Fausto Sanz

Fine structure study of Aβ1-42 fibrillogenesis with atomic force microscopy

One of the hallmarks of Alzheimers disease is the self‐aggregation of the amyloid β peptide (Aβ) in extracellular amyloid fibrils. Among the different forms of Aβ, the 42‐residue fragment (Aβ1–42) readily self‐associates and forms nucleation centers from where fibrils can quickly grow. The strong tendency of Aβ1–42 to aggregate is one of the reasons for the scarcity of data on its fibril formation process. We have used atomic force microscopy (AFM) to visualize in liquid environment the fibrillogenesis of synthetic Aβ1–42 on hydrophilic and hydrophobic surfaces. The results presented provide nanometric resolution of the main structures characteristic of the several steps from monomeric Aβ1–42 to mature fibrils in vitro. Oligomeric globular aggregates of Aβ1–42 precede the appearance of protofibrils, the first fibrillar species, although we have not obtained direct evidence of oligomer‐protofibril interconversion. The protofibril dimensions deduced from our AFM images are consistent with a model that postulates the stacking of the peptide in a hairpin conformation perpendicular to the long axis of the protofibril, forming single β‐sheets ribbon‐shaped. The most abundant form of Aβ1–42 fibril exhibits a nodular structure with a ∼100‐nm periodicity. This length is very similar 1) to the length of protofibril bundles that are the dominant feature at earlier stages in the aggregation process, 2) to the period of helical structures that have been observed in the core of fibrils, and 3) to the distance between regularly spaced, structurally weak fibril points. Taken together, these data are consistent with the existence of a ∼100‐nm long basic protofibril unit that is a key fibril building block.

Nanomechanics of lipid bilayers by force spectroscopy with AFM: a perspective.

Lipid bilayers determine the architecture of cell membranes and regulate a myriad of distinct processes that are highly dependent on the lateral organization of the phospholipid molecules that compose the membrane. Indeed, the mechanochemical properties of the membrane are strongly correlated with the function of several membrane proteins, which demand a very specific, highly localized physicochemical environment to perform their function. Several mesoscopic techniques have been used in the past to investigate the mechanical properties of lipid membranes. However, they were restricted to the study of the ensemble properties of giant bilayers. Force spectroscopy with AFM has emerged as a powerful technique able to provide valuable insights into the nanomechanical properties of supported lipid membranes at the nanometer/nanonewton scale in a wide variety of systems. In particular, these measurements have allowed direct measurement of the molecular interactions arising between neighboring phospholipid molecules and between the lipid molecules and the surrounding solvent environment. The goal of this review is to illustrate how these novel experiments have provided a new vista on membrane mechanics in a confined area within the nanometer realm, where most of the specific molecular interactions take place. Here we report in detail the main discoveries achieved by force spectroscopy with AFM on supported lipid bilayers, and we also discuss on the exciting future perspectives offered by this growing research field.

Nanomechanics of lipid bilayers: heads or tails?

Understanding the effect of mechanical stress on membranes is of primary importance in biophysics. Here we use force spectroscopy AFM to quantitatively characterize the nanomechanical stability of supported lipid bilayers as a function of their chemical composition. The onset of plastic deformation reveals itself as a repetitive jump in the approaching force curve, which represents a molecular fingerprint for the bilayer mechanical stability. By systematically probing a set of chemically distinct supported lipid bilayers (SLBs), we first show that both the headgroup and tail have a decisive effect on their mechanical properties. While the mechanical stability of the probed SLBs linearly increases by 3.3 nN upon the introduction of each additional -CH(2)- in the chain, it exhibits a significant dependence on the phospholipid headgroup, ranging from 3 nN for DPPA to 66 nN for DPPG. Furthermore, we also quantify the reduction of the membrane mechanical stability as a function of the number of unsaturations and molecular branching in the chemical structure of the apolar tails. Finally, we demonstrate that, upon introduction of cholesterol and ergosterol, contrary to previous belief the mechanical stability of membranes not only increases linearly in the liquid phase (DLPC) but also for phospholipids present in the gel phase (DPPC). Our results are discussed in the framework of the continuum nucleation model. This work highlights the compelling effect of subtle variations in the chemical structure of phospholipid molecules on the membrane response when exposed to mechanical forces, a mechanism of common occurrence in nature.

Influence of cholesterol on the phase transition of lipid bilayers: a temperature-controlled force spectroscopy study.

Cholesterol (Chol) plays the essential function of regulating the physical properties of the cell membrane by controlling the lipid organization and phase behavior and, thus, managing the membrane fluidity and its mechanical strength. Here, we explore the model system DPPC:Chol by means of temperature-controlled atomic force microscopy (AFM) imaging and AFM-based force spectroscopy (AFM-FS) to assess the influence of Chol on the membrane ordering and stability. We analyze the system in a representative range of compositions up to 50 mol % Chol studying the phase evolution upon temperature increase (from room temperature to temperatures high above the T(m) of the DPPC bilayer) and the corresponding (nano)mechanical stability. By this means, we correlate the mechanical behavior and composition with the lateral order of each phase present in the bilayers. We prove that low Chol contents lead to a phase-segregated system, whereas high contents of Chol can give a homogeneous bilayer. In both cases, Chol enhances the mechanical stability of the membrane, and an extraordinarily stable system is observed for equimolar fractions (50 mol % Chol). In addition, even when no thermal transition is detected by the traditional bulk analysis techniques for liposomes with high Chol content (40 and 50 mol %), we demonstrate that temperature-controlled AFM-FS is capable of identifying a thermal transition for the supported lipid bilayers. Finally, our results validate the AFM-FS technique as an ideal platform to differentiate phase coexistence and transitions in lipid bilayers and bridge the gap between the results obtained by traditional methods for bulk analysis, the theoretical predictions, and the behavior of these systems at the nanoscale.

