Search

When a denatured protein isoform (i.e., a proteoform) immersed in electrolyte is impelled by an electric field through a sub-nanometer-diameter pore (i.e., a sub-nanopore) spanning a thin membrane, the sequence of amino acid (AA) residues constituting the proteoform can be directly “read” one at a time by measuring fluctuations in the electrolytic current. Corroborating this assertion, an analysis of the pore current with molecular dynamic (MD) simulations reveals that the fluctuations are correlated to the sequence of AA volumes, the water in the pore and acid mobility. After alignment to account for variations in the acid mobility, the simulated pore current is nearly perfectly correlated to the pattern of empirical fluctuations. To prove out the prospects for decoding proteoforms this way, site-specific post-translational modifications (PTMs) and point mutations in amyloid-beta (Aβ1-42) were analyzed with a sub-nanopore. The results show that single acids can be resolved in proteoforms with a dynamic range limited by the size of phenylalanine and glycine. With this sensitivity and single acid resolution, the sequence of a scrambled variant of Aβ1-42was discriminated with a p-value < 10-5.

Rose petals may involve high water contact angles together with drop adhesion which are antagonistic wetting properties. Petal surfaces have a cuticle which is generally considered a continuous, hydrophobic lipid coating. The peculiar properties of rose petals are not fully understood and have been associated with high surface roughness at different scales. Here, the chemical and structural features of natural upper and lower petal surfaces are analyzed by atomic force microscopy (AFM). Both rose petal surfaces are statistically equivalent and have very high roughness at all scales from 5 nm to 10 μm. At the nanoscale, surfaces are fractal-like with an extreme fractal dimension close to df = 2.5. A major nanoscale variability is also observed which leads to large (nanoscale) wettability changes. To model the effect of roughness and chemical variability on wetting properties, a single wetting parameter is introduced. This approach enables to explain the Rose petal effect using a conceptually simple scheme. The described fundamental mechanisms leading to high contact angles together with drop adhesion can be applied to any natural and synthetic surface. Apart from introducing a new approach for characterizing a biological surface, these results can trigger new developments on nanoscale wetting and bio-inspired functional surfaces.

Under ambient conditions, surfaces are rapidly modified and contaminated by absorbance of molecules and a variety of nanoparticles that drastically change their chemical and physical properties. The atomic force microscope tip–sample system can be considered a model system for investigating a variety of nanoscale phenomena. In the present work we use atomic force microscopy to directly image nanoscale contamination on surfaces, and to characterize this contamination by using multidimensional spectroscopy techniques. By acquisition of spectroscopy data as a function of tip–sample voltage and tip–sample distance, we are able to determine the contact potential, the Hamaker constant and the effective thickness of the dielectric layer within the tip–sample system. All these properties depend strongly on the contamination within the tip–sample system. We propose to access the state of contamination of real surfaces under ambient conditions using advanced atomic force microscopy techniques.

The size of an ion affects everything from the structure of water to life itself. In this report, to gauge their size, ions dissolved in water are forced electrically through a sub-nanometer-diameter pore spanning a thin membrane and the current is measured. The measurements reveal an ion-selective conductance that vanishes in pores, The size of an ion affects everything from the structure of water to life itself. Here, a sub-nanometer diameter pore sputtered through a thin silicon nitride membrane is used to systematically test ion permeability by measuring the electrolytic current and current noise and show that the ions move with a grossly distorted hydration shell in a correlated way.

Chemical information can be obtained by using atomic force microscopy (AFM) and force spectroscopy (FS) with atomic or molecular resolution, even in liquid media. The aim of this paper is to demonstrate that single molecules of avidin and streptavidin anchored to a biotinylated bilayer can be differentiated by using AFM, even though AFM topographical images of the two proteins are remarkably alike. At physiological pH, the basic glycoprotein avidin is positively charged, whereas streptavidin is a neutral protein. This charge difference can be determined with AFM, which can probe electrostatic doublelayer forces by using FS. The force curves, owing to the electrostatic interaction, show major differences when measured on top of each protein as well as on the lipid substrate. FS data show that the two proteins are negatively charged. Nevertheless, avidin and streptavidin can be clearly distinguished, thus demonstrating the sensitivity of AFM to detect small changes in the charge state of macromolecules., We acknowledge funding by the MICINN through projects FIS2012‐35248 and CSD2009‐00024 and by the CAM through project Nanoobjetos (S2009/MAT‐1467).