Adsorption Of Polypeptides Research Articles

Investigating the Sequence Specific Adsorption Behavior of Polypeptides at the Solid/Liquid Interface

Polymer adsorption at the solid/liquid interface depends not only on the chemical composition of the polymer but also on the specific placement of the monomers along the polymer sequence. However, challenges in designing polymers with well-controlled sequences have limited explorations into the role of polymer sequence on adsorption behavior to molecular simulations. Here, we demonstrate how the sequence control offered by polypeptide synthesis can be utilized to study the effects small changes in polymer sequence have on polymer adsorption behavior at the solid/liquid interface. Through a combination of quartz crystal microbalance with dissipation monitoring and total internal reflection ellipsometry, we study the adsorption behavior of three polypeptides, consisting of 90% lysine and 10% cysteine, onto a gold surface. We find different mechanisms are responsible for the adsorption of polypeptides and the resulting conformation on the surface. The initial adsorption of the polypeptides is driven by electrostatic interactions between the polylysine and the gold surface. Once adsorbed, the cysteine undergoes a thiol-Au reaction with the surface, altering the conformation of the polymer layer. Our findings suggest the conformation of the polypeptide layer is dependent on the placement of the cysteines within the sequence; polypeptide chains with evenly spaced cysteine groups adopt a more tightly bound "train" conformation as compared to polypeptides with closely grouped cysteine groups. We envision that the methodologies presented here to study sequence specific adsorption behaviors using polypeptides could be a valuable tool to complement molecular simulations studies.

Molecular mechanisms of pH-tunable stability and surface coverage of polypeptide films

Streaming potential and quartz crystal microbalance measurements, combined with all-atom molecular dynamics simulations, were used to study the pH dependency of the adsorption of two basic homopolypeptides, poly-L-lysine (PLL) and poly-L-arginine (PARG), on α-quartz surface. We report that the observed adsorption behavior rises from an interplay of i) the change in the number of possible peptide-surface ion pairs between the charged moieties and ii) repulsive electrostatic interactions between the polypeptide molecules. For low pH values, polypeptide adsorption was strongest and stable monolayers were formed. However, electrostatic repulsion between the polypeptides led to a relatively low maximum surface coverage. On the other hand, higher pH led to more weakly bound, but significantly denser, peptide films with limited stability. Simulations indicate that electrostatic interactions are the main driving force for adsorption, while hydrogen bonding and non-specific interactions also contribute. Additionally, the important role of the counterions of the negatively charged quartz surface that form a positively charged ion adlayer is highlighted. Ion release of the condensed sodium ions at the charged surface occurs via displacement by polypeptide adsorption. The mechanisms revealed by this work provide systematic guidelines to engineering active surfaces of charged peptides with controlled surface coverage and reversible binding.

A Molecular Theory of Polypeptide Adsorption at Inorganic Surfaces

A faithful description of polypeptide adsorption at ionizable surfaces remains a theoretical challenge from a molecular perspective due to the strong coupling of local thermodynamic nonideality and ionizations of both the adsorbate and substrate that are sensitive to the solution condition such as pH, ion valence, and salt concentration. Building upon a recently developed coarse-grained model for natural amino acids in bulk electrolyte solutions, here we report a molecular theory applicable to polypeptide adsorption on ionizable inorganic surfaces over a broad range of inhomogeneous conditions. Our thermodynamic model is able to account for diverse solution effects as well as the amino-acid sequence on polypeptide adsorption and surface association such as hydrogen bonding or bidentate coordination. The theoretical predictions have been validated by extensive comparison with experimental data for the adsorption isotherms of three representative polypeptides at a titanium surface.

X-ray Reflectivity Study of Polylysine Adsorption on the Surface of DMPS Monolayers.

