Fibronectin adsorption, conformation, and orientation on polystyrene substrates studied by radiolabeling, XPS, and ToF SIMS

Surfaces that sequester serum-borne heparin amplify growth factor activity

Surfaces presenting a heparin-binding peptide can non-covalently sequester heparin from culture supplements, such as fetal bovine serum. In turn, sequestered, serum-borne heparin can non-covalently localize growth factors at the cell-material interface, resulting in amplified growth factor bioactivity.

Effects of electrode surface modification with chlorotoxin on patterning single glioma cells

A microchip patterned with arrays of single cancer cells can be an effective platform for the study of tumor biology, medical diagnostics, and drug screening. However, patterning and retaining viable single cancer cells on defined sites of the microarray can be challenging. In this study we used a tumor cell-specific peptide, chlorotoxin (CTX), to mediate glioma cell adhesion on arrays of gold microelectrodes and investigated the effects of three surface modification schemes for conjugation of CTX to the microelectrodes on single cell patterning, which include physical adsorption, covalent bonding mediated by N-hydroxysuccinimide (NHS), and covalent bonding via crosslinking succinimidyl iodoacetate and Traut's (SIA-Traut) reagents. The CTX immobilization to microelectrodes was confirmed by high-resolution X-ray photoelectron spectroscopy. Physically adsorbed CTX showed better support for cell adhesion and is more effective in confining adhered cells on the electrodes than covalently-bound CTX. Furthermore, cell adhesion and spreading on microelectrodes were quantified in real-time by impedance measurements, which revealed an impedance signal from physically adsorbed CTX electrodes four times greater than the signal from covalently-bound CTX electrodes.

Probing Albumin Adsorption onto Calcium Phosphates by XPS and ToF-SIMS

In this study the binding and assembly of bovine serum albumin (BSA) onto three different calcium phosphate phases (hydroxyapatite, dibasic calcium phosphate dihydrate, and β-tricalcium phosphate) was investigated using a combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS was used to record adsorption isotherms and to quantify the amount of BSA adsorbed onto the different CaP surfaces. On all three surfaces a monolayer of adsorbed BSA was formed. ToF-SIMS was then used to investigate how the structure of BSA changes upon surface binding. ToF-SIMS data from BSA films on the three CaP surfaces showed intensity differences of secondary ions originating from both hydrophobic and hydrophilic amino acids. For a more quantitative examination of structural changes, we developed a ratio comparing the sum of intensities of secondary ions from hydrophobic and hydrophilic residues. A small, but statistically significant, increase in the value of this ratio (7%) was observed between a BSA film on hydroxyapatite versus dibasic calcium phosphate dihydrate. From this ratio we can make some initial hypotheses about what specific changes in BSA structure relate to these differences observed in the ToF-SIMS data.

Orientation-regulated immobilization of Jagged1 on glass substrates for ex vivo proliferation of a bone marrow cell population containing hematopoietic stem cells

Notch signaling has been recognized as a key pathway to regulate the proliferation and differentiation of hematopoietic stem cells (HSC). In this study, the orientation-regulated immobilization of a Notch ligand was designed to achieve the efficient Notch ligand-receptor recognition for the ex vivo proliferation of a bone marrow cell population containing HSC. Protein A was chemically conjugated onto aminated glass substrates, followed by immobilizing a recombinant chimeric protein of Jagged1 and Fc domain (Jagged1-Fc) through the biospecific binding between protein A and Fc domain. Protein A adsorption was suppressed for the Jagged1-Fc-immobilized substrates, in contrast to the Jagged1-Fc-coated ones, indicating the orientation-regulated immobilization of Jagged1-Fc for the substrates. Mouse lineage negative cells (Lin(-)) were cultured on the Jagged1-Fc-immobilized substrates. Flow cytometric analyses demonstrated that c-Kit(+), Sca-1(+), Lin(-), and CD34(-) cells of an HSC population was significantly proliferated on the Jagged1-Fc-immobilized substrates 6 days after culture, whereas no proliferation was observed for the Jagged1-Fc-coated substrates in a random manner or Jagged1-Fc-immobilized ones with a Notch signaling inhibitor. It is concluded that the orientation-regulated immobilization of Jagged1-Fc increased the efficiency of Jagged1 to recognize the Notch receptors, resulting in the promoted ex vivo proliferation of the HSC population.

Protein denaturation detected by time-of-flight secondary ion mass spectrometry

In the present work we investigate the denaturation of a functional protein, horseradish peroxidase (HRP), under various experimental conditions using time-of-flight secondary ion mass spectrometry. HRP was immobilized on TiO(2), and the samples were stored under different conditions. The activity of the enzyme was assessed colorimetrically and compared to ToF-SIMS spectra. We show that denaturation of the protein can be monitored using the ToF-SIMS signal of the disulfide bonds, which is related to the tertiary structure of the protein. As disulfide bonds appear in a vast range of proteins, the present findings may be of wide significance; i.e., a tool is provided that can allow the investigation of the presence of an active protein structure by a comparably simple surface analytical method.

