Ka Yee C. Lee

pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli

Growing evidence supports a critical role of metal-ligand coordination in many attributes of biological materials including adhesion, self-assembly, toughness, and hardness without mineralization [Rubin DJ, Miserez A, Waite JH (2010) Advances in Insect Physiology: Insect Integument and Color, eds Jérôme C, Stephen JS (Academic Press, London), pp 75–133]. Coordination between Fe and catechol ligands has recently been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties [Harrington MJ, Masic A, Holten-Andersen N, Waite JH, Fratzl P (2010) Science 328:216–220]. Inspired by the pH jump experienced by proteins during maturation of a mussel byssus secretion, we have developed a simple method to control catechol-Fe3+ interpolymer cross-linking via pH. The resonance Raman signature of catechol-Fe3+ cross-linked polymer gels at high pH was similar to that from native mussel thread cuticle and the gels displayed elastic moduli (G′) that approach covalently cross-linked gels as well as self-healing properties.

Stress and Fold Localization in Thin Elastic Membranes

Thin elastic membranes supported on a much softer elastic solid or a fluid deviate from their flat geometries upon compression. We demonstrate that periodic wrinkling is only one possible solution for such strained membranes. Folds, which involve highly localized curvature, appear whenever the membrane is compressed beyond a third of its initial wrinkle wavelength. Eventually the surface transforms into a symmetry-broken state with flat regions of membrane coexisting with locally folded points, reminiscent of a crumpled, unsupported membrane. We provide general scaling laws for the wrinkled and folded states and proved the transition with numerical and experimental supported membranes. Our work provides insight into the interfacial stability of such diverse systems as biological membranes such as lung surfactant and nanoparticle thin films.

Interaction of Lung Surfactant Proteins with Anionic Phospholipids

Langmuir isotherms, fluorescence microscopy, and atomic force microscopy were used to study lung surfactant specific proteins SP-B and SP-C in monolayers of dipalmitoylphosphatidylglycerol (DPPG) and palmitoyloleoylphosphatidylglycerol (POPG), which are representative of the anionic lipids in native and replacement lung surfactants. Both SP-B and SP-C eliminate squeeze-out of POPG from mixed DPPG/POPG monolayers by inducing a two- to three-dimensional transformation of the fluid-phase fraction of the monolayer. SP-B induces a reversible folding transition at monolayer collapse, allowing all components of surfactant to remain at the interface during respreading. The folds remain attached to the monolayer, are identical in composition and morphology to the unfolded monolayer, and are reincorporated reversibly into the monolayer upon expansion. In the absence of SP-B or SP-C, the unsaturated lipids are irreversibly lost at high surface pressures. These morphological transitions are identical to those in other lipid mixtures and hence appear to be independent of the detailed lipid composition of the monolayer. Instead they depend on the more general phenomena of coexistence between a liquid-expanded and liquid-condensed phase. These three-dimensional monolayer transitions reconcile how lung surfactant can achieve both low surface tensions upon compression and rapid respreading upon expansion and may have important implications toward the optimal design of replacement surfactants. The overlap of function between SP-B and SP-C helps explain why replacement surfactants lacking in one or the other proteins often have beneficial effects.

Phase and Morphology Changes in Lipid Monolayers Induced by SP-B Protein and Its Amino-Terminal Peptide

Both human lung surfactant protein, SP-B, and its amino-terminal peptide, SP-B1-25, inhibit the formation of condensed phases in monolayers of palmitic acid, resulting in a new fluid phase. This fluid phase forms a network, separating condensed-phase domains at coexistence. The network persists to high surface pressures, altering the nucleation, growth, and morphology of monolayer collapse structures, leading to lower surface tensions on compression and more reversible respreading on expansion. The network is stabilized by the low line tension between the fluid phase and the condensed phase as confirmed by the formation of “stripe” phases.

Interaction of antimicrobial peptide protegrin with biomembranes.

The antimicrobial peptide protegrin-1 (PG-1) interacts with membranes in a manner that strongly depends on membrane lipid composition. In this research we use an approach representing the outer layers of bacterial and red blood cell membranes with lipid monolayers and using a combination of insertion assay, epifluorescence microscopy, and surface x-ray scattering to gain a better understanding of antimicrobial peptides mechanism of action. We find that PG-1 inserts readily into anionic dipalmitoyl-phosphatidylglycerol, palmitoyl-oleoyl-phosphatidylglycerol, and lipid A films, but significantly less so into zwitterionic dipalmitoyl-phosphatidylcholine, palmitoyl-oleoyl-phosphatidylcholine, and dipalmitoyl-phosphatidylethanolamine monolayers under similar experimental conditions. Epifluorescence microscopy shows that the insertion of PG-1 into the lipid layer results in the disordering of lipid packing; this disordering effect is corroborated by grazing incidence x-ray diffraction data. X-ray reflectivity measurements further point to the location of the peptide in the lipid matrix. In a pathologically relevant example we show that PG-1 completely destabilizes monolayer composed of lipid A, the major component in the outer membrane of Gram-negative bacteria, which is likely to be the mechanism by which PG-1 disrupts the outer membrane, thus allowing it to reach the target inner membrane.

