repeat distance and the amount of phases are found to vary systematically as a function of the added hybrid lipids. This experimental study opens up an opportunity to verify the theoretical ideas about the role of hybrid lipids in forming the ‘raft’ in cellular membranes.[1] R. Brewster, et al., Biophys. J. (2009), 97, 1087-1094.[2] T. Yamamoto et al., Euro. Phys. Lett. (2010), 91, 28002.

First Observation of Dynamics in Lipid Multilayers using X-ray Photon Correlation Spectroscopy (XPCS)

Collective modes of layer undulations in lipid multilayers are of considerable interest because they can be used to measure the elastic moduli and viscosity of the lipid bilayers as a function of interlamellar spacing, properties of the interlayer aqueous channels, and temperature. These are fundamental quantities required to calculate the configurations and fluctuations of lipid membranes, relevant in modeling many biomembrane functionalities (e.g., intermembrane interactions and polyvalent ligand recognition) that depend on elasticity and dynamics of membrane phases. However, only relatively few investigations have been made of lipid systems, with e.g. dynamical light scattering, neutron spin echo and inelastic neutron scattering. There remains a gap in time scales and length scales which the technique of x-ray photon correlation spectroscopy (XPCS) can fill.We present here the first XPCS measurements of the dynamics of 1,2-dioleoyl-sn-glycero-3-phtosphocholine (DOPC) multilayers at relative humidity of 99% and temperature of 28°C. The measurements were done at and near the 1st Bragg peak of the multilayer, in which range the intensity-intensity autocorrelation function includes heterodyne oscillations [1] due to a large static component as well as homodyne oscillations predominated at q-values off the Bragg peak. According to de Jeus theory [2], there are two different modes of relaxation time in the system: a slow and a fast one. Our experiment reveals the existence of the slow mode, which exhibits a plateau in relaxation time over a range of q (10e-5 A-1∼10e-4 A-1). The results of the analysis of these correlation functions according to the model of de Jeu et al. will be presented.1. C. Gutt et al, Phys. Rev. Lett. 91, 076104 (2003)2. W. H. de Jue et al, Rev. Mod. Phys., 75, 181-235 (2003)