High-density fabrication of normally closed microfluidic valves by patterned deactivation of oxidized polydimethylsiloxane

The use of polydimethylsiloxane (PDMS) in microfluidic devices is extensive in academic research. One of the most fundamental treatments is to expose PDMS to plasma oxidation in order to render its surface temporarily hydrophilic and capable of permanent bonding. Here, we show that changes in the surface chemistry induced by plasma oxidation can spatially be counteracted very cleanly and reliably in a scalable manner by subsequent microcontact printing of residual oligomers from a PDMS stamp. We characterize the surface modifications through contact angle, atomic force microscopy, X-ray photoelectron spectroscopy, and bond-strength measurements. We utilize this approach for negating the bonding of a flexible membrane layer within an elastomeric valve and demonstrate its effectiveness by integration of over one thousand normally closed elastomeric valves within a single substrate. In addition, we demonstrate that surface energy patterning can be used for "open microfluidic" applications that utilize spatial control of surface wetting.

Probing orientation of immobilized humanized anti-lysozyme variable fragment by time-of-flight secondary-ion mass spectrometry

As methods to orient proteins are conceived, techniques must also be developed that provide an accurate characterization of immobilized protein orientation. In this study, X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to probe the orientation of a surface immobilized variant of the humanized anti-lysozyme variable fragment (HuLys Fv, 26 kDa). This protein contained both a hexahistidine tag and a cysteine residue, introduced at opposite ends of the HuLys Fv, for immobilization onto nitrilotriacetic acid (NTA) and maleimide oligo(ethylene glycol) (MEG)-terminated substrates, respectively. The thiol group on the cysteine residue selectively binds to the MEG groups, while the his-tag selectively binds to the Ni-loaded NTA groups. XPS was used to monitor protein coverage on both surfaces by following the change in the nitrogen atomic %. SPR results showed a 10-fold difference in lysozyme binding between the two different HuLys Fv orientations. The ToF-SIMS data provided a clear differentiation between the two samples due to the intensity differences of secondary ions originating from asymmetrically located amino acids in HuLys Fv (histidine: 81, 82, and 110 m/z; phenylalanine: 120 and 131 m/z). An intensity ratio of the secondary ion peaks from the histidine and phenylalanine residues at either end of the protein was then calculated directly from the ToF-SIMS data. The 45% change in this ratio, observed between the NTA and MEG substrates with similar HuLys Fv surface coverages, indicates that the HuLys Fv fragment has opposite orientations on two different surfaces.

Protein adhesion on dental surfaces-a combined surface analytical approach

Protein adsorption is a field of huge interest in a number of application fields. Information on protein adhesion is accessible by a variety of methods. However, the results obtained are significantly influenced by the applied technique. The objective of this work was to understand the role of adhesion forces (obtained by scanning force spectroscopy, SFS) in the process of protein adsorption and desorption. In SFS, the protein is forced to and retracted from the surface, even under unfavorable conditions, in contrast to the natural situation. Furthermore, adhesion forces are correlated with adhesion energies, neglecting the entropic part in the Gibbs enthalpy. In this context, dynamic contact angle (DCA) measurements were performed to identify the potential of this method to complement SFS data. In DCA measurements, the protein diffuses voluntarily to the surface and information on surface coverage and reversibility of adsorption is obtained, including entropic effects (conformational changes and hydrophobic effect). It could be shown that the surface coverage (by DCA) of bovine serum albumin on dental materials correlates well with the adhesion forces (by SFS) if no hydrophobic surface is involved. On those, the entropic hydrophobic effect plays a major role. As a second task, the reversibility of the protein adsorption, i.e., the voluntary desorption as studied by DCA, was compared to the adhesion forces. Here, a correlation between low adhesion forces and good reversibility could be found as long as no covalent bonds were involved. The comparative study of DCA and SFS, thus, leads to a more detailed picture of the complete adsorption/desorption cycle.

Probing the orientation of surface-immobilized protein G B1 using ToF-SIMS, sum frequency generation, and NEXAFS spectroscopy

The ability to orient active proteins on surfaces is a critical aspect of many medical technologies. An important related challenge is characterizing protein orientation in these surface films. This study uses a combination of time-of-flight secondary ion mass spectrometry (ToF-SIMS), sum frequency generation (SFG) vibrational spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to characterize the orientation of surface-immobilized Protein G B1, a rigid 6 kDa domain that binds the Fc fragment of IgG. Two Protein G B1 variants with a single cysteine introduced at either end were immobilized via the cysteine thiol onto maleimide-oligo(ethylene glycol)-functionalized gold and bare gold substrates. X-ray photoelectron spectroscopy was used to measure the amount of immobilized protein, and ToF-SIMS was used to measure the amino acid composition of the exposed surface of the protein films and to confirm covalent attachment of protein thiol to the substrate maleimide groups. SFG and NEXAFS were used to characterize the ordering and orientation of peptide or side chain bonds. On both substrates and for both cysteine positions, ToF-SIMS data showed enrichment of mass peaks from amino acids located at the end of the protein opposite to the cysteine surface position as compared with nonspecifically immobilized protein, indicating end-on protein orientations. Orientation on the maleimide substrate was enhanced by increasing pH (7.0-9.5) and salt concentration (0-1.5 M NaCl). SFG spectral peaks characteristic of ordered α-helix and β-sheet elements were observed for both variants but not for cysteine-free wild type protein on the maleimide surface. The phase of the α-helix and β-sheet peaks indicated a predominantly upright orientation for both variants, consistent with an end-on protein binding configuration. Polarization dependence of the NEXAFS signal from the N 1s to π* transition of β-sheet peptide bonds also indicated protein ordering, with an estimated tilt angle of inner β-strands of 40-50° for both variants (one variant more tilted than the other), consistent with SFG results. The combined results demonstrate the power of using complementary techniques to probe protein orientation on surfaces.