Charge Exchange Processes during the Open‐Circuit Deposition of Nickel on Silicon from Fluoride Solutions

The open‐circuit potential deposition of nickel on silicon from fluoride solutions has been studied under potentiostatic control for two extreme values of pH. At pH 1.2, nickel ions are unable to exchange charge with the silicon substrate, and thus deposition is not observed. At pH 8.0, nickel ions in solution can exchange electrons with both the conduction band and the valence band of the silicon substrate in order to be reduced and deposit on the surface. The results are interpreted in terms of the coupling between the anodic dissolution of silicon in fluoride media and two competing cathodic reactions, hydrogen evolution and nickel deposition. The role of silicon surface states as dissolution intermediates is recalled and their interplay with the cathodic reactions is discussed as a function of solution pH. Surface states must also be relevant in the deposition process from other metallic ion solutions of high technological interest whose energy levels lie within the silicon bandgap.

How changes in the sequence of the peptide CLPFFD-NH2 can modify the conjugation and stability of gold nanoparticles and their affinity for β-amyloid fibrils

In a previous work, we studied the interaction of beta-amyloid fibrils (Abeta) with gold nanoparticles (AuNP) conjugated with the peptide CLPFFD-NH2. Here, we studied the effect of changing the residue sequence of the peptide CLPFFD-NH2 on the efficiency of conjugation to AuNP, the stability of the conjugates, and the affinity of the conjugates to the Abeta fibrils. We conjugated the AuNP with CLPFFD-NH 2 isomeric peptides (CDLPFF-NH2 and CLPDFF-NH2) and characterized the resulting conjugates with different techniques including UV-Vis, TEM, EELS, XPS, analysis of amino acids, agarose gel electrophoresis, and CD. In addition, we determined the proportion of AuNP bonded to the Abeta fibrils by ICP-MS. AuNP-CLPFFD-NH2 was the most stable of the conjugates and presented more affinity for Abeta fibrils with respect to the other conjugates and bare AuNP. These findings help to better understand the way peptide sequences affect conjugation and stability of AuNP and their interaction with Abeta fibrils. The peptide sequence, the steric effects, and the charge and disposition of hydrophilic and hydrophobic residues are crucial parameters when considering the design of AuNP peptide conjugates for biomedical applications.

First Stages of Electrochemical Growth of the Passive Film on Iron

The first stages of electrochemical growth of passive film on iron have been studied electrochemically by in situ electrochemical scanning tunneling microscopy (ECSTM) A freshly polished iron surface has been cathodically reduced in a borate buffer solution (pH 7.51 to get an oxide-free surface, and the passive film has been subsequently formed by applying short anodic potential steps. ECSTM has been used to follow the evolution of the oxide starting with growth on the surface at very negative potentials. We associate the changes observed on the surface to formation of iron hydroxides from Fe 2+ (aq). The growth rate and film thickness have been measured from scanning tunneling microscopy images and compared with literature values. Cyclic voltammetric results obtained at pH 7.5 were compared with those in commonly used borate buffer pH 8.4. X-ray photoelectron spectroscopy measurements provide additional information about the oxidation state of the iron passive film formed potentiostatically.

Nanometer-scale oxidation of Si(100) surfaces by tapping mode atomic force microscopy

The nanometer‐scale oxidation of Si(100) surfaces in air is performed with an atomic force microscope working in tapping mode. Applying a positive voltage to the sample with respect to the tip, two kinds of modifications are induced on the sample: grown silicon oxide mounds less than 5 nm high and mounds higher than 10 nm (which are assumed to be gold depositions). The threshold voltage necessary to produce the modification is studied as a function of the average tip‐to‐sample distance.

Nanoindentation: Toward the sensing of atomic interactions.

The mechanical properties of surfaces of layered materials (highly oriented pyrolytic graphite, InSe, and GaSe) and single-crystal ionic materials (NaCl, KBr, and KCl) have been investigated at the nanometer scale by using nanoindentations produced with an atomic force microscope with ultrasharp tips. Special attention has been devoted to the elastic response of the materials before the onset of plastic yield. A new model based on an equivalent spring constant that takes into account the changes in in-plane interactions on nanoindentation is proposed. The results of this model are well correlated with those obtained by using the Debye model of solid vibrations.

Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport

Controlling the spin of electrons in nanoscale electronic devices is one of the most promising topics aiming at developing devices with rapid and high density information storage capabilities. The interface magnetism or spinterface resulting from the interaction between a magnetic molecule and a metal surface, or vice versa, has become a key ingredient in creating nanoscale molecular devices with novel functionalities. Here, we present a single-molecule wire that displays large (>10000%) conductance switching by controlling the spin-dependent transport under ambient conditions (room temperature in a liquid cell). The molecular wire is built by trapping individual spin crossover Fe(II) complexes between one Au electrode and one ferromagnetic Ni electrode in an organic liquid medium. Large changes in the single-molecule conductance (>100-fold) are measured when the electrons flow from the Au electrode to either an α-up or a β-down spin-polarized Ni electrode. Our calculations show that the current flowing through such an interface appears to be strongly spin-polarized, thus resulting in the observed switching of the single-molecule wire conductance. The observation of such a high spin-dependent conductance switching in a single-molecule wire opens up a new door for the design and control of spin-polarized transport in nanoscale molecular devices at room temperature.