XPS studies of amino acids adsorbed on titanium dioxide surfaces

Energy Landscape of Relaxation and Interaction of an Amino Acid, Glutamine (L), on Pristine and Au/Ag/Cu-Doped TiO2 Surfaces

Studying the interaction of inorganic systems with organic ones is a highly important avenue for finding new drugs and treatment methods. Tumor cells show an increased demand for amino acids due to their rapid proliferation; thus, targeting their metabolism is becoming a potential oncological therapeutic strategy. One of the inorganic materials that show antitumor properties is titanium dioxide, while its doping was found to enhance interactions with biological systems. Thus, in this study, we investigated the energy landscape of glutamine (L), an amino acid, on pristine and doped TiO2 surfaces. We first locally optimized 2D-slab structures of pristine and Au/Ag/Cu-doped anatase (001 and 101 surfaces) and similarly optimized a single molecule of glutamine in vacuum. Next, we placed the pre-optimized glutamine molecule in various orientations and on a variety of locations onto the relaxed substrate surfaces (in vacuum) and performed ab initio relaxations of the molecule on the substrate slabs. We employed the DFT method with a GGA-PBE functional implemented in the Quantum Espresso code. Comparisons of the optimized conformations and electronic structures of the amino acid in vacuum and on the surfaces yield useful insights into various biological processes.

Amorphous Calcium Phosphate and Amorphous Calcium Phosphate Carboxylate: Synthesis and Characterization

Amorphous calcium phosphate (ACP) is the first solid phase precipitated from a supersaturated calcium phosphate solution. Naturally, ACP is formed during the initial stages of biomineralization and stabilized by an organic compound. Carboxylic groups containing organic compounds are known to regulate the nucleation and crystallization of hydroxyapatite. Therefore, from a biomimetic point of view, the synthesis of carboxylate ions containing ACP (ACPC) is valuable. Usually, ACP is synthesized with fewer steps than ACPC. The precipitation reaction of ACP is rapid and influenced by pH, temperature, precursor concentration, stirring conditions, and reaction time. Due to phosphates triprotic nature, controlling pH in a multistep approach becomes tedious. Here, we developed a new ACP and ACPC synthesis approach and thoroughly characterized the obtained materials. Results from vibration spectroscopy, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), true density, specific surface area, and ion release studies have shown a difference in the physiochemical properties of the ACP and ACPC. Additionally, the effect of a carboxylic ion type on the physiochemical properties of ACPC was characterized. All of the ACPs and ACPCs were synthesized in sterile conditions, and in vitro analysis was performed using MC-3T3E1 cells, revealing the cytocompatibility of the synthesized ACPs and ACPCs, of which the ACPC synthesized with citrate showed the highest cell viability.

Adsorption and Mechanism of Glycine on the Anatase with Exposed (001) and (101) Facets

As a widely existing mineral types on Earth, semiconductor minerals play an important role in the origin of life and the material geochemical cycle. The first step of peptide formation is amino acid adsorption on the mineral surface, but the role and mechanism of different crystal facets of semiconductor minerals are not well understood. Anatase (TiO2) with exposed (001) facets was synthesized by a hydrothermal method, and then analyzed and compared with the purchased ordinary anatase (TiO2) for the adsorption of glycine, the simplest amino acid. XRD, SEM and TEM results show that the hydrothermally synthesized anatase (TiO2) has a good anatase crystal form, which is micro-nano-scale flake particles and mainly composed of (001) facets. The results of HPLC used in the adsorption experiment showed that under optimal conditions (pH 5 to 6, an adsorption time of 24 h, and an initial concentration of 0.09 mol/L), the adsorption quantity of glycine on anatase (TiO2) with exposed (001) facets may reach 10 mg/m2, which is larger than that for ordinary anatase (TiO2) with exposed (101) facets. Based on a combination of various characterizations and simulation calculations, the results proved that anatase can activate thermodynamically stable γ-glycine to β-glycine. The adsorption of glycine on anatase (TiO2) has two forms, one is the zwitterionic form in which the carboxyl group forms a bridge structure with two Ti atoms connected by surface bridging oxygen, and the dissociated form is in which the amino group forms a bond with the surface Ti atom. Among these, glycine is mainly adsorbed to anatase by dissociative molecules on the anatase (TiO2) with exposed (001) facets and by zwitterion adsorption on the anatase (TiO2) with exposed (101) facets. This research elucidates the conditions and mechanism of amino acid adsorption by semiconductor minerals in weak acidic environment, which is similar to the environmental pH that was beneficial to the formation of life on the early Earth. Therefore, these can provide a reference for the further study of the role of semiconductor minerals in the adsorption and polymerization of small biomolecules in the origin of life.