Multi-technique Characterization of Adsorbed Peptide and Protein Orientation: LK310 and Protein G B1

The ability to orient biologically active proteins on surfaces is a major challenge in the design, construction, and successful deployment of many medical technologies. As methods to orient biomolecules are developed, it is also essential to develop techniques that can an accurately determine the orientation and structure of these materials. In this study, two model protein and peptide systems are presented to highlight the strengths of three surface analysis techniques for characterizing protein films: time-of-flight secondary ion mass spectrometry (ToF-SIMS), sum-frequency generation (SFG) vibrational spectroscopy, and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. First, the orientation of Protein G B1, a rigid 6 kDa domain covalently attached to a maleimide-functionalized self-assembled monolayer, was examined using ToF-SIMS. Although the thickness of the Protein G layer was similar to the ToF-SIMS sampling depth, orientation of Protein G was successfully determined by analyzing the C2H5S+ intensity, a secondary ion derived from a methionine residue located at one end of the protein. Next, the secondary structure of a 13-mer leucine-lysine peptide (LK310) adsorbed onto hydrophilic quartz and hydrophobic fluorocarbon surfaces was examined. SFG spectra indicated that the peptide's lysine side chains were ordered on the quartz surface, while the peptide's leucine side chains were ordered on the fluorocarbon surface. NEXAFS results provided complementary information about the structure of the LK310 film and the orientations of amide bonds within the LK310 peptide.

Ion-specific effects on the interaction between fibronectin and negatively charged mica surfaces

Atomic force microscopy (AFM) imaging and subsequent image analysis have been used to measure the ion-specific and ionic strength effects on the adsorption of fibronectin to mica surfaces in buffer solution. Increasing the concentration of monovalent Na(+) salt solutions is shown to cause a transition from tightly aggregated and "string-of-beads" structures on the mica surface to well dispersed single-molecule adsorption. Studying the effect of two divalent salts, Ni(2+) and Ca(2+), reveals a dramatic enhancement of fibronectin adsorption to mica in buffer solutions containing Ni(2+), but not for Ca(2+). The origin of this ion-specific effect is discussed.

Time-of-flight secondary-ion mass spectrometry for the surface characterization of solid-state pharmaceuticals

Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) is a highly surface sensitive analytical method for surface chemical identification and surface chemical distribution analysis (mapping). Here we have explored the application of ToF-SIMS for the characterization of solid-state pharmaceuticals and highlight specific case studies concerning the distribution and stability of pharmaceutical actives within solid matrices (pellets and polymeric carriers) and the face-specific properties of pharmaceutical crystals.

Effect of functional end groups of silane self-assembled monolayer surfaces on apatite formation, fibronectin adsorption and osteoblast cell function

Bioactive glass (BG) can directly bond to living bone without fibrous tissue encapsulation. Key mechanistic steps of BG's activity are attributed to calcium phosphate formation, surface hydroxylation and fibronectin (FN) adsorption. In the present study, self-assembled monolayers (SAMs) of alkanesilanes with different surface chemistry (OH, NH(2) and COOH) were used as a model system to mimic BG's surface activity. Calcium phosphate (Ca-P) was formed on SAMs by immersion in a solution that simulates the electrolyte content of physiological fluids. FN adsorption kinetics and monolayer coverage was determined on SAMs with or without Ca-P coating. The surface roughness was also examined on these substrates before and after FN adsorption. The effects of FN-adsorbed, Ca-P-coated SAMs on the function of MC3T3-E1 were evaluated by cell growth, expression of alkaline phosphatase activity and actin cytoskeleton formation. We demonstrate that, although the FN monolayer coverage and the root mean square (rms) roughness are similar on --OH and --COOH terminated SAMs with or without Ca-P coating, higher levels of ALP activity, more actin cytoskeleton formation and more cell growth are obtained on --OH- and --COOH-terminated SAMs with Ca-P coating. In addition, although the FN monolayer coverage is higher on Ca-P-coated --NH(2)-terminated SAMs and SiO(x) surfaces, higher levels of ALP activity and more cell growth are obtained on Ca-P-coated --OH- and --COOH-terminated SAMs. Thus, with the same Ca-P coatings, different surface functional groups have different effects on the function of osteoblastic cells. These findings represent new insights into the mechanism of bioactivity of BG and thereby may lead to designing superior constructs for bone grafting.

