UV-induced reaction kinetics of dilinoleoylphosphatidylethanolamine monolayers

Characterizing ultrathin and thick organic layers by surface plasmon resonance three-wavelength and waveguide mode analysis

A three-wavelength angular-scanning surface plasmon resonance based analysis has been utilized for characterizing optical properties of organic nanometer-thick layers with a wide range of thicknesses. The thickness and refractive index were determined for sample layers with thicknesses ranging from subnanometer to hundreds of nanometers. The analysis approach allows for simultaneous determination of both the refractive index and thickness without prior knowledge of either the refractive index or the thickness of the sample layers and without the help of other instruments, as opposed to current methods and approaches for characterizing optical properties of organic nanometer-thick layers. The applicability of the three-wavelength angular-scanning surface plasmon resonance approach for characterizing thin and thick organic layers was demonstrated by ex situ deposited mono- and multilayers of stearic acid and hydrogenated soy phosphatidylcholine and in situ layer-by-layer deposition of two different polyelectrolyte multilayer systems. In addition to the three-wavelength angular-scanning surface plasmon resonance approach, another surface plasmon resonance optical phenomenon, i.e., the surface plasmon resonance waveguide mode, was utilized to characterize organic sample layers whose thicknesses border the micrometer scale. This was demonstrated by characterizing both in situ layer-by-layer deposited polyelectrolyte multilayer systems and an ex situ deposited spin-coated polymer layer.

Oxide nanocrystal based nanocomposites for fabricating photoplastic AFM probes

We report on the synthesis, characterization and application of a novel nanocomposite made of a negative tone epoxy based photoresist modified with organic-capped Fe(2)O(3) nanocrystals (NCs). The mechanical properties of the nanocomposite drastically improve upon incorporation of a suitable concentration of NCs in the polymer, without deteriorating its photolithography performance. High aspect ratio 3D microstructures made of the nanocomposite have been fabricated with a uniform surface morphology and with a resolution down to few micrometres. The embedded organic-capped Fe(2)O(3) NCs drastically increase the stiffness and hardness of the epoxy based photoresist matrix, making the final material extremely interesting for manufacturing miniaturized polymer based mechanical devices and systems. In particular, the nanocomposite has been used as structural material for fabricating photoplastic Atomic Force Microscopy (AFM) probes with integrated tips showing outstanding mechanical response and high resolution imaging performance. The fabricated probes consist of straight cantilevers with low stress-gradient and high quality factors, incorporating sharp polymeric tips. They present considerably improved performance compared to pure epoxy based photoresist AFM probes, and to commercial silicon AFM probes.

Adsorption of a phospholipid-hydroperoxide glutathione peroxidase into phospholipid monolayers at the air–water interface

The interfacial behavior differences of two glutathione peroxidase isoforms have been investigated. The first isoform is the phospholipid-hydroperoxide glutathione peroxidase (EC 1.11.1.12) (GPx-4) isolated from rat testes and the second one is the cytosolic glutathione peroxidase (EC 1.11.1.9) (GPx-1) from bovine erythrocytes. Injected in the subphase buffer of a Langmuir trough, GPx-4 was able to adsorb quickly at the air-water interface whereas the GPx-1 was not. Then, the protein interaction with phospholipid monolayers was explored. Indeed, a monolayer of phospholipids containing a different number of polyunsaturated fatty acyl chains was prepared at the air-water interface. Under each kind of monolayer, the protein solution was injected and its adsorption was visualized by the measurement of successive pressure-area isotherms. We have, then, determined the molecular area increase due to the protein adsorption. It was found that the GPx-4 is adsorbed in each kind of monolayer tested whereas no molecular area increase was detected with the GPx-1. This indicates that the GPx-4 has a higher affinity for the interface, recovered or not by lipids, than the GPx-1. Moreover, the GPx-4 presents a different affinity for the phospholipid monolayers depending on the number of polyunsaturated fatty acyl chains.

Plasmalogens protect unsaturated lipids against UV-induced oxidation in monolayer

Oxidative stress results from the attack by free radicals of several cellular targets (proteins, DNA and lipids). The cell equilibrium is a direct consequence of the pro-/antioxidant balance. In order to understand the physiological processes involved in oxidative stress, we followed oxidation of unsaturated lipids using a biomimetic system: Langmuir monolayers. The oxidation mode chosen was UV-irradiation and the lipid model was a polyunsaturated phospholipid: 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC). The monomolecular film technique was used to measure membrane rheology before and after UV-irradiation. We showed that the UV-irradiation of a DLPC monomolecular film led to a molecular area and surface elasticity modulus decrease that attests to the apparition of new molecular species at the air-water interface. The antioxidant effect of a synthetic plasmalogen (1-O-(1'-(Z)-hexadecenyl)-2-O-oleoyl-sn-glycero-3-phosphocholine or P(PLM)OPE) was tested on the oxidation of DLPC. Indeed, for about 25% mol P(PLM)OPE in mixed DLPC/P(PLM)OPE monolayers, a complete inhibition of the molecular area and the surface elasticity modulus decreases was observed in our experimental conditions. Lower P(PLM)OPE quantities delayed but did not prevent the DLPC oxidation in mixed monolayers.

Rupture of molecular thin films observed in atomic force microscopy. I. Theory

In atomic force microscope studies of molecular thin films, a defined jump of the tip through the film is often observed once a certain threshold force has been exceeded. Here, we present a theory to describe this film rupture and to relate microscopic parameters to measurable quantities. We assume that the tip has to overcome an activation energy before the film ruptures. A universal relation between the force dependence of the activation energy and the approaching velocity of the tip is derived. Two complementary models for calculating the activation energy are presented: a continuum nucleation model and a discrete molecular model. Both models predict a narrow distribution of yield forces in agreement with experimental results.

Atomic force microscopy of macromolecular interactions

The introduction of functional imaging tools and techniques that operate at molecular-length scales has provided investigators with unique approaches to characterizing biomolecular structure and function relationships. Recent advances in the field of scanning probe techniques and, in particular, atomic force microscopy have yielded tantalizing insights into the dynamics of protein self-assembly and the mechanics of protein unfolding.

Advances in the characterization of supported lipid films with the atomic force microscope

During the past decade, the atomic force microscope (AFM) has become a key technique in biochemistry and biophysics to characterize supported lipid films, as testified by the continuous growth in the number of papers published in the field. The unique capabilities of AFM are: (i) capacity to probe, in real time and in aqueous environment, the surface structure of lipid films; (ii) ability to directly measure physical properties at high spatial resolution; (iii) possibility to modify the film structure and biophysical processes in a controlled way. Such experiments, published up to June 2000, are the focus of the present review. First, we provide a general introduction on the preparation and characterization of supported lipid films as well as on the principles of AFM. The section 'Structural properties' focuses on the various applications of AFM for characterizing the structure of supported lipid films: visualization of molecular structure, formation of structural defects, effect of external agents, formation of supported films, organization of phase-separated films (coexistence region, mixed films) and, finally, the use of supported lipid bilayers for anchoring biomolecules such as DNA, enzymes and crystalline protein arrays. The section 'Physical properties' introduces the principles of force measurements by AFM, interpretation of these measurements and their recent application to supported lipid films and related structures. Finally, we highlight the major achievements brought by the technique and some of the current limitations.