32 research outputs found
Dynamics of Associative Polymers with High Density of Reversible Bonds
We design and synthesize unentangled associative polymers carrying
unprecedented high fractions of stickers, up to eight per Kuhn segment, that
can form strong pairwise hydrogen bonding of without microphase
separation. The reversible bonds significantly slow down the polymer dynamics
but nearly do not change the shape of linear viscoelastic spectra. Moreover,
the structural relaxation time of associative polymers increases exponentially
with the fraction of stickers and exhibits a universal yet non-Arrhenius
dependence on the distance from polymer glass transition temperature. These
results cannot be understood within the framework of the classic sticky-Rouse
model but are rationalized by a renormalized Rouse model, which highlights an
unexpected influence of reversible bonds on the structural relaxation rather
than the shape of viscoelastic spectra for associative polymers with high
concentrations of stickers.Comment: 4 figure
Antiferroelectric Smectic Ordering as a Prelude to the Ferroelectric Nematic:Introducing the Smectic Phase
We have structurally characterized the liquid crystal phase that appears as
an intermediate state when a dielectric nematic, having polar disorder of its
molecular dipoles, transitions to the almost perfectly polar-ordered
ferroelectric nematic. This intermediate phase, which fills a 100-year-old void
in the taxonomy of smectics and which we term the "smectic ", is
antiferroelectric, with the nematic director and polarization oriented parallel
to smectic layer planes, and the polarization alternating in sign from layer to
layer. The period of this polarization wave (180 A) is mesoscopic,
corresponding to 40 molecules side-by-side, indicating that this lamellar
structure is collectively stabilized. A Landau free energy, originally
formulated to model incommensurate antiferroelectricity in crystals, describes
the key features of the nematic-Sm-ferroelectric nematic phase sequence.Comment: main paper (with 7 figures) and supplement (with 12 figures) comprise
65 page
Measuring calcium content in plants using NEXAFS spectroscopy
Calcium is important for the growth and development of plants. It serves crucial functions in cell wall and cell membrane structure and serves as a secondary messenger in signaling pathways relevant to nutrient and immunity responses. Thus, measuring calcium levels in plants is important for studies of plant biology and for technology development in food, agriculture, energy, and forest industries. Often, calcium in plants has been measured through techniques such as atomic absorption spectrophotometry (AAS), inductively coupled plasma–mass spectrometry (ICP-MS), and electrophysiology. These techniques, however, require large sample sizes, chemical extraction of samples or have limited spatial resolution. Here, we used near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the calcium L- and K-edges to measure the calcium to carbon mass ratio with spatial resolution in plant samples without requiring chemical extraction or large sample sizes. We demonstrate that the integrated absorbance at the calcium L-edge and the edge jump in the fluorescence yield at the calcium K-edge can be used to quantify the calcium content as the calcium mass fraction, and validate this approach with onion epidermal peels and ICP-MS. We also used NEXAFS to estimate the calcium mass ratio in hypocotyls of a model plant, Arabidopsis thaliana, which has a cell wall composition that is similar to that of onion epidermal peels. These results show that NEXAFS spectroscopy performed at the calcium edge provides an approach to quantify calcium levels within plants, which is crucial for understanding plant physiology and advancing plant-based materials
Revealing the mechanism of passive transport in lipid bilayers via phonon-mediated nanometre-scale density fluctuations
International audienceThe passive transport of molecules through a cell membrane relies on thermal motions of the lipids. However, the nature of transmembrane transport and the precise mechanism remain elusive and call for a comprehensive study of phonon excitations. Here we report a high resolution inelastic X-ray scattering study of the in-plane phonon excitations in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine above and below the main transition temperature. In the gel phase, for the first time, we observe low-frequency transverse modes, which exhibit a phonon gap when the lipid transitions into the fluid phase. We argue that the phonon gap signifies the formation of short-lived nanometre-scale lipid clusters and transient pores, which facilitate the passive molecular transport across the bilayer plane. Our findings suggest that the phononic motion of the hydrocarbon tails provides an effective mechanism of passive transport, and illustrate the importance of the collective dynamics of biomembrane
Influence of Lipid Membrane Rigidity on Properties of Supporting Polymer
Temperature-sensitive hydrogel polymers are utilized as responsive layers in various applications. Although the polymer's native characteristics have been studied extensively, details concerning its properties during interaction with biorelated structures are lacking. This work investigates the interaction between a thermoresponsive polymer cushion and different lipid membrane capping layers probed by neutron reflectometry. N-isopropylacrylamide copolymerized with methacroylbenzophenone first supported a lipid bilayer composed of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) and subsequently 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The polymer-membrane systems were investigated above and below the polymer transition temperature (37 and 25°C). Although the same cushion supported each lipid membrane, the polymer hydration profile and thickness were markedly different for DPPE and DPPC systems. Because DPPE and DPPC have different bending rigidities, these results establish that the polymer-membrane interaction is critically mediated by the mechanics of the membrane, providing better insight into cell-hydrogel interactions
Functional lipid pairs as building blocks of phase-separated membranes
Biological membranes exhibit a great deal of compositional and phase heterogeneity due to hundreds of chemically distinct components. As a result, phase separation processes in cell membranes are extremely difficult to study, especially at the molecular level. It is currently believed that the lateral membrane heterogeneity and the formation of domains, or rafts, are driven by lipid–lipid and lipid–protein interactions. Nevertheless, the underlying mechanisms regulating membrane heterogeneity remain poorly understood. In the present work, we combine inelastic X-ray scattering with molecular dynamics simulations to provide direct evidence for the existence of strongly coupled transient lipid pairs. These lipid pairs manifest themselves experimentally through optical vibrational (a.k.a. phononic) modes observed in binary (1,2-dipalmitoyl-sn-glycero-3-phosphocholine [DPPC]–cholesterol) and ternary (DPPC–1,2-dioleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-glycero-3-phosphocholine [DOPC/POPC]–cholesterol) systems. The existence of a phononic gap in these vibrational modes is a direct result of the finite size of patches formed by these lipid pairs. The observation of lipid pairs provides a spatial (subnanometer) and temporal (subnanosecond) window into the lipid–lipid interactions in complex mixtures of saturated/unsaturated lipids and cholesterol. Our findings represent a step toward understanding the lateral organization and dynamics of membrane domains using a well-validated probe with a high spatial and temporal resolution