Immuno-atomic force microscopy characterization of adsorbed fibronectin

The fibronectin (Fn) binding conformation on mica and ultraflat poly(D,L-lactide-co-glycolide) (UPLGA) was characterized using atomic force microscopy (AFM). AFM topographic images showed that Fn was in an extended form on mica and in a compact structure on UPLGA. With immuno-AFM, an antibody (Ab(hep)) was used to characterize the Fn binding conformation. When Fn opens its binding site for an antibody upon adsorption to a surface, the resulting Fn-antibody complex creates an additional peak in the sample's height distribution. Immuno-AFM uses this change to detect antigen-antibody binding. In this letter, height histograms (distributions) were generated using the mean true height of molecules, which was measured by examining the histogram for each individual molecule and subtracting the mica background. Mean true height values were obtained from the histograms and showed that Fn and Ab(hep) formed complexes on mica, signifying that one of the heparin binding sites on Fn was open when Fn was adsorbed to mica. The mean true height of the Fn-antibody complex from the histogram is greater than expected, suggesting that the antibody had pulled the extended "arms" of Fn together and caused an Fn conformation change upon binding. The height histograms can illustrate the Fn binding conformation and other antigen-antibody binding.

Protein immobilization on epoxy-activated thin polymer films: effect of surface wettability and enzyme loading

A series of epoxy-activated polymer films composed of poly(glycidyl methacrylate/butyl methacrylate/hydroxyethyl methacrylate) were prepared. Variation in comonomer composition allowed exploration of relationships between surface wettability and Candida antartica lipase B (CALB) binding to surfaces. By changing solvents and polymer concentrations, suitable conditions were developed for preparation by spin-coating of uniform thin films. Film roughness determined by AFM after incubation in PBS buffer for 2 days was less than 1 nm. The occurrence of single CALB molecules and CALB aggregates at surfaces was determined by AFM imaging and measurements of volume. Absolute numbers of protein monomers and multimers at surfaces were used to determine values of CALB specific activity. Increased film wettability, as the water contact angle of films increased from 420 to 550, resulted in a decreased total number of immobilized CALB molecules. With further increases in the water contact angle of films from 55 degrees to 63 degrees, there was an increased tendency of CALB molecules to form aggregates on surfaces. On all flat surfaces, two height populations, differing by more than 30%, were observed from height distribution curves. They are attributed to changes in protein conformation and/or orientation caused by protein-surface and protein-protein interactions. The fraction of molecules in these populations changed as a function of film water contact angle. The enzyme activity of immobilized films was determined by measuring CALB-catalyzed hydrolysis of p-nitrophenyl butyrate. Total enzyme specific activity decreased by decreasing film hydrophobicity.

Nanopatterning of fibronectin and the influence of integrin clustering on endothelial cell spreading and proliferation

Investigating stages of maturation of cellular adhesions to the extracellular matrix from the initial binding events to the formation of small focal complexes has been challenging because of the difficulty in fabricating the necessary nanopatterned substrates with controlled biochemical functionality. We present the fabrication and characterization of surfaces presenting fibronectin nanopatterns of controlled size and pitch that provide well-defined cellular adhesion sites against a nonadhesive polyethylene glycol background. The nanopatterned surfaces allow us to control the number of fibronectin proteins within each adhesion site from 9 to 250, thereby limiting the number of integrins involved in each cell-substrate adhesion. We demonstrate the presence of fibronectin on the nanoislands, while no protein was observed on the passivated background. We show that the cell adheres to the nanopatterns with adhesions that are much smaller and more evenly distributed than on a glass control. The nanopattern influences cellular proliferation only at longer times, but influences spreading at both early and later times, indicating adhesion size and adhesion density play a role in controlling cell adhesion and signaling. However, the overall density of fibronectin on all patterns is far lower than on homogeneously coated control surfaces, showing that the local density of adhesion ligands, not the average density, is the important parameter for cell proliferation and spreading.

Antifouling properties of poly(methyl methacrylate) films grafted with poly(ethylene glycol) monoacrylate immersed in seawater

Biofouling of all structures immersed in seawater constitutes an important problem, and many strategies are currently being developed to tackle it. In this context, our previous work shows that poly(ethylene glycol) monoacrylate (PEGA) macromonomer grafted on preoxidized poly(methyl methacrylate) (PMMAox) films exhibits an excellent repellency against the bovine serum albumin used as a model protein. This study aims to evaluate the following: (1) the prevention of a marine extract material adsorption by the modified surfaces and (2) the antifouling property of the PEGA-g-PMMAox substrates when immersed in natural seawater during two seasons (season 1: end of April-beginning of May 2007, and season 2: end of October-beginning of November 2007). The antifouling performances of the PEGA-g-PMMAox films are investigated for different PEG chain lengths and macromonomer concentrations into the PEGA-based coatings. These two parameters are followed as a function of the immersion time, which evolves up to 14 days. The influence of the PEGA layer on marine compounds (proteins and phospholipids) adsorption is evidenced by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). It was found that the antifouling efficiency of the PEGA-grafted surfaces increases with both PEGA concentration and PEG chain length.

