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- Correlation between Surface Morphology and Surface Forces of...Correlation between Surface Morphology and Surface Forces of Protein A Adsorbed on Mica
Biophysical Journal, Volume 74, Issue 1, 1 January 1998, Pages 455-465
Satomi Ohnishi, Masami Murata and Masakatsu Hato
AbstractWe have investigated the morphology and surface forces of protein A adsorbed on mica surface in the protein solutions of various concentrations. The force-distance curves, measured with a surface force apparatus (SFA), were interpreted in terms of two different regimens: a “large-distance” regimen in which an electrostatic double-layer force dominates, and an “adsorbed layer” regimen in which a force of steric origin dominates. To further clarify the forces of steric origin, the surface morphology of the adsorbed protein layer was investigated with an atomic force microscope (AFM) because the steric repulsive forces are strongly affected by the adsorption mode of protein A molecules on mica. At lower protein concentrations (2ppm, 10ppm), protein A molecules were adsorbed “side-on” parallel to the mica surfaces, forming a monolayer of ∼2.5nm. AFM images at higher concentrations (30ppm, 100ppm) showed protruding structures over the monolayer, which revealed that the adsorbed protein A molecules had one end oriented into the solution, with the remainder of each molecule adsorbed side-on to the mica surface. These extending ends of protein A overlapped each other and formed a “quasi-double layer” over the mica surface. These AFM images proved the existence of a monolayer of protein A molecules at low concentrations and a “quasi-double layer” with occasional protrusions at high concentrations, which were consistent with the adsorption mode observed in the force-distance curves.
Abstract | Full Text | PDF (438 kb) - Adsorption of DNA to Mica Mediated by Divalent Counterions: ...Adsorption of DNA to Mica Mediated by Divalent Counterions: A Theoretical and Experimental Study
Biophysical Journal, Volume 85, Issue 4, 1 October 2003, Pages 2507-2518
David Pastré, Olivier Piétrement, Stéphane Fusil, Fabrice Landousy, Josette Jeusset, Marie-Odile David, Loïc Hamon, Eric Le Cam and Alain Zozime
AbstractThe adsorption of DNA molecules onto a flat mica surface is a necessary step to perform atomic force microscopy studies of DNA conformation and observe DNA-protein interactions in physiological environment. However, the phenomenon that pulls DNA molecules onto the surface is still not understood. This is a crucial issue because the DNA/surface interactions could affect the DNA biological functions. In this paper we develop a model that can explain the mechanism of the DNA adsorption onto mica. This model suggests that DNA attraction is due to the sharing of the DNA and mica counterions. The correlations between divalent counterions on both the negatively charged DNA and the mica surface can generate a net attraction force whereas the correlations between monovalent counterions are ineffective in the DNA attraction. DNA binding is then dependent on the fractional surface densities of the divalent and monovalent cations, which can compete for the mica surface and DNA neutralizations. In addition, the attraction can be enhanced when the mica has been pretreated by transition metal cations (Ni, Zn). Mica pretreatment simultaneously enhances the DNA attraction and reduces the repulsive contribution due to the electrical double-layer force. We also perform end-to-end distance measurement of DNA chains to study the binding strength. The DNA binding strength appears to be constant for a fixed fractional surface density of the divalent cations at low ionic strength (<0.1M) as predicted by the model. However, at higher ionic strength, the binding is weakened by the screening effect of the ions. Then, some equations were derived to describe the binding of a polyelectrolyte onto a charged surface. The electrostatic attraction due to the sharing of counterions is particularly effective if the polyelectrolyte and the surface have nearly the same surface charge density. This characteristic of the attraction force can explain the success of mica for performing single DNA molecule observation by AFM. In addition, we explain how a reversible binding of the DNA molecules can be obtained with a pretreated mica surface.
Abstract | Full Text | PDF (306 kb) - Interaction Forces and Morphology of a Protein-Resistant Pol...Interaction Forces and Morphology of a Protein-Resistant Poly(ethylene glycol) Layer
Biophysical Journal, Volume 88, Issue 1, 1 January 2005, Pages 495-504
M. Heuberger, T. Drobek and N.D. Spencer
AbstractThe molecular interactions on a protein-resistant surface coated with low-molecular-weight poly(ethylene glycol) (PEG) copolymer brushes are investigated using the extended surface forces apparatus. The observed interaction force is predominantly repulsive and nearly elastic. The chains are extended with respect to the Flory radius, which is in agreement with qualitative predictions of scaling theory. Comparison with theory allows the determination of relevant quantities such as brush length and adsorbed mass. Based on these results, we propose a molecular model for the adsorbed copolymer morphology. Surface-force isotherms measured at high resolution allow distinctive structural forces to be detected, suggesting the existence of a weak equilibrium network between poly(ethylene glycol) and water—a finding in accordance with the remarkable solution properties of PEG. The occurrence of a fine structure is interpreted as a water-induced restriction of the polymer's conformational space. This restriction is highly relevant for the phenomenon of PEG protein resistance. Protein adsorption requires conformational transitions, both in the protein as well as in the PEG layer, which are energetically and kinetically unfavorable.
Abstract | Full Text | PDF (273 kb) - …more
Copyright © 1996 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 71, Issue 3, 1267-1272, 1 September 1996
doi:10.1016/S0006-3495(96)79326-8
Research Article
Formation of ordered domains in membrane-bound DNA
N. Dan
Department of Chemical Engineering, University of Delaware, Newark 19716, USA. dan@che.udel.edu
Abstract
The interactions between DNA molecules adsorbed on fluid membranes are calculated. The adsorbing DNA perturbs the equilibrium packing of the lipids, thereby giving rise to membrane-induced, attractive interactions. These balance the direct repulsive interactions between DNA molecules. As a result, DNA adsorbed on membranes is predicted to form ordered domains characterized by a finite spacing, which varies with the membrane characteristics and the solution Debye screening length. Comparing the model predictions to recent experiments (Yang et al. 1996) yields excellent agreement with only one free (i.e., experimentally unknown) parameter.