The results of a systematic study on the adsorption of polylysine molecules of different lengths on the surface of a 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) monolayer in the liquid (LE) and condensed (LC) states are presented. A compressibility diagram and the Volta potential were recorded with the Langmuir monolayer technique and further analyzed with the empirical approach. The structure of the monolayer films with adsorbed polypeptides was studied with synchrotron X-ray reflectometry. Two- and three-layer slab models describe the reflectivity data fairly well and reveal both the significant structural changes and the dehydration of the polar groups induced by all polylysines used at the maximal coverage of the monolayer interface in both the LE and LC states. On the one hand, in the LE phase of the monolayer (area per molecule A ≅ 70 Ǻ2), the integrated electron density of the lipid headgroup region is approximately half the density contained in the clean monolayer. This indicates both significant compaction and dehydration in the polar groups of the lipids, caused by the adsorption of polypeptides. On the other hand, in the LC state (A ≅ 40 Ǻ2), the degree of the hydration of the polar region is similar to that for the initial DMPS monolayer. However, both the electron density and the thickness of the head group region differ significantly from the values of these parameters for the clean monolayer in the LC state.

Influence of Ionic Strength on Adsorption of Polypeptides on Lipid Membranes: Theoretical Analysis

Influence of Ionic Strength on Adsorption of Polypeptides on Lipid Membranes: Theoretical Analysis

Polypeptide-Nanoparticle Interactions and Corona Formation Investigated by Monte Carlo Simulations.

Biomacromolecule activity is usually related to its ability to keep a specific structure. However, in solution, many parameters (pH, ionic strength) and external compounds (polyelectrolytes, nanoparticles) can modify biomacromolecule structure as well as acid/base properties, thus resulting in a loss of activity and denaturation. In this paper, the impact of neutral and charged nanoparticles (NPs) is investigated by Monte Carlo simulations on polypeptide (PP) chains with primary structure based on bovine serum albumin. The influence of pH, salt valency, and NP surface charge density is systematically studied. It is found that the PP is extended at extreme pH, when no complex formation is observed, and folded at physiological pH. PP adsorption around oppositely-charged NPs strongly limits chain structural changes and modifies its acid/base properties. At physiological pH, the complex formation occurs only with positively-charged NPs. The presence of salts, in particular those with trivalent cations, introduces additional electrostatic interactions, resulting in a mitigation of the impact of negative NPs. Thus, the corona structure is less dense with locally-desorbed segments. On the contrary, very limited impact of salt cation valency is observed when NPs are positive, due to the absence of competitive effects between multivalent cations and NP.

Unraveling origins of the heterogeneous curvature dependence of polypeptide interactions with carbon nanostructures.

Emerging nanotechnology has rapidly broadened interfacial prospects of biological molecules with carbon nanomaterials (CNs). A prerequisite for effectively harnessing such hybrid materials is a multi-faceted understanding of their complex interfacial interactions as functions of the physico-chemical characteristics and the surface topography of the individual components. In this article, we address the origins of the curvature dependence of polypeptide adsorption on CN surfaces (CNSs), a phenomenon bearing an acute influence upon the behavior and activity of CN-protein conjugates. Our benchmark molecular dynamics (MD) simulations with the amphiphilic full-length amyloid beta (Aβ) peptide demonstrate that protein adsorption is strongest on the concave (inner) CN surface, weakest on the convex (outer) surface, and intermediary on the planar surface, in agreement with recent experimental reports. The curvature effects, however, are found to manifest non-uniformly between the amino acid subtypes. To understand the underlying interplay of the chemical nature of the amino acids and surface topography of the CNs, we performed high-level quantum chemical (QM) calculations with amino acid analogs (AAA) representing their five prominent classes, and convex, concave and planar CN fragments. Molecular electrostatic potential maps reveal pronounced curvature dependence in the mixing of electron densities, and a resulting variance in the stabilization of the non-covalently bound molecular complexes. Interestingly, our study revealed that the interaction trends of the high-level QM calculations were captured well by the empirical force field. The findings in this study have important bearing upon the design of carbon based bio-nanomaterials, and additionally, provide valuable insights into the accuracy of various computational techniques for probing non-bonded interfacial interactions.