Local electronic structure of histidine in aqueous solution

X-Ray spectroscopy coupled with DFT calculations reveals the pH dependent electronic structure of an amino acid in an aqueous environment.

On interaction of arginine, cysteine and guanine with a nano-TiO2 cluster

Nanoscopic properties of TiO₂ augmented with its physicochemical properties and biocompatibility make it a material interest in the biomedical field. Efficient methods to design of such materials require a thorough understanding of associated nano-bio interfaces. In the present study, density functional theory calculations were performed to study the interactions of arginine, cysteine and guanine with a nano-TiO₂ cluster. Different configurations were sampled for the adsorption of arginine, cysteine and guanine to probe the nano-bio interface via the interaction of various functional groups present on biomolecules. Adsorption energies for arginine, cysteine and guanine were in a range of -25.0 to -57.6, -12.1 to -29.6 and -45.6 to -58.7 kcal/mol, respectively. From the change in adsorption energies and free energies, interaction of amino acids with carboxylic (COOH), thiol (SH) and amine (NH₂) groups while the interaction of the nucleobase via O bonded to C and N of purine ring was found to be essential for thermodynamically stable and energetically favorable states. Density of states analysis also disclosed the prominent interactions of the biomolecules with the nano-TiO₂ cluster. Decrease in band gaps on adsorption of the biomolecules was a pertinent phenomenon indicating the strong chemical interactions of the biomolecules with the nanoscopic TiO₂ chosen for analysis in this study.

Adsorption of Native Amino Acids on Nanocrystalline TiO2: Physical Chemistry, QSPR, and Theoretical Modeling

The affinity of biomolecules, such as peptides and proteins, with inorganic surfaces, is a fundamental topic in biotechnology and bionanotechnology. Amino acids are often used as "model" bits of peptides or proteins for studying their properties in different environments and/or developing functional surfaces. Despite great demand for knowledge about amino acid interactions with metal oxide surfaces, studies on the issue represent a fragmentary picture. In this paper, we describe amino acid adsorption on nanocrystalline anatase systematically at uniform conditions. Analysis of the Gibbs free adsorption energy indicated how the aliphatic, aromatic, polar, and charged side chain groups affect the binding affinity of the amino acids. Thermodynamic features of the l-amino acid adsorption receive thorough interpretation with calculated molecular descriptors. Theoretical modeling shows that amino acids complex with TiO₂ nanoparticles as zwitterions via ammonium group.

Electrochemical behavior of titanium and platinum in dicarboxilic amino acids solution

Titanium and platinum samples as components of bimetallic implants for the osteoregeneration process have been modified in solutions modeling biological systems and studied by means of cyclic voltammetry, electrochemical impedance spectroscopy and scanning probe microscopy. While aspartic and glutamic acids did not adsorb significantly on platinum in the potential region investigated, the presence of the amino acids affects oxide layer growth on the titanium surface under anodic polarization. The two studied amino acids behave differently on the titanium electrode surface due to differences in adsorption modes of these substances. The adsorption of the glutamic acid depends on the polarization potential to a large extent, and most of quantitative adsorption characteristics (EIS data, the surface roughness) undergo drastic change at the polarization potential value of 750 mV (vs. Ag/AgCl) in the presence of this substance. Equivalent circuit modeling of the surface processes has been carried out, and a scheme for aspartic and glutamic acid adsorption onto the titanium surface has proposed.

