Two-dimensional crystallization of streptavidin: in pursuit of the molecular origins of structure, morphology, and thermodynamics

Heteroepitaxial streptavidin nanocrystals reveal critical role of proton "fingers" and subsurface atoms in determining adsorbed protein orientation

Characterization of noncovalent interactions between nanometer-sized structures, such as proteins, and solid surfaces is a subject of intense interest of late owing to the rapid development of numerous solid materials for medical and technological applications. Yet the rational design of these surfaces to promote the adsorption of specific nanoscale complexes is hindered by a lack of an understanding of the noncovalent interactions between nanostructures and solid surfaces. Here we take advantage of the unexpected observation of two-dimensional nanocrystals of streptavidin on muscovite mica to provide details of the streptavidin-mica interface. Analysis of atomic force microscopic images together with structural modeling identifies six positively charged residues whose terminal amine locations match the positions of the single atom-sized anionic cavities in the basal mica surface to within 1 Å. Moreover, we find that the streptavidin crystallites are oriented only along a single direction on this surface and not in either of three different directions as they must be if the protein interacted solely with the 3-fold symmetric basal surface atoms. Hence, this broken symmetry indicates that the terminal amine protons must also interact directly with the subsurface hydroxide atoms that line the bottom of these anionic cavities and generate only a single axis of symmetry. Thus, in total, these results reveal that subsurface atoms can have a significant influence on protein adsorption and orientation and identify the insertion of proton "fingers" as a means by which proteins may generally interact with solid surfaces.

Manipulating energy landscapes to tune ordering in biotemplated nanoparticle arrays

Two-dimensional non-close-packed crystals of the protein streptavidin, grown on phospholipid membranes, can serve as nanoscale templates capable of directing the formation of ordered nanoparticle arrays through site-specific electrostatic adsorption. Here we examine the effects of both interparticle and nanoparticle/lipid membrane electrostatic interactions on the degree of structural order exhibited by the templated nanoparticle array. Interparticle electrostatic repulsion is shown to have only marginal influence on nanoparticle ordering. In contrast, the degree of order exhibited by the templated array can be tuned by controlling the charge on the lipid membrane. Analysis of the local and global structure of arrays generated with negatively charged gold nanoparticles (∼6 nm) indicate improved long-range order when the lipid membrane supporting the protein crystal is derived from cationic lipid molecules as opposed to zwitterionic phospholipids. Furthermore, as nanoparticle size is reduced (∼3 nm), the presence of a charged lipid membrane is found to be essential, as smaller particles do not adhere to streptavidin crystals grown on zwitterionic membranes. These findings demonstrate that the composition of the lipid support can influence the efficacy of directed-assembly processes which utilize protein templates and are important results toward enhancing control over bottom-up nanofabrication applications.

Characterization of streptavidin binding to biotinylated, binary self-assembled thiol monolayers--influence of component ratio and solvent

Many biosensor applications are based on streptavidin (SA) binding to partially biotinylated self-assembled thiol monolayers (SAMs). In our study, binary SAMs on gold were prepared from solutions containing 16-mercapto-1-hexadecanol (thiol I) and N-(8-biotinyl-3,6-dioxa-octanamidyl)-16-mercaptohexadecanamide (thiol II) in varying component ratios. Either chloroform or ethanol was used as solvent. After 24 h thiol incubation, SA was immobilized on the resulting SAMs using the strong SA-biotin interaction. The SA binding process was monitored by QCM-D (quartz crystal microbalance monitoring dissipation factor). It is shown that the Sauerbrey equation is valid to calculate the mass quantities of the immobilized SA layers. Under the chosen incubation conditions, marginal fractions of the biotinylated component II in chloroform ((n(I)/n(II))(solution) approximately = 1000) lead to SAMs which ensure a maximal SA binding quantity of m(Sauerbrey SA) approximately = 400 ng x cm(-2), being equivalent to a SA single-layer arrangement on the SAM surface. In case of incubations from ethanolic solutions, a complete SA layer formation needs significantly higher amounts of the biotinylated component II during SAM preparation ((n(I)/n(II))(solution) approximately = 50). X-ray photoelectron spectroscopy data show that the fraction of biotinylated thiol II in the SAM determines the amount of surface-bound SA. The SAM thiol ratio ((n(I)/n(II))(SAM)) not only depends on the corresponding component ratio in the incubation solution, but is also strongly influenced by the solvent. Using chloroform as solvent during SAM preparation significantly increased the fraction of biotinylated thiol II in the SAMs compared to ethanol.

Modulating colloidal adsorption on a two-dimensional protein crystal

The geometric and physicochemical properties of the protein streptavidin make it a useful building block in the construction and manipulation of nanoscale structures and devices. However, one requirement in exploiting streptavidin for "bottom-up" assembly is the capability to modulate protein-nanoparticle interactions. This work examines the effects of pH and the biotin-streptavidin interaction on the adsorption of colloidal gold onto a two-dimensional streptavidin crystal. Particle deposition was carried out below (pH 6), at (pH 7), and above (pH 8) the protein's isoelectric point with both biotinylated and nonbiotinylated nanoparticles. Particle surface coverage depends on deposition time and pH, and increases by 1.4-10 times when biotin is incorporated onto the particle surface. This coverage is highest for both particle types at pH 6 and decreases monotonically with increasing pH. Calculations of interparticle potentials based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory demonstrate that this trend in surface coverage is most likely due to alterations in particle-surface electrostatic interactions and not a result of changes in interparticle electrostatic repulsion. Furthermore, post-adsorption alterations in pH demonstrate that electrostatically adsorbed particles can be selectively desorbed from the surface. Evaluation of the nonspecifically adsorbed fraction of biotinylated particles indicates that the receptor-ligand adsorption mechanism gives a higher rate of attachment to the substrate than nonspecific, electrostatic adsorption. This results in faster adsorption kinetics and higher coverages for biotinylated particles relative to the nonbiotinylated case.

Molecular bond formation between surfaces: anchoring and shearing effects

Specific molecular bonds between apposing surfaces play a central role in many biological structures and functions. They display a widely varying anchoring to the cell surface, and they are subject to forces that affect their binding characteristics due to their hydrodynamic environments. Here, we examine both anchoring and shearing aspects using simplified model systems aimed at gaining insight into the formation of a 2D bond collection under stress using two different surface anchors. The highly specific streptavidin-biotin molecular bond was chosen as the model receptor-ligand pair, and grafted colloids were used as model surfaces. To explore the role of the surface anchor, we grafted biotin onto the particle surface following two different approaches: first, the grafting was performed directly on the particle amine functions; second, a 35-nm-long PEG spacer was used. Hybrid particle classes were brought into contact in a homogeneous shear (between 200 s(-)(1) and 1200 s(-)(1)) using a cone plate geometry. The bond association and dissociation kinetics were given by the time course assemblage of hybrid particles into doublets. We observed saturating kinetics profiles that we interpreted as a linkage-breakage equilibrium, which yielded the on and off rates. We found that the biotin-PEG spacer was needed in order to observe significant binding at any shear rate. We also showed that only the number of collisions per unit time, generated by the shear, affected the on rate of the binding. Neither the exerted forces nor the collision lifetime had any effect. The off rate decreased with shear, possibly because of the shortening of the force duration, which results from the increasing shear rate.

Immobilization and surface characterization of NeutrAvidin biotin-binding protein on different hydrogel interlayers

For a number of potential applications, it is desirable to immobilize avidin class molecules onto solid supports and exploit their ability to bind biotinylated molecules with high affinity. NeutrAvidin molecules were surface immobilized in various ways. In this study, NeutrAvidin was covalently attached by carbodiimide chemistry onto carboxyl groups of polyacrylic acid and carboxymethyl-dextran hydrogel interlayers. A third strategy involved the affinity "docking" of NeutrAvidin onto a biotinylated poly(ethylene glycol) interlayer. These three interlayers were selected for their low nonspecific binding of proteins, which was expected to minimize surface binding of NeutrAvidin by nonspecific interfacial adsorption. X-ray photoelectron spectroscopy (XPS) analyses allowed detailed characterization of the multilayer fabrication steps. An ELISA assay was used to measure NeutrAvidin activity, which varied with the surface immobilization route. Atomic force microcopy (AFM) force measurements showed that the hydrogel interlayer contributed to a repulsive force and verified the specific interaction between biotinylated AFM tips and the NeutrAvidin surfaces. When a solution of free biotin was injected into the AFM liquid cell, the force curve changed substantially and became identical to that recorded between surfaces carrying no NeutrAvidin, indicating that the free solution biotin had displaced NeutrAvidin proteins off the PEG-biotin layer.

Single molecule research on surfaces: from analytics to construction and back

The study of single molecules opens a new dimension in understanding nature down to its finest ramifications. While much progress was achieved in the last decade concerning the detection techniques, suitable techniques for manipulating and handling the biomolecules still bear a challenge. Primarily, the task is keeping an individual, active molecule of a certain lifespan in the spot. Here, we will focus on techniques for the functional immobilization of (single) molecules on surfaces to enable their observation at one position over a time period. Presenting the main methods of reversible immobilization we will accentuate the chelator lipid concept as combining all features prerequisite for functional, reversible and well-defined immobilization. This will also show that single molecule research in principle is the synthesis of an insight into the function of nature and nano-biotechnology (manipulation): thus of analytics, construction, and back.