Interactions of Amino Acids and Polypeptides with Metal Oxide Nanoparticles Probed by Fluorescent Indicator Adsorption and Displacement

The adsorption of polypeptides containing an N-terminal tryptophan (Trp) residue attached to a hexa-backbone of alanine, serine, lysine, histidine, and aspartate was investigated by monitoring the fluorescence response of the Trp chromophore upon titration with metal oxide nanoparticles (MOx-NPs: CuO, Co(3)O(4), TiO(2), MgO, and CeO(2)). After correction for light-scattering effects, a strong static fluorescence quenching was observed upon addition of CuO and Co(3)O(4) to the peptides. The interaction of MOx-NPs with the peptides was assigned to an adsorption of the peptide backbone on the nanoparticle surface. The method was refined using a derivatized amino acid, 5-fluoro-Trp (5F-Trp), which resulted in a stronger fluorescence response. The use of the fluorescent amino acid labels allowed the direct assessment of the adsorption propensities of Trp-containing peptides in dependence on the backbone, which was verified by zeta-potential measurements. Moreover, upon addition of different analytes to nanoparticles with preadsorbed Trp-containing polypeptides, adsorption propensities of the analytes were assessed by an indicator displacement strategy; that is, addition of increasing amounts of analyte resulted in a continuous fluorescence enhancement/recovery. This method afforded adsorption propensities for several analytes. The relative binding constants for the MOx-NPs, obtained from the competitive titrations, varied by more than 6 orders of magnitude for CuO (5F-TrpHis(6)-NH(2) > TrpAsp(6)-NH(2), TrpSer(6)-NH(2) > TrpLys(6)-NH(2), Trp, 5F-Trp > TrpAla(6)-NH(2)) but only 4 for Co(3)O(4) (TrpHis(6)-NH(2), TrpAsp(6)-NH(2) ≫ TrpLys(6)-NH(2), TrpAla(6)-NH(2), TrpSer(6)-NH(2), Trp, 5F-Trp). The study reveals that MOx-NPs adsorb biomolecular analytes with high selectivity, which has immediate implications for their applications in protein purification, drug delivery, and, potentially, for the assessment of their toxicology.

Spectral dependence of gold nanorods on the optical properties of substrates and adsorption of polypeptides

Gold nanorods (NRs) wrapped with poly(styrene sulfonate) were fixed on glass or indium tin oxide (ITO) substrates that had been modified with polycationic polymer or 3-aminopropyltrimethoxysilane layers. The spectral properties of the NRs were affected by the refractive indices of the substrates and their surface conditions. Surface modification with poly(styrene sulfonate) and avidin molecules shifted the longitudinal surface plasmon (SP) bands to longer wavelength when the substrates were placed in air. In water and a buffered solution, red shifts of the SP bands were also observed, but the shift shifts were not as large as that in the air. It was found that refractive indices around the polymer-wrapped NRs were sensitive to the adsorption of organic molecules even in the aqueous media.

Crystallization of Calcium Oxalates Is Controlled by Molecular Hydrophilicity and Specific Polyanion-Crystal Interactions

To gain more insight into protein structure-function relationships that govern ectopic biomineralization processes in kidney stone formation, we have studied the ability of urinary proteins (Tamm-Horsfall protein, osteopontin (OPN), prothrombin fragment 1 (PTF1), bikunin, lysozyme, albumin, fetuin-A), and model compounds (a bikunin fragment, recombinant-, milk-, bone osteopontin, poly-L-aspartic acid (poly asp), poly-L-glutamic acid (poly glu)) in modulating precipitation reactions of kidney stone-related calcium oxalate mono- and dihydrates (COM, COD). Combining scanning confocal microscopy and fluorescence imaging, we determined the crystal faces of COM with which these polypeptides interact; using scanning electron microscopy, we characterized their effects on crystal habits and precipitated volumes. Our findings demonstrate that polypeptide adsorption to COM crystals is dictated first by the polypeptide's affinity for the crystal followed by its preference for a crystal face: basic and relatively hydrophobic macromolecules show no adsorption, while acidic and more hydrophilic polypeptides adsorb either nonspecifically to all faces of COM or preferentially to {100}/{121} edges and {100} faces. However, investigating calcium oxalates grown in the presence of these polypeptides showed that some acidic proteins that adsorb to crystals do not affect crystallization, even if present in excess of physiological concentrations. These proteins (albumin, bikunin, PTF1, recombinant OPN) have estimated total hydrophilicities from 200 to 850 kJ/mol and net negative charges from -9 to -35, perhaps representing a "window" in which proteins adsorb and coat urinary crystals (support of excretion) without affecting crystallization. Strongest effects on crystallization were observed for polypeptides that are either highly hydrophilic (>950 kJ/mol) and highly carboxylated (poly asp, poly glu), or else highly hydrophilic and highly phosphorylated (native OPN isoforms), suggesting that highly hydrophilic proteins strongly affect precipitation processes in the urinary tract. Therefore, the level of hydrophilicity and net charge is a critical factor in the ability of polypeptides to affect crystallization and to regulate biomineralization processes.

