Sonia Antoranz Contera

Direct mapping of the solid-liquid adhesion energy with subnanometre resolution

Solid-liquid interfaces play a fundamental role in surface electrochemistry, catalysis, wetting, self-assembly and biomolecular functions. The interfacial energy determines many of the properties of such interfaces, including the arrangement of the liquid molecules at the surface of the solid. Diffraction techniques are often used to investigate the structure of solid-liquid interfaces, but measurements of irregular or inhomogeneous interfaces remain challenging. Here, we report atomic- and molecular-resolution images of various organic and inorganic samples in liquids, obtained with a commercial atomic force microscope operated dynamically with small-amplitude modulation. This approach uses the structured liquid layers close to the solid to enhance lateral resolution. We propose a model to explain the mechanism dominating the image formation, and show that the energy dissipated during this process is related to the interfacial energy through a readily achievable calibration curve. Our topographic images and interfacial energy maps could provide insights into important interfaces.

Role of the Trans-activation Response Element in Dimerization of HIV-1 RNA

The HIV-1 genome consists of two identical RNA strands that are linked together through non-covalent interactions. A major determinant for efficient dimerization of the two RNA strands is the interaction between palindromic sequences in the dimerization initiation site. Here we use an interplay of bioinformatics, biochemistry, and atomic force microscopy to describe another conserved palindrome in the trans-activation response element (TAR) that functions as a strong dimerization site when transiently exposed to the viral nucleocapsid protein. In conjunction with the DIS interaction, the TAR dimerization induces the formation of a 65-nm higher-order circular structure in the dimeric HIV-1 RNA. Our results provide a molecular model for the role of TAR in packaging and reverse transcription of the viral genome. The unique structure of the TAR-TAR dimer renders it an intriguing therapeutic target for the treatment of HIV-1 infection.

Dynamics of bacteriorhodopsin 2D crystal observed by high-speed atomic force microscopy

We have used high-speed atomic force microscopy to study the dynamics of bacteriorhodopsin (bR) molecules at the free interface of the crystalline phase that occurs naturally in purple membrane. Our results reveal temporal fluctuations at the crystal edges arising from the association and dissociation of bR molecules, most predominantly pre-formed trimers. Analysis of the dissociation kinetics yields an estimate of the inter-trimer single-bond energy of -0.9kcal/mol. Rotational motion of individual bound trimers indicates that the inter-trimer bond involves W10-W12 tryptophan residues.

Bilayer-Mediated Clustering and Functional Interaction of MscL Channels

Mechanosensitive channels allow bacteria to respond to osmotic stress by opening a nanometer-sized pore in the cellular membrane. Although the underlying mechanism has been thoroughly studied on the basis of individual channels, the behavior of channel ensembles has yet to be elucidated. This work reveals that mechanosensitive channels of large conductance (MscL) exhibit a tendency to spatially cluster, and demonstrates the functional relevance of clustering. We evaluated the spatial distribution of channels in a lipid bilayer using patch-clamp electrophysiology, fluorescence and atomic force microscopy, and neutron scattering and reflection techniques, coupled with mathematical modeling of the mechanics of a membrane crowded with proteins. The results indicate that MscL forms clusters under a wide range of conditions. MscL is closely packed within each cluster but is still active and mechanosensitive. However, the channel activity is modulated by the presence of neighboring proteins, indicating membrane-mediated protein-protein interactions. Collectively, these results suggest that MscL self-assembly into channel clusters plays an osmoregulatory functional role in the membrane.

Three strategies to stabilise nearly monodispersed silver nanoparticles in aqueous solution

Silver nanoparticles are extensively used due to their chemical and physical properties and promising applications in areas such as medicine and electronics. Controlled synthesis of silver nanoparticles remains a major challenge due to the difficulty in producing long-term stable particles of the same size and shape in aqueous solution. To address this problem, we examine three strategies to stabilise aqueous solutions of 15 nm citrate-reduced silver nanoparticles using organic polymeric capping, bimetallic core-shell and bimetallic alloying. Our results show that these strategies drastically improve nanoparticle stability by distinct mechanisms. Additionally, we report a new role of polymer functionalisation in preventing further uncontrolled nanoparticle growth. For bimetallic nanoparticles, we attribute the presence of a higher valence metal on the surface of the nanoparticle as one of the key factors for improving their long-term stability. Stable silver-based nanoparticles, free of organic solvents, will have great potential for accelerating further environmental and nanotoxicity studies.PACS: 81.07.-b; 81.16.Be; 82.70.Dd.