Effects of Lung Surfactant Proteins, SP-B and SP-C, and Palmitic Acid on Monolayer Stability

Langmuir isotherms and fluorescence and atomic force microscopy images of synthetic model lung surfactants were used to determine the influence of palmitic acid and synthetic peptides based on the surfactant-specific proteins SP-B and SP-C on the morphology and function of surfactant monolayers. Lung surfactant-specific protein SP-C and peptides based on SP-C eliminate the loss to the subphase of unsaturated lipids necessary for good adsorption and respreading by inducing a transition between monolayers and multilayers within the fluid phase domains of the monolayer. The morphology and thickness of the multilayer phase depends on the lipid composition of the monolayer and the concentration of SP-C or SP-C peptide. Lung surfactant protein SP-B and peptides based on SP-B induce a reversible folding transition at monolayer collapse that allows all components of surfactant to be retained at the interface during respreading. Supplementing Survanta, a clinically used replacement lung surfactant, with a peptide based on the first 25 amino acids of SP-B also induces a similar folding transition at monolayer collapse. Palmitic acid makes the monolayer rigid at low surface tension and fluid at high surface tension and modifies SP-C function. Identifying the function of lung surfactant proteins and lipids is essential to the rational design of replacement surfactants for treatment of respiratory distress syndrome.

Collapse Mechanisms of Langmuir Monolayers

When a two-dimensional (2D) film is compressed to its stability limit, it explores the third dimension via collapse. Understanding this 2D-to-3D transition is of great importance as it provides insight into the origin of defects in thin films. This review draws attention to a reversible folding collapse first discovered in model lung surfactant systems and explores the driving forces for this mechanism. The mode of collapse can be tuned by varying the mechanical properties of the film. I present a continuum elastic theory that captures the onset of the observed folding instability and use digital image analysis to analyze the folding dynamics. This article further explores factors that determine the maximum surface pressure a mixed monolayer can sustain and explains the observed phenomenon using the principle of rigidity percolation. The folding transition observed in lipid monolayers described here has also been observed in other systems, including monolayers of nanoparticles.

Lipid membrane templates the ordering and induces the fibrillogenesis of Alzheimer's disease amyloid-β peptide

The lipid membrane has been shown to mediate the fibrillogenesis and toxicity of Alzheimers disease (AD) amyloid‐β (Aβ) peptide. Electrostatic interactions between Aβ40 and the phospholipid headgroup have been found to control the association and insertion of monomeric Aβ into lipid monolayers, where Aβ exhibited enhanced interactions with charged lipids compared with zwitterionic lipids. To elucidate the molecular‐scale structural details of Aβ‐membrane association, we have used complementary X‐ray and neutron scattering techniques (grazing‐incidence X‐ray diffraction, X‐ray reflectivity, and neutron reflectivity) in this study to investigate in situ the association of Aβ with lipid monolayers composed of either the anionic lipid 1,2‐dipalmitoyl‐sn‐glycero‐3‐[phospho‐rac‐(1‐glycerol)] (DPPG), the zwitterionic lipid 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC), or the cationic lipid 1,2‐dipalmitoyl 3‐trimethylammonium propane (DPTAP) at the air‐buffer interface. We found that the anionic lipid DPPG uniquely induced crystalline ordering of Aβ at the membrane surface that closely mimicked the β‐sheet structure in fibrils, revealing an intriguing templated ordering effect of DPPG on Aβ. Furthermore, incubating Aβ with lipid vesicles containing the anionic lipid 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐[phospho‐rac‐(1‐glycerol)] (POPG) induced the formation of amyloid fibrils, confirming that the templated ordering of Aβ at the membrane surface seeded fibril formation. This study provides a detailed molecular‐scale characterization of the early structural fluctuation and assembly events that may trigger the misfolding and aggregation of Aβ in vivo. Our results implicate that the adsorption of Aβ to anionic lipids, which could become exposed to the outer membrane leaflet by cell injury, may serve as an in vivo mechanism of templated‐aggregation and drive the pathogenesis of AD. Proteins 2008.

Influence of palmitic acid and hexadecanol on the phase transition temperature and molecular packing of dipalmitoylphosphatidyl-choline monolayers at the air–water interface

Palmitic acid (PA) and 1-hexadecanol (HD) strongly affect the phase transition temperature and molecular packing of dipalmitoylphosphatidylcholine (DPPC) monolayers at the air–water interface. The phase behavior and morphology of mixed DPPC/PA as well as DPPC/HD monolayers were determined by pressure-area-isotherms and fluorescence microscopy. The molecular organization was probed by synchrotron grazing incidence x-ray diffraction using a liquid surface diffractometer. Addition of PA or HD to DPPC monolayers increases the temperature of the liquid-expanded to condensed phase transition. X-ray diffraction shows that DPPC forms mixed crystals both with PA and HD over a wide range of mixing ratios. At a surface pressure (π) of 40 mN/m, increasing the amount of the single chain surfactant leads to a reduction in tilt angle of the aliphatic chains from nearly 30° for pure DPPC to almost 0° in a 1:1 molar ratio of DPPC and PA or HD. At this composition we also find closest packing of the aliphatic chains. Furth...

Metal-coordination: using one of nature’s tricks to control soft material mechanics

Growing evidence supports a critical role of dynamic metal-coordination crosslinking in soft biological material properties such as self-healing and underwater adhesion1. Using bio-inspired metal-coordinating polymers, initial efforts to mimic these properties have shown promise2. Here we demonstrate how bio-inspired aqueous polymer network mechanics can be easily controlled via metal-coordination crosslink dynamics; metal ion-based crosslink stability control allows aqueous polymer network relaxation times to be finely tuned over several orders of magnitude. In addition to further biological material insights, our demonstration of this compositional scaling mechanism should provide inspiration for new polymer material property-control designs.