Synthetic polypeptide adsorption to Cu-IDA containing lipid films: a model for protein-membrane interactions.

Adsorption of synthetic alanine-rich peptides to lipid monolayers was studied by X-ray and neutron reflectivity, grazing incidence X-ray diffraction (GIXD), and circular dichroic spectroscopy. The peptides contained histidine residues to drive adsorption to Langmuir monolayers of lipids with iminodiacetate headgroups loaded with Cu2+. Adsorption was found to be irreversible with respect to bulk peptide concentration. The peptides were partially helical in solution at room temperature, the temperature of the adsorption assays. Comparisons of the rate of binding and the structure of the adsorbed layer were made as a function of the number of histidines (from 0 to 2) and also as a function of the positioning of the histidines along the backbone. For peptides containing two histidines on the same side of the helical backbone, large differences were observed in the structure of the adsorbed layer as a function of the spacing of the histidines. With a spacing of 6 A, there was a substantial increase in helicity upon binding (from 17% to 31%), and the peptides adsorbed to a final density approaching that of a nearly completed monolayer of alpha-helices adsorbed side-on. The thickness of the adsorbed layer (17 +/- 2.5 A) was slightly greater than the diameter of alpha-helices, suggesting that the free, unstructured ends extended into solution. With a spacing of 30 A between histidines, a far weaker increase in helicity upon binding was observed (from 13% to 19%) and a much lower packing density resulted. The thickness of the adsorbed layer (10 +/- 4 A) was smaller, consistent with the ends being bound to the monolayer. Striking differences were observed in the interaction of the two types of peptide with the lipid membrane by GIXD, consistent with binding by two correlated sites only for the case of 6 A spacing. All these results are attributed to differences in spatial correlation between the histidines as a function of separation distance along the backbone for these partially helical peptides. Finally, control over orientation was demonstrated by placing a histidine on an end of the sequence, which resulted in adsorbed peptides oriented perpendicular to the membrane.

Highly sensitive and quantitative analysis of polyeptides using a new gradient system based on an abrupt change in adsorption of polypeptide to the reversed‐phase column packing

During a study of 100 microL aliquots of urocortin containing various acetonitrile contents, we hypothesized that a change in the acetonitrile content in the solution across a specific content of acetonitrile (critical threshold) causes an abrupt change in adsorption capacity to the column packing. Circular dichroism measurements suggest that the conformational change induced by acetonitrile in the solution causes the abrupt change in adsorption capacity, and this solvent-induced conformational change is reversible across the critical threshold. This hypothesis can apply to various polypeptides with molecular weights range from 1007 to 6789 and to other organic solvents. A new gradient system utilizing the instant recovery of the adsorption capacity across the critical threshold was designed, and applied to the analysis of a 100 microL aliquot of various polypeptide solutions. The results suggest that use of a solution containing organic solvents more than the critical threshold allows successful dilution of polypeptides up to picomolar concentration range without any loss due to its adsorption during handling procedures and loading onto the LC system, and that a new gradient system enables quantitative analysis of polypeptides at picomolar concentrations in such solutions.