Urban PM2.5Surface Chemistry and Interactions with Bronchoalveolar Lavage Fluid

This study investigated the surface chemistry of urban fine particles (PM(2.5)), and quantified the adsorbed and desorbed species after exposure to bronchoalveolar lavage fluid (BALF). Urban background and roadside PM(2.5) samples of different mass concentration and total weight were collected in triplicate in the South Bronx region of New York City. Simultaneously, the concentrations of other atmospheric pollutants (CO, NO(x), SO(2), O(3), elemental carbon) were measured, and weather conditions were recorded. The collected PM(2.5) samples underwent one of three treatments: no treatment, treatment in vitro with BALF, or treatment in a saline solution (control). The surfaces of untreated, saline-treated, and BALF-treated PM(2.5) samples were analyzed using x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). These results were then compared with ambient air pollutant concentrations, weather variables, selected BALF characteristics, and results from a previous London study conducted using identical preparation methods by XPS analysis only. Both XPS and ToF-SIMS detected PM(2.5) surface species and observed changes in surface concentrations after treatment. XPS analysis showed the surface of untreated urban PM(2.5) consisted of 79 to 87% carbon and 10 to 16% oxygen with smaller contributions of N, S, Si, and P in the samples from both background and roadside locations. A wider variety of other inorganic and organic species (including metals, aliphatic and aromatic hydrocarbons, and nitrogen-containing molecules) was detected with ToF-SIMS. Surface characteristics of particles from the roadside and background sites were very similar, except for higher (p <.05) nitrate concentrations at the roadside, which were attributable to higher roadside NO(x) concentrations. Comparable species and quantities were identified in a previous study of London PM(2.5), where PM(2.5) surface chemistry differed considerably depending on the source, particularly in surface concentrations of oxygen and trace species. After treatment with BALF the N-C signal detected by XPS analysis increased in the average by 372 +/- 203%, indicating significant surface adsorption of protein or other N-containing biomolecules. Lower (nonsignificant) N-C signals were observed for smoker BALF, compared to nonsmoker BALF. ToF-SIMS data confirmed protein adsorption after BALF treatment--smoker BALF resulted in lower levels of adsorbed proteins compared to nonsmoker BALF. ToF-SIMS also indicated an adsorption of phospholipid on the treated PM(2.5) surfaces. The primary phospholipid in BALF is dipalmitoylphospatidylcholine (DPPC), although positive identification was not possible due to low concentrations at the PM(2.5) surface. Oxygen content of PM(2.5) surfaces was the most significant determinant of both N-C and phospholipid adsorption. The XPS signal of the soluble species NH(+)(4), NO(2-)(3), Si, and S decreased in both saline- and BALF-treated samples, showing that these species may be bioavailable in the lung. Similarly, ToF-SIMS analysis suggests the bioavailability of Na(+) and Al(+) as well as NH(+)(4) and Si(+).

The development of peptide-based interfacial biomaterials for generating biological functionality on the surface of bioinert materials

Biomaterials used in implants have traditionally been selected based on their mechanical properties, chemical stability, and biocompatibility. However, the durability and clinical efficacy of implantable biomedical devices remain limited in part due to the absence of appropriate biological interactions at the implant interface and the lack of integration into adjacent tissues. Herein, we describe a robust peptide-based coating technology capable of modifying the surface of existing biomaterials and medical devices through the non-covalent binding of modular biofunctional peptides. These peptides contain at least one material binding sequence and at least one biologically active sequence and thus are termed, "Interfacial Biomaterials" (IFBMs). IFBMs can simultaneously bind the biomaterial surface while endowing it with desired biological functionalities at the interface between the material and biological realms. We demonstrate the capabilities of model IFBMs to convert native polystyrene, a bioinert surface, into a bioactive surface that can support a range of cell activities. We further distinguish between simple cell attachment with insufficient integrin interactions, which in some cases can adversely impact downstream biology, versus biologically appropriate adhesion, cell spreading, and cell survival mediated by IFBMs. Moreover, we show that we can use the coating technology to create spatially resolved patterns of fluorophores and cells on substrates and that these patterns retain their borders in culture.

Differential binding of factor XII and activated factor XII to soluble and immobilized fibronectin--localization of the Hep-1/Fib-1 binding site for activated factor XII

Fibronectins (FNs) are dimeric glycoproteins that adopt a globular conformation when present in plasma and solution and an extended conformation in the extracellular matrix. Factor XII (FXII) is a zymogen of the proteolytically active FXIIa that plays a role in thrombus stabilization by enhancing clot formation and in inflammation by enhancing bradykinin formation. To investigate whether the extracellular matrix could play a role in these events, we have recently shown that FXIIa, but not FXII, binds to the extracellular matrix (ECM), and suggested that FN may be the target for the binding. Immunofluorescence microscopy has in the present investigation confirmed that FXIIa added to the ECM colocalizes with FN deposited during growth of human umbilical vein endothelial cells. The aim of the present study, therefore, was to further elucidate the interaction between FXIIa and FN by the use of a solid face binding assay. This showed, like the binding to the ECM, that FXIIa, but not FXII, binds in a Zn2+-independent manner to immobilized FN. The K(D) for the binding was 8.5 +/- 0.9 nM (n = 3). The binding was specific for the immobilized FN, as the binding could not be inhibited by soluble FN. Furthermore, soluble FN did not bind to immobilized FXIIa. However, soluble FN could bind to FXII, and this binding inhibited the surface-induced autoactivation of FXII and subsequent binding of the generated FXIIa to immobilized FN. The presence of FXII in an anti-FN immunoprecipitate of plasma indicated that some FXII in plasma circulates bound to FN. The binding of FXIIa to FN was inhibited by gelatine and fibrin but not by heparin, indicating that FXIIa binds to immobilized FN through the type I repeat modules. Accordingly, FXIIa was found to bind to immobilized fragments of FN containing the type I repeat modules in the N-terminal domain to which fibrin and gelatine bind.