Biofunctionalization of TiO2 surfaces with self-assembling oligopeptides in different pH and Ionic Strength conditions: Charge effects and molecular organization

Self-assembling peptides (SAPs) were investigated by means of XPS and Angular Dependent NEXAFS spectroscopies, with the aim to probe the influence of pH and Ionic Strength conditions on the chemical structure and molecular organization of SAPs anchored on titania surfaces. XPS at the C1s, N1s, O1s core levels allowed to study surfaces and biomolecule/substrate interfaces. NEXAFS data allowed ascertaining that SAPs molecular structure is preserved upon grafting to the titania surface. Angular Dependent NEXAFS was used to investigate the influence of environmental conditions on the molecular organization behaviour. The objective of our study was to establish a set of methodologies for obtaining arrangements of well-organized biomolecules on scaffolds surfaces as a basic technology to develop and optimize cells adhesion and proliferation for tissue engineering applications.

Adsorption of Amino Acids and Peptides on Metal and Oxide Surfaces in Water Environment: A Synthetic and Prospective Review

Amino acids and peptides are often used as "model" segments of proteins for studying their behavior in various types of environments, and/or elaborating functional surfaces. Indeed, though the protein behavior is much more complex than that of their isolated segments, knowledge of the binding mode as well as of the chemical structure of peptides on metal or oxide surfaces is a significant step toward the control of materials in a biological environment. Such knowledge has considerably increased in the past few years, thanks to the combination of advanced characterization techniques and of modeling methods. Investigations of biomolecule-surface interactions in water/solvent environments are quite numerous, but only in a few cases is it possible to reach an understanding of the molecule-(water)-surface interaction with a level of detail comparable to that of the UHV studies. This contribution aims at reviewing the recent data describing the amino acid and peptide interaction with metal or oxide surfaces in the presence of water.

Formation of a thermally stable bilayer of coadsorbed intact and deprotonated thymine exploiting the surface corrugation of rutile TiO2(110)

Adsorption of thymine on rutile TiO₂(110) leads to a room temperature stable bilayer which follows the corrugation of the oxide surface and consists of both intact and deprotonated molecules.

Binding affinities of amino acid analogues at the charged aqueous titania interface: implications for titania-binding peptides

Despite the extensive utilization of biomolecule-titania interfaces, biomolecular recognition and interactions at the aqueous titania interface remain far from being fully understood. Here, atomistic molecular dynamics simulations, in partnership with metadynamics, are used to calculate the free energy of adsorption of different amino acid side chain analogues at the negatively-charged aqueous rutile TiO2 (110) interface, under conditions corresponding with neutral pH. Our calculations predict that charged amino acid analogues have a relatively high affinity to the titania surface, with the arginine analogue predicted to be the strongest binder. Interactions between uncharged amino acid analogues and titania are found to be repulsive or weak at best. All of the residues that bound to the negatively-charged interface show a relatively stronger adsorption compared with the charge-neutral interface, including the negatively-charged analogue. Of the analogues that are found to bind to the titania surface, the rank ordering of the binding affinities is predicted to be "arginine" > "lysine" ≈ aspartic acid > "serine". This is the same ordering as was found previously for the charge-neutral aqueous titania interface. Our results show very good agreement with available experimental data and can provide a baseline for the interpretation of peptide-TiO2 adsorption data.

Cysteine on TiO2(110): a theoretical study by reactive dynamics and photoemission spectra simulation

Owing to the importance of bioinorganic interface properties for the biocompatibility of implants and for biosensing technology, it has become indispensable to gain understanding of their crucial structure-property relations at the atomistic level. Motivated by this fact, we use cysteine amino acid on perfect and defective TiO2(110) surfaces as model systems and study adsorption by means of classical all-atom reactive molecular dynamics and ab initio O 1s, N 1s, and S 2p photoemission spectra (XPS) simulations of the most relevant adsorbate structures. By analysis of the dynamics results and a detailed comparison with spectra recently collected for this adsorbate, we obtain conclusions of both general and particular character. It is shown that the interaction of cysteine with the TiO2(110) surface has multipoint character involving the carboxylic group as well as the amino and sulfur groups. The proton-transfer reactivity of cysteine is enhanced by the presence of the surface, and different forms of cysteines are confirmed to be present in the adsorbate. A general conclusion is that reactive force field dynamics combined with selected spectroscopy provides a viable path to understanding bioinorganic surfaces with ramifications for the design of such surfaces for future technological applications.

Selective adsorption ofl-serine functional groups on the anatase TiO2(101) surface in benthic microbial fuel cells

In unmediated benthic microbial fuel cells, the titania anode surface as a promising candidate can have effective interactions with the carboxylic and hydroxyl groups of bacteria or pili.

Effect of Manufacturing Process on the Biocompatibility and Mechanical Properties of Ti-30Ta Alloy

Ti alloys have been widely used in the aerospace, chemical, and biomedical industries for their high strength/weight ratio and corrosion resistance. However, Nitinol's usage in the latter industry has been fraught with concerns of allergic and toxic effects of Ni released from implants. Recently, much attention has been placed on the development of Ni-free Ti-Ta alloys, which are considered prime candidates for applications such as metal-on-metal spinal disk replacements, orthopedic implants, cardiovascular stents, dental posts, and guide wires. In this research, the biocompatibility of Ti-30Ta alloys manufactured by powder metallurgy (PM) and arc melting (ARC) were investigated. The corrosion resistance of each alloy was determined in accordance with ASTM F 2129-08 in phosphate buffered saline (PBS) and PBS with amino acids at 37 °C. The concentration of metal ions released during corrosion was measured by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). Scanning Electron Microscopy (SEM) was used to assess the morphology of the alloys before and after corrosion. Vicker's hardness tests were performed to compare the hardness and tensile strength of the alloys. Human osteoblast cells were successfully grown on the surface of both alloys.

A new route to size and population control of silver clusters on colloidal TiO₂ nanocrystals

Formation of hybrid Ag-TiO(2) nanocrystals (NCs) in which Ag clusters are uniformly deposited on individual TiO(2) NC surface has been achieved by using hydrophobic surfactant-capped TiO(2) NCs in combination with a photodeposition technique. The population of Ag clusters on the individual TiO(2) NC surface can be controlled by the degree of hydrophobicity (e.g., the number of vacant sites) on the TiO(2) NC surface while their size may be altered simply by varying irradiation time. A reversible change in color of the resulting hybrid Ag-TiO(2) NCs is induced by alternating UV light and visible-light illumination; however, the size and population of Ag clusters on TiO(2) NCs are almost unchanged. Furthermore, these materials also exhibit much higher photocatalytic performance as compared to that of Ag supported on commercial TiO(2)-P25.

Biomimetic formation of titania thin films: effect of amino acids on the deposition process

Different types of amino acids have been used as additives to control the aqueous deposition of titanium dioxide thin films on single-crystal Si wafers. Thin titania films can be obtained through a chemical bath deposition (CBD) process using TiCl₄ as a precursor in an aqueous solution at temperatures below 100 °C. The addition of amino acids to the deposition solution was shown to reduce the thickness and roughness of the films and to increase their density. These protein building blocks were employed to modify the deposition rate as well as the size of aggregates that form the film. The thickness, crystallinity, morphology and composition of the grown films were characterized by a variety of techniques, including XRD, XPS, AFM and SEM. The consequences of the type of the amino acid additive (and its concentration in the solution) on the microstructural evolutions of the deposed films are thus revealed and discussed on the basis of the organic-inorganic interactions in solution and at the film surface.

Corrosion behavior of nitinol in blood serum and PBS containing amino acids

The susceptibility to localized corrosion of metallic, implant medical devices is typically evaluated in a simulated physiological solution, such as phosphate buffered saline (PBS). For implant devices in contact with blood, the underlying premise is that proteins and other components in blood do not significantly change the corrosion susceptibility. This study examined the corrosion behavior of nitinol in bovine serum and PBS containing amino acids (cysteine, glutamine, and tryptophan). Tests were performed on mechanically polished (MP) and black oxide (BO) nitinol wire using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy. They showed that the susceptibility to pitting corrosion of MP nitinol in PBS can be influenced by the presence of amino acids, depending on the type of amino acid. However, the pitting susceptibility of MP and BO nitinol did not differ significantly (based on a t-test, with p < 0.05 being statistically significant) between serum and PBS, suggesting that the combination of proteins and amino acids in serum had little effect. The impedance spectra showed near-capacitive behavior in serum and in PBS alone and with amino acids, and the data could be fitted by a parallel resistance-capacitance (as a constant phase element) circuit associated with the passive oxide film. The capacitance indicated that serum proteins were adsorbed on nitinol but that little, if any, adsorption of the three amino acids under study occurred at the corrosion potential. There did not appear to be a correlation between breakdown and the adsorption of organic compounds in serum.

Hydrophobic amino acid adsorption on surfaces of varying wettability

The adsorption behavior of the model nonpolar amino acid leucine at a solid surface of tunable wetting ability has been examined by means of molecular dynamics simulation. When leucine adsorbs at a highly hydrophobic surface it is found to orient itself with its nonpolar side chain toward the surface and its charged amine and carboxyl groups directed toward the bulk. When the surface is less hydrophobic, leucine alternates between two stable orientations, a standing orientation like that observed at nonwetting surfaces, and a laying orientation in which its side chain and charged groups are located nearly the same distance from the surface. These results are rationalized by a water-density-dependent ordering scheme in which interfacial water structure governs the adsorbed structures. We pay particular attention to how the structure of surface water is changed in the presence of adsorbed leucine in both orientations. We propose that a water density model may be applied as a general technique for understanding adsorption from solution at solid hydrophobic surfaces.

Characterization of surfaces presenting covalently immobilized oligopeptides using near-edge X-ray absorption fine structure spectroscopy

This study addresses the need for methods that validate the surface chemistry leading to the immobilization of biomolecules and provide information about the resulting structural configurations. We report on the use of near-edge X-ray absorption fine structure spectroscopy (NEXAFS) to characterize a widely employed immobilization chemistry that leads to the covalent attachment of a biologically relevant oligopeptide to a surface. The oligopeptide used in this study is a kinase substrate of the epidermal growth factor receptor (EGFR), a protein that is a common target for cancer therapeutics. By observing changes in the pi* and sigma* orbitals of specific nitrogen and carbon atoms (amide, imide, carbonyl), we are able to follow the sequential reactions leading to immobilization of the oligopeptide. We also show that it is possible to use NEXAFS to extend this characterization method to submonolayer densities that are relevant to biological assays. Such an element-specific chemical characterization of small peptides on surfaces fills an unmet need and establishes NEXAFS as useful technique for characterizing the immobilization of small biomolecules on surfaces.

Adsorption of Collagen Fragments on Titanium Oxide Surfaces: A Molecular Dynamics Study

The adsorption of flexible collagen I triple helices with lengths of 8 nm in aqueous solution on a partially hydroxylated rutile layer (pH = 7.4) is modelled by classical molecular dynamics simulation with trajectories of up to 5 ns lengths. The carboxyl groups of glutamic and aspartic acids form hydrogen bonds with surface protons, which result in stable contact points at 300 K, if the bond length is smaller than 2 Å. Lysine side chains bind to surface hydroxyl groups. In spite of rotational motion around the C–N figure axis and opening and closing of hydrogen bonds, the amino groups are immobilized at the surface at N–O distances below 4 Å. In all runs the proteins have few contact points to the surface even though it is perfectly planar in our simulation. We suggest that the experimentally observed increase of adsorption energy and area per molecule over time may be associated to an increase of the number of contact points, which induces spreading of the adsorbate.

Molecular simulation of bovine beta-lactoglobulin adsorbed onto a positively charged solid surface

To obtain detailed insight into the mechanism of beta-lactoglobulin (beta-Lg) adsorption to a stainless steel surface at acidic pH, the adsorption of positively charged beta-Lg to a positively charged surface (Au (100) surface with virtual positive charge) was simulated using classical molecular dynamics. The initial orientation and position of beta-Lg on the surface were determined using Monte Carlo simulation using the implicit water system. Molecular dynamics simulation with the explicit water system was conducted for a 5 ns simulation time to monitor beta-Lg adsorption. To investigate surface charge density effects on adsorption of beta-Lg, the positive charge number per Au atom on the (100) surface, C, was varied from 0 to +0.0250|e|. Stable adsorption occurred in MD simulations when C was equal to or less than +0.0200|e|. Among these surface Au charge conditions, no large difference was observed in the vertical separation distance between the surface and the protein's center of mass, and the orientation angle. This fact indicates that the main interactions contributing to the adsorption were van der Waals interactions. The protein domain contacting the surface was near Thr125, agreeing with previous experimental studies. Considering simulation results and those previous experimental studies suggests a detailed adsorption mechanism of beta-Lg at acidic pH: beta-Lg molecule is adsorbed initially with the specific part of 125-135th residues close to the surface by van der Waals interactions. Simultaneously or subsequently, side carboxylic groups of acidic amino acid residues near the surface in 125-135th residues dissociate, leading to firmer adsorption by attractive electrostatic residue-surface interaction.

Biomechanical evaluation of the interfacial strength of a chemically modified sandblasted and acid-etched titanium surface

The functional capacity of osseointegrated dental implants to bear load is largely dependent on the quality of the interface between the bone and implant. Sandblasted and acid-etched (SLA) surfaces have been previously shown to enhance bone apposition. In this study, the SLA has been compared with a chemically modified SLA (modSLA) surface. The increased wettability of the modSLA surface in a protein solution was verified by dynamic contact angle analysis. Using a well-established animal model with a split-mouth experimental design, implant removal torque testing was performed to determine the biomechanical properties of the bone-implant interface. All implants had an identical cylindrical shape with a standard thread configuration. Removal torque testing was performed after 2, 4, and 8 weeks of bone healing (n = 9 animals per healing period, three implants per surface type per animal) to evaluate the interfacial shear strength of each surface type. Results showed that the modSLA surface was more effective in enhancing the interfacial shear strength of implants in comparison with the conventional SLA surface during early stages of bone healing. Removal torque values of the modSLA-surfaced implants were 8-21% higher than those of the SLA implants (p = 0.003). The mean removal torque values for the modSLA implants were 1.485 N m at 2 weeks, 1.709 N m at 4 weeks, and 1.345 N m at 8 weeks; and correspondingly, 1.231 N m, 1.585 N m, and 1.143 N m for the SLA implants. The bone-implant interfacial stiffness calculated from the torque-rotation curve was on average 9-14% higher for the modSLA implants when compared with the SLA implants (p = 0.038). It can be concluded that the modSLA surface achieves a better bone anchorage during early stages of bone healing than the SLA surface; chemical modification of the standard SLA surface likely enhances bone apposition and this has a beneficial effect on the interfacial shear strength.

Enhancing surface free energy and hydrophilicity through chemical modification of microstructured titanium implant surfaces

Roughness-induced hydrophobicity, well-known from natural plant surfaces and intensively studied toward superhydrophobic surfaces, has currently been identified on microstructured titanium implant surfaces. Studies indicate that microstructuring by sandblasting and acid etching (SLA) enhances the osteogenic properties of titanium. The undesired initial hydrophobicity, however, presumably decelerates primary interactions with the aqueous biosystem. To improve the initial wettability and to retain SLA microstructure, a novel surface modification was tested. This modification differs from SLA by its preparation after acid etching, which was done under protective gas conditions following liquid instead of dry storage. We hypothesized that this modification should have increased wettability due to the prevention of contaminations that occurs during air contact. The main outcome of dynamic wettability measurements was that the novel modification shows increased surface free energy (SFE) and increased hydrophilicity with initial water contact angles of 0 degrees compared to 139.9 degrees for SLA. This hydrophilization was kept even after any drying. Reduced hydrocarbon contaminations were identified to play a possible role in altered surface thermodynamics. Such surfaces aim to retain the hydrophilicity and natural high surface energy of the Ti dioxide surface until surgical implants' insertion and are compared in this in vitro study with structural surface variants of titanium to compare roughness and chemically induced wettability.

Soft X-Ray-Induced Decomposition of Amino Acids: An XPS, Mass Spectrometry, and NEXAFS Study

Decomposition of five amino acids, alanine, serine, cysteine, aspartic acid, and asparagine, under irradiation with soft X rays (magnesium Kalpha X-ray source) in ultra-high vacuum was studied by means of X-ray photoelectron spectrometry (XPS) and mass spectrometry. A comparative analysis of changes in XPS line shapes, stoichiometry and residual gas composition indicates that the molecules decompose by several pathways. Dehydration, decarboxylation, decarbonylation, deamination and desulfurization of pristine molecules accompanied by desorption of H2, H2O, CO2, NH3 and H2S are observed with rates depending on the specific amino acid. NEXAFS spectra of cysteine at the carbon, oxygen and nitrogen K-shell and sulfur L2,3 edges complement the XPS and mass spectrometry data and show that the exposure of the sample to an intense soft X-ray synchrotron beam results in the formation of C-C and C-N double and triple bonds. Qualitatively, the amino acids studied can be arranged in the following ascending order of radiation stability: serine<alanine<aspartic acid<cysteine<asparagine.

Adsorption of human serum albumin (HSA) onto colloidal TiO2 particles, Part I

The adsorption of human serum albumin (HSA) onto colloidal TiO2 (P25 Degussa) particles was studied in NaCl electrolyte at different solution pH and ionic strength. The HSA-TiO2 interactions were studied using adsorption isotherms and the electrokinetic properties of HSA-covered TiO2 particles were monitored by electrophoretic mobility measurements. The adsorption behavior shows a remarkable dependence of the maximum coverage degree on pH and was almost independent of the ionic strength. Other characteristic features such as maximum adsorption values at the protein isoelectric point (IEP approximately 4.7) and low-affinity isotherms that showed surface saturation even under unfavorable electrostatic conditions (at pH values far away from the HSA IEP and TiO2 PZC) were observed. Structural and electrostatic effects can explain the diminution of HSA adsorption under these conditions, assuming that protein molecules behave as soft particles. Adsorption reactions are discussed, taking into account acid-base functional groups of the protein and the surface oxide in different pH ranges, considering various types of interactions.

Study of human serum albumin-TiO(2) nanocrystalline electrodes interaction by impedance electrochemical spectroscopy

The adsorption of human serum albumin (HSA) onto nanocrystalline TiO(2) electrodes was studied by electrochemical impedance spectroscopy (EIS) in function of pH and electrode potential. The characterization and physico-chemical properties of the TiO(2) electrode were investigated by scanning electron microscopy (SEM), UV-photoelectron spectroscopy (UPS), cyclic voltammetry and capacitance measurements. The impedance response of the particulate TiO(2) electrode/protein interface was fitted using an equivalent circuit model to describe the adsorption process. The adsorbed protein layer, which is formed as soon as the protein is injected into the solution and becomes in contact with the electrode, was investigated as a function of electrode potential and solution pH. The measurements were performed under pseudo-steady-state and steady-state conditions, which gave information about the different states of the system. With the pseudo-steady state measurements, it was possible to determine two rate constants of the protein adsorption process, which correspond to two different states of the protein. The shortest one was associated with the first contact between the protein and the substrate and the second relaxation time, with the protein suffering an structural rearrangement due to the interaction with the TiO(2) electrode. It was detected that at sufficiently long times (approx. 1 h, where the system was under steady state conditions), a quasi-reversible protein adsorption mechanism was established. The measurements performed as a function of frequency under steady-state conditions, an equivalent circuit with a Warburg element gave the better fitting to data taken at -0.585 V closer to the oxide flat band potential and it was associated with protein diffusion. Experimental results obtained at only one frequency as a function of potential could be fitted to a model that takes into account non-specific and probable specific protein adsorption, which renders to be potential- and pH-dependent. Low capacity values were obtained in the whole potential range, which were measured in the presence and in the absence of the protein layer. The capacity dependence on potential and pH were associated with the generation of surface states on TiO(2). A surface state concentration of 4.1x10(18) cm(-2) was obtained by relating the parallel capacitance with oxide surface states arising from the protein-oxide interaction.

Surface reactions of a plasma-sprayed CaO-P2O5-SiO2-based glass with albumin, fibroblasts and granulocytes studied by XPS, fluorescence and chemiluminescence

X-ray photoelectron spectroscopy (XPS) was used to define the chemical composition of the outermost surface layer and the surface modification of a plasma-coated phospho-silicate glass (identified as BVA) when immersed in K-phosphate buffer or in phosphate buffered human albumin solution. Its behavior was compared with that of a soda-lime-based glass (identified as BVH) treated in the same way. The surface % composition of plasma-sprayed glass was consistent with bulk composition. After incubation with buffer, a Ca-P-rich layer developed only on the surface of BVA glass. Human serum albumin was bound reversibly to both glasses maintaining its native state. However, the protein completely covered the BVA glass surface within 24 h, with the formation of a mixed albumin-Ca-P layer, while 4 days incubation was necessary for complete coverage of BVH glass surface. Murine fibroblasts seeded on plasma-coated BVA glass showed a proliferation pattern similar to that of control cells grown on Petri dish, while cells seeded on BVH had more restricted growth. A limited response was induced in polymorphonuclear granulocytes by both bulk glasses powder. In conclusion, the glass identified as BVA has the suitable characteristics of its surface layers to be considered biologically active from both a chemical and a cellular point of view.

Adsorption of Sodium Dodecyl Sulfate to Colloidal Titanium Dioxide: An Electrophoretic Fingerprinting Investigation

The adsorption of sodium dodecyl sulfate to colloidal titanium dioxide was investigated using the electrophoretic fingerprinting approach. An electrophoretic fingerprint is a contour diagram of the observed electrophoretic mobility as a function of the bulk solution pH and plambda, the log of the bulk solution conductivity. Surfactant adsorption was observed to be strong under acidic conditions, as illustrated in the dramatic changes in the electrophoretic fingerprints. Electrokinetic data were compared with adsorption isotherm data obtained by a depletion method and good qualitative agreement was found. The observed pH changes associated with surfactant adsorption suggested ligand exchange as a possible mechanism of adsorption. Electrophoretic fingerprinting was shown to be a powerful means of examining surfactant adsorption to colloidal particles. Copyright 2000 Academic Press.

Metal ion release from titanium with active oxygen species generated by rat macrophages in vitro

The release of metal ions due to active oxygen species generated by macrophages (Mphi) phagocytosing high-density polyethylene (HDPE) particles was studied in vitro to investigate the mechanism behind the release of metal ions from titanium implants into nearby tissues in the absence of wear and fretting in vivo. To determine the effects of Mphis on metal ion release, titanium disks were immersed in different solutions and the titanium ions released from the titanium disks into each solution were quantified. The results revealed that active oxygen species generated by Mphis induced the metal ion release. In particular, the ion release was accelerated with HDPE because the Mphis that phagocytosed HDPE generated more active oxygen species than Mphis that did not phagocytose any HDPE. Metal ions were also released by organic species in the absence of Mphis. These are some of the causes for metal ion release from titanium implants in the absence of wear and fretting in vivo.