Salt Concentration and pH-Dependent Adsorption of Two Polypeptides on Planar and Divided Alumina Surfaces. In Situ IR Investigations

The adsorption of proteins is the first process to take place when a solid is immersed in a biological fluid; though not yet thoroughly understood at a molecular level, this process is also known to be strongly influenced by the presence of salt in solution or by pH changes. In the present work, poly-L-glutamic acid (PG) and poly-L-lysine (PL) were selected to mimic the behavior of some protein fragments. Their adsorption was investigated by infrared spectroscopy in various modes, both on planar and on divided (powder) surfaces of aluminum oxide. These two peptides were shown to have different behaviors when adsorbed from solutions with or without CaCl2 and at various pH values. Polarization modulation-reflection absorption infrared spectroscopy, applied in a special cell designed to characterize the solid surface in contact with the liquid, enabled the observation of the influence of pH and salts upon polypeptide adsorption. At pH values higher than 5 and in the presence of CaCl2 in solution, a net increase of the PG adsorbed amount is observed, whereas no such effect could be detected for PL. Specific interactions between the COO- groups on the side chains and the surface, or between those of two different molecules, was inferred. Interestingly, similar conclusions could be drawn for the surface of alumina powders contacted with solutions of PG and PL and characterized by attenuated total reflectance IR. This work demonstrates the potential for IR investigations of solid oxide-liquid interfaces combining the study of planar and finely divided surfaces.

Computer simulation of polypeptide adsorption on model biomaterials

When biomaterials are inserted in a biological environment, for instance in a body implant, proteins do quickly adsorb on the exposed surface. Such process is of fundamental importance, since it directs the subsequent cell adhesion. Here we review recent advances in this field obtained with molecular simulations. While coarse-grained models can provide important general results, as it has long been recognized in polymer science, the hierarchical structure of a very complex copolymer such as a protein, together with the nature of the biomaterial surface suggest that atomistic models are better suited to investigate these phenomena. Thus, after briefly mentioning some common features of coarse-grained and atomistic force fields, we first discuss early theoretical and coarse-grained simulation results about protein adsorption, and then we highlight the main results recently obtained by us with atomistic models. In particular, we discuss some conformational and energetic aspects of the adsorption of protein fragments with different secondary structure on surfaces of different wettability, including hydrophobic graphite and hydrophilic poly(vinylalcohol). We also consider other features, such as the simulation of the materials wettability, the hydration of the adsorbed fragments, their kinetics of spreading, and the sequential adsorption of two protein fragments on top of each other, highlighting the results of general interest.

Polypeptide Multilayer Films: Role of Molecular Structure and Charge

The role of molecular structure, charge, and hydrophobicity in polyelectrolyte layer-by-layer assembly (LbL) of thin films has been studied using the model polypeptides poly-L-glutamatic acid (PLGA) and poly-L-lysine (PLL), quartz crystal microbalance (QCM), and circular dichroism spectroscopy (CD). The adsorption behavior of PLGA and PLL has been compared with the structure of these molecules in aqueous solution under the same conditions. The data show that the deposition of polypeptide per adsorption step scales with average secondary structure content, whether alpha helix or beta sheet. This is contrary to the expectation based on the view that hydrogen bonds are crucial to polypeptide film assembly, because secondary structure formation in a polypeptide reduces its intermolecular hydrogen-bonding potential. The data also show that polypeptide adsorption scales with ionic strength and chain length. Taken together, the results increase knowledge of polypeptide-based LbL thin film fabrication and will help to provide a firmer foundation for the use of natural or designed polypeptides in LbL.

Polypeptide Interactions at Ice and Biomineral Interfaces Are Defined by Secondary Structure-Dependent Chain Orientations

The stereospecific interaction of polypeptides with biominerals and ice represents a unique phenomenon in nature. Here, using the theoretical geometric lattice matching algorithm, CHASM (abbreviation for chain alignment on the surface of materials), we explore the geometric complementarity of specific Cα polypeptide secondary structures (α-helix, β-sheet, γ-turn) with the [001] planes of the biominerals, aragonite and calcite, and the [001], [100], and [201] faces of hexagonal ice. We find that certain secondary structure−surface pairings exhibit a defined set of orientational minima that agree closely with published results. Moreover, no two secondary structures feature the exact same set of global and local orientational minima for a given surface. This suggests that the molecular complementarity of each secondary structure with a given surface is highly specific and cannot be mimicked by another secondary structure. These results suggest that the site and orientation of polypeptide adsorption onto biominerals or ice may be influenced by secondary structure type.