Electrical conductance and breakdown in individual CNx multiwalled nanotubes

Doping of carbon nanotubes with nitrogen during growth strongly modifies their electronic structure through n-type doping. This provides the possibility of producing nanotubes with high conductances, independent of tube chirality. To date, electrical measurements on individual nitrogen-doped multiwalled nanotubes (CNx MWNTs) have reported surprisingly low conductances (∼0.01G0). Here the authors present high conductance (1.0±0.3G0) measurements at low bias for individual CNx MWNTs. Conductance increases linearly with voltage at a rate of 0.7±0.2G0∕V until the threshold for electrical breakdown is reached. Discrete current steps of 20±10μA are then observed.

Doping of carbon nanotubes with nitrogen improves protein coverage whilst retaining correct conformation

Relevant parameters for non-covalent protein functionalization of carbon nanotubes are explored. Multiwalled carbon nanotubes are carboxylated and functionalized with metalloproteins. Using atomic force microscopy (AFM) we quantitatively determine that coverage with nitrogen-doped multiwalled carbon nanotubes is superior compared to coverage with un-doped multiwalled carbon nanotubes, due to enhanced carboxylation. Conformational analysis using a combination of AFM, antibody binding assays, circular dichroism and UV-visible spectroscopy demonstrates that the metalloproteins retain their native structure when adsorbed to nitrogen-doped multiwalled carbon nanotubes irrespective of their size, charge or folding motif.

Temperature-dependent phase transitions in zeptoliter volumes of a complex biological membrane

Phase transitions in purple membrane have been a topic of debate for the past two decades. In this work we present studies of a reversible transition of purple membrane in the 50-60 °C range in zeptoliter volumes under different heating regimes (global heating and local heating). The temperature of the reversible phase transition is 52 ± 5 °C for both local and global heating, supporting the hypothesis that this transition is mainly due to a structural rearrangement of bR molecules and trimers. To achieve high resolution measurements of temperature-dependent phase transitions, a new scanning probe microscopy-based method was developed. We believe that our new technique can be extended to other biological systems and can contribute to the understanding of inhomogeneous phase transitions in complex systems.

Imaging the proteins pseudoazurin and apo-pseudoazurin on gold by STM in air: effect of the bias voltage.

We have applied scanning tunnelling microscopy (STM) to the study of two proteins: pseudoazurin and apo-pseudoazurin. Both proteins adsorbed onto a Au (1 1 1) surface are visible to STM individually, forming into layers and multilayers, with currents from about 55 to 600 pA. The images reproduce well the expected dimensions laterally but not in the z direction. The apparent height of the proteins varies with the voltage polarity, being higher at negative sample voltages. The bias also affects their shape. Negative sample voltages of more than 1.5 V orient the proteins present on a gold terrace in parallel rows. The layer of water adsorbed on surfaces in ambient conditions can be related to our results to explain the reduced z dimensions, the asymmetry with the voltage polarity and the alignment of proteins at voltages more negative than -1.5 V.

Nanotribology of Clean and Oxide-Covered Silicon Surfaces Using Atomic Force Microscopy.

Atomic force microscopy (AFM) has been used for tribological studies of silicon surfaces both with and without an oxide layer on the surface. Three different types of surfaces were prepared: a silicon surface with a chemical oxide made by the SC1 process, a silicon surface with a thermal oxide, and a H-terminated silicon surface without an oxide layer. Only in the case of the chemical oxide, scratching of the oxide and ploughing of the silicon by the Si3N4 AFM tip were observed. On the other hand, no wear of the sample was noted on the other surfaces. On these surfaces, the AFM often produced elevated patterns in the shape of the scanned area, which were no longer visible after HF etching. The difference between the tribological behavior of the chemical-oxide-covered surface and that of the other surfaces is discussed in relation to the presence of hydroxyl groups in the oxide layer.