Cell adhesion and proliferation on hydrophilic dendritically modified surfaces

Dendritically modified, or "dendronized" surfaces are generated by modification of a substrate with perfectly branched polymers, known as dendrimers. Here, such dendronized surfaces were prepared by initial chemisorption of poly(ethylene glycol)-mono-thiol (HS-PEG(650)-OH) onto gold-coated silicon wafers, followed by divergent synthesis of aliphatic polyester dendrons, generation 1-4, starting from the terminal PEG OH- group. The adhesion and proliferation of human corneal epithelial cells (HCEC) and mouse 3T3 fibroblasts (M-3T3) as model cells on these hydroxyl-terminated dendronized surfaces were investigated. In addition, the effect of covalently attaching PEG mono-methyl ether (PEG-OMe) chains (M(n)=2000 Da) to the peripheral hydroxyl groups of G1- and G2-dendronized surfaces on adhesion and proliferation of the same cell lines was studied. Little or no HCEC adhesion was noted on gold surfaces modified with PEG mono-thiol (HO-PEG-SH) in serum-free medium. These cells showed a greater affinity for the dendronized surfaces compared to the control Au surfaces at early incubation stages (1 day). At longer incubation times, HCEC proliferation increased exponentially on the dendronized surfaces. However, when G1- and G2-dendronized surfaces were modified with PEG-OMe chains, adhesion of both HCEC and M-3T3 cells was significantly reduced. Cell studies with M-3T3 fibroblasts, carried out in serum-containing medium, showed that cell attachment was diminished for the PEG-grafted Au surfaces compared to the control Au and G1-G4 dendronized surfaces.

Combined use of atomic force microscopy, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry for cell surface analysis

Understanding the surface properties of microbial cells is a major challenge of current microbiological research and a key to efficiently exploit them in biotechnology. Here, we used three advanced surface analysis techniques with different sensitivity, probing depth, and lateral resolution, that is, in situ atomic force microscopy, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry, to gain insight into the surface properties of the conidia of the human fungal pathogen Aspergillus fumigatus. We show that the native ultrastructure, surface protein and polysaccharide concentrations, and amino acid composition of three mutants affected in hydrophobin production are markedly different from those of the wild-type, thereby providing novel insight into the cell wall architecture of A. fumigatus. The results demonstrate the power of using multiple complementary techniques for probing microbial cell surfaces.

Fine airborne urban particles (PM2.5) sequester lung surfactant and amino acids from human lung lavage

Components of surfactant act as opsonins and enhance phagocytosis of bacteria; whether this process occurs with atmospheric fine particles has not been shown. We have studied the interactions of fine particles (urban PM2.5) and surfactant removed from normal human lungs by lavage, using a surface analysis technique. The aim was to identify which of the chemical components of brochoalveolar lavage (BAL) deposit on the surfaces of urban PM2.5. Deposition of surfactant components on urban PM2.5 surfaces was reported in previous studies, but molecular identification and relative quantification was not possible using simple data analysis. In this study, we were able to identify adsorbed components by applying an appropriate statistical technique, factor analysis. In this study, the most strongly associated mass fragment on PM2.5 surfaces exposed to BAL (and undetected on both untreated samples and saline controls) was di-palmitoyl-phosphatidylcholine, a component of lung surfactant. Amino acids were also strongly associated with BAL-exposed PM2.5 surfaces and not other sample types. Thirteen mass fragments were identified, diagnostic of the amino acids alanine, arginine, asparagine, aspartic acid, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, serine, and valine. This study provides evidence that lung surfactant and amino acids related to opsonin proteins adsorb to nonbiological particle surfaces exposed to human lung lining fluid. Disruption of normal surfactant function, both physical and immunological, is possible but unproven. Further work on this PM-opsonin interaction is recommended.

PM-IRRAS Investigation of the Interaction of Serum Albumin and Fibrinogen with a Biomedical-Grade Stainless Steel 316LVM Surface

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the interaction of bovine serum albumin (BSA) and fibrinogen with a biomedical-grade 316LVM stainless steel surface, in terms of the adsorption thermodynamics and adsorption-induced secondary structure changes of the proteins. Highly negative apparent Gibbs energy of adsorption values revealed a spontaneous adsorption of both proteins onto the surface, accompanied by significant changes in their secondary structure. It was determined that, at saturated surface coverages, lateral interactions between the adsorbed BSA molecules induced rather extensive secondary structure changes. Fibrinogen's two coiled coils appeared to undergo negligible secondary structure changes upon adsorption of the protein, while large structural rearrangements of the protein's globular domains occurred upon adsorption. The secondary structure of adsorbed fibrinogen was not influenced by lateral interactions between the adsorbed fibrinogen molecules. PM-IRRAS was deemed to be viable for investigating protein adsorption and for obtaining information on adsorption-induced changes in their secondary structures.

Effects of surface wettability and contact time on protein adhesion to biomaterial surfaces

Atomic force microscopy (AFM) was used to directly measure the adhesion forces between three test proteins and low density polyethylene (LDPE) surfaces treated by glow discharge plasma to yield various levels of water wettability. The adhesion of proteins to the LDPE substrates showed a step dependence on the wettability of surfaces as measured by the water contact angle (theta). For LDPE surfaces with theta> approximately 60-65 degrees , stronger adhesion forces were observed for bovine serum albumin, fibrinogen and human FXII than for the surfaces with theta<60 degrees . Smaller adhesion forces were observed for FXII than for the other two proteins on all surfaces although trends were identical. Increasing the contact time from 0 to 50s for each protein-surface combination increased the adhesion force regardless of surface wettability. Time varying adhesion data was fit to an exponential model and free energies of protein unfolding were calculated. This data, viewed in light of previously published studies, suggests a 2-step model of protein denaturation, an early stage on the order of seconds to minutes where the outer surface of the protein interacts with the substrate and a second stage involving movement of hydrophobic amino acids from the protein core to the protein/surface interface. Impact statement: The work described in this manuscript shows a stark transition between protein adherent and protein non-adherent materials in the range of water contact angles 60-65 degrees , consistent with known changes in protein adsorption and activity. Time-dependent changes in adhesion force were used to calculate unfolding energies relating to protein-surface interactions. This analysis provides justification for a 2-step model of protein denaturation on surfaces.

Dynamics of fibronectin adsorption on TiO2 surfaces

In the present work we analyze the dynamics of fibronectin (FN) adsorption on two different stable titanium oxides, with varied surface roughness, and chemically similar to those used in clinical practice. The two types of titanium oxide surfaces used were TiO2 sputtered on Si (TiO2 sp) and TiO2 formed on commercially pure titanium after immersion in H2O2 (TiO2 cp). Surface characterization was previously carried out using different techniques (Sousa, S. R.; Moradas-Ferreira, P.; Melo, L. V.; Saramago, B.; Barbosa, M. A. Langmuir 2004, 20 (22), 9745-9754). Imaging and roughness analysis before and after FN adsorption used atomic force microscopy (AFM) in tapping mode, in air, and in magnetic alternating current mode, in liquid (water). FN adsorption as a function of time was followed by X-ray photoelectron spectroscopy (XPS), by radiolabeling of FN with 125I (125I-FN), and by ellipsometry. Exchangeability studies were performed using FN and HSA. AFM roughness analysis revealed that, before FN adsorption, both TiO2 surfaces exhibited a lower root-mean-square (Rq) and maximum peak with the depth of the maximum valley (Rmax) roughness in air than in water, due to TiO2 hydration. After protein adsorption, the same behavior was observed for the TiO2 sp substrate, while Rq and Rmax roughness values in air and in water were similar in the case of the TiO2 cp substrate, for the higher FN concentration used. Surface roughness was always significantly higher on the TiO2 cp surfaces. AFM led to direct visualization of adsorbed FN on both surfaces tested, indicating that after 10 min of FN incubation the TiO2 sp surface was partially covered by FN. The adsorbed protein seems to form globular aggregates or ellipsoids, and FN aggregates coalesce, forming clusters as the time of adsorption and the concentration increase. Radiolabeling of FN revealed that a rapid adsorption occurs on both surfaces and the amount adsorbed increased with time, reaching a maximum after 60 min of incubation. Time dependence is also observed for the evolution of the atomic (%) of N determined by XPS and by the increase of the thickness by ellipsometry. TiO2 cp adsorbs more FN than the TiO2 sp surfaces, after 60 min of adsorption, as shown by the radiolabeling data. FN molecules are also more strongly attached to the former surface as indicated by the exchangeability studies. The overall results provide novel evidence that FN spontaneously adsorbs as a self-assembly at TiO2 surfaces as a function of time. The aggregate structure is an intermediate feature shared by some protein fibrillar assemblies at interfaces, which is believed to promote cell adhesion and cytoskeleton organization (Pellenc, D.; Berry, H.; Gallet, O. J. Colloid Interface Sci. 2006, 298 (1), 132-144. Maheshwari, G.; Brown, G.; Lauffenburger, D. A.; Wells, A.; Griffith, L. G. J. Cell Sci. 2000, 113 (10), 1677-1686).

Adhesion of mouse fibroblasts on hexamethyldisiloxane surfaces with wide range of wettability

Surface wettability is an important physicochemical property of biomaterials, and it would be more helpful for understanding this property if a wide range of wettability are employed. This study focused on the effect of surface wettability on fibroblast adhesion over a wide range of wettability using a single material without changing surface topography. Plasma polymerization with hexamethyldisiloxane followed by oxygen (O2)-plasma treatment was employed to modify the surfaces. The water contact angle of sample surfaces varied from 106 degrees (hydrophobicity) to almost 0 degrees (super-hydrophilicity). O2 functional groups were introduced on polymer surfaces during O2-plasma treatment. The cell attachment study confirmed that the more hydrophilic the surface, the more fibroblasts adhered in the initial stage that includes super-hydrophilic surfaces. Cells spread much more widely on the hydrophilic surfaces than on the hydrophobic surfaces. There was no significant difference in fibroblast proliferation, but cell spreading was much greater on the hydrophilic surfaces. The fibronectin adsorbed much more on a hydrophilic surface while albumin dominated on a hydrophobic surface in a competing mode. These findings suggest the importance of the surface wettability of biomaterials on initial cell attachment and spreading. The degree of wettability should be taken into account when a new biomaterial is to be employed.

The interaction of the polyphenylacetylene surface with biological environments studied by XPS, RAIRS and biological tests

A pi-conjugated polymer, polyphenylacetylene or PPA, has been tested for its possible applications as biosensor or biomaterial. Protein adsorption was investigated by incubating PPA films in solutions of bovine serum albumin (BSA) dissolved in phosphate buffer (PBS) having increasing protein concentration. Investigations on the PPA films were carried out by means of two surface analysis techniques, X-ray photoelectron spectroscopy (XPS) and reflection-absorption infrared spectroscopy (RAIRS). Desorption of BSA from the PPA surface was also investigated. Finally, the cytototoxicity of the PPA surface was checked by measuring viability and proliferation of lymphoma macrophages and SAOS osteoblasts grown in the presence of the polymer.

Influence of PEG architecture on protein adsorption and conformation

Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers with various grafting ratios were adsorbed to niobium pentoxide-coated silicon wafers and characterized before and after protein adsorption using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Three proteins of different sizes, myoglobin (16 kD), albumin (67 kD), and fibrinogen (340 kD), were studied. XPS was used to quantify the amount of protein adsorbed to the bare and PEGylated surfaces. ToF-SIMS and principal component analysis (PCA) were used to study protein conformational changes on these surfaces. The smallest protein, myoglobin, generally adsorbed in higher numbers than the much larger fibrinogen. Protein adsorption was lowest on the surfaces with the highest PEG chain surface density and increased as the PEG layer density decreased. The highest adsorption was found on lysine-coated and bare niobium surfaces. ToF-SIMS and PCA data evaluation provided further information on the degree of protein denaturation, which, for a particular protein, were found to decrease with increasing PEG surface density and increase with decreasing protein size.

Single molecule fluorescence studies of surface-adsorbed fibronectin

The conformation of the extracellular matrix protein fibronectin plays a critical role in regulating cell function, including cell adhesion and migration. While average conformations of large ensembles of adhesion proteins have been previously measured, cells may sensitively respond to conformational outliers. We therefore applied both single molecule imaging and spectroscopy techniques to map a range of conformational states of individual fibronectin molecules adsorbed to glass, as well as to measure their conformational fluctuations in time. Single-step photobleaching experiments confirmed single molecule sensitivity. Single molecule spectra showed fluctuations in the peak wavelength, both over a spatial ensemble of molecules and in a single molecule over time, most likely indicating the different conformational states fibronectin can assume upon surface adsorption. Single-pair fluorescence resonance energy transfer (FRET) revealed that a fraction of fibronectin molecules existed in conformations that allowed for energy transfer between the labeled cysteine residues of the two dimeric arms folded upon each other, and that fluctuations occurred in the FRET efficiency. Fluorescence polarization experiments identified two possible sources of FRET fluctuations: changes in fluorophore orientation and conformational fluctuations of fibronectin over a time scale of seconds.

Effect of magnetic field on the fibronectin adsorption, cell attachment and proliferation on titanium surface

OBJECTIVES

We studied the effect of various static magnetic fields (SMFs) on the adsorption of specific recombinant fibronectin (FN) peptide (hFNIII9-10) on the titanium surface. Furthermore, the responses of human osteosarcoma TE-85 cells in the SMF were observed.

MATERIAL AND METHODS

Various magnetic fields--1, 2, 3, 5, 7, 10 mT--were established by controlling the distance from Nd-Fe-B magnet to the disks. For FN adsorption experiment, machined titanium disks were incubated in 1 microM hFNIII9-10 at 37 degrees C overnight under magnetic field. The adsorbed hFNIII9-10 was measured as optical density (OD). For attachment study, TE-85 cells were incubated for 2 h on the hFNIII9-10 coated machined titanium disks and OD values were measured. As for proliferation study, titanium disks were incubated for 48 h after washing unattached cells in 2 h. The amount of proliferated TE-85 cell was also measured as OD value. Attachments of TE-85 cells under various intensities of magnetic field were observed using a scanning electron microscope.

RESULTS

The amount of adsorbed hFNIII9-10 showed no significant difference between control (0 mT) and six experimental groups (1, 2, 3, 5, 7, 10 mT). However, TE-85 cells attached significantly higher in groups of 1, 2, 5, 10 mT than in control group (P=0). Cell attachment in groups of 3, 7 mT showed no significant difference with that of control group. TE-85 cells were observed to attach through filopodia. Especially in 1 mT, flattened cells were predominant. In proliferation assay, 1 mT stimulated TE-85 cells showed significantly higher proliferation than those in 2, 3 and 7 mT (P=0).

CONCLUSION

Magnetic fields under 10 mT did not influence FN adsorption on the titanium surface. However, a significant effect was found on cell attachment and proliferation.

Effective monitoring of protein reaction on glass plate surfaces by TOF-SIMS

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is capable of chemically visualizing proteins on insulated samples. Distribution of an immobilized probe protein, fluorescent-labeled protein A-immobilized on a glass plate, and that of a sample protein, immunogloblin G (IgG) in solution, reacting with protein A on the biosensor surface, were evaluated with TOF-SIMS (TFS-