Surface-Enhanced Raman Scattering (SERS) Spectroscopy with Borohydride-Reduced Silver Colloids: Controlling Adsorption of the Scattering Species by Surface Potential of Silver Colloid

Formation of Ag colloid/adsorbate SERS-active systems (upon adsorption of the selected adsorbates on the surface of Ag colloidal particles) as a function of (i) NaBH4 to AgNO3 molar ratio in the preparation protocol of Ag colloid, and (ii) aging of the colloid has been investigated by Surface-enhanced Raman scattering (SERS) spectroscopy. Oligomeric synthetic polypeptides, bovine serum albumin, phosphate coadsorbed with CuTMePyP [copper(II) derivative of 5,10,15,20-tetrakis-(N-methylpyridinium-4-yl)porphyrin chloride] and borates in systems with N-containing bases were selected as model adsorbates. Both (i) a decrease of NaBH4 to AgNO3 molar ratio upon preparation and (ii) aging of Ag colloid affect adsorption of the adsorbates and consequently, their SERS spectra, in the same manner. Aging of Ag colloid is thus viewed as a slow hydrolysis of BH4- anions. The actual concentration of BH4- in the system is identified as the most important factor controlling adsorption of all the selected adsorbates on the surface of Ag colloid. As this factor can be related to the surface potential, the conditions controlling adsorption of the selected adsorbates are specified in terms of a more negative and/or more positive surface potential of Ag colloidal particles. A more positive surface potential promotes adsorption of polypeptides, bovine serum albumin and phosphate while observation of spectral features of borates in the SERS spectra of N-containing bases in alkaline solutions is conditioned by a more negative surface potential.

Adsorption of polypeptides on a solid surface. IV. Adsorption and β‐structure–coil transition

AbstractA theory of adsorption of a polypetide chain capable of undergoing the coil–β‐structure transition on a solid planar surface has been developed. The mutual influence of two order–disorder phase transitions, a conformational and an adsorption transition, was investigated. Various types of adsorption transitions are possible, depending on the initial conformational state (partly or completely β‐structured) and the selectivity of adsorption: (a) the second‐order phase transition, in which the chain is partly structured, both in adsorbed and desorbed states; and (b) the first‐order phase transition, in which the chain exhibits a regular β‐structure, at least on one side of the adsorption transition boundary. The chain bonding to the surface alters the degree of β‐structure, both in the case of selective and nonselective adsorption (similar to the adsorption of the chains with other types of secondary structure). We show that the slope of the adsorption curves for partly β‐structural chains increases as a result of an increase in the degree of β‐structuring, and this effect is even stronger than the analogous effect of β‐structuring.

The Adsorption of Polypeptides on Polystyrene Latices and Their Effects on Colloid Stability

Abstract The adsorption behavior of poly(l-lysine) and poly(l-glutamic acid) on negatively and positively charged latices was examined in combination with their stabilizing effects on the latex dispersion. It was realized that the polypeptide adsorption was, to a large extent, determined by the coulombic interaction between the polypeptide and the charged surface, and by the conformational change (helix-coil transition) in the polypeptide in the solution. The flocculation behavior of the dispersion in the presence of polypeptide was also explained reasonably by taking account of the results of electrophoretic measurements of the polypeptide-coated particles.

Adsorption of polypeptides on the solid surface. II. Effect of secondary chain structure and helix–coil transition

AbstractThe theory of adsorption of semistiff chains on a planar surface developed by the authors previously has been used to consider the helix–coil transition in single‐stranded macromolecule interacting with an adsorbent plane. The cases of nonselective interaction when the adsorption energy is independent of the unit conformation (a) and selective interaction with only helical (b) or coiled (c) sequences active in adsorption were investigated. In case (b) the existence of secondary structure favors chain bonding to the surface. This leads to the increase in the stability of the helical state and complete polypeptide chain spiralization. The profile of the conformational helix–coil transition acquires an asymmetrical shape inherent to the second‐order phase transition.In case (c) the bonding of a partially helical chain to the surface is similar to the adsorption of Gaussian coils and is accompanied by the destruction of secondary structure, this destruction being appreciable even if the helical state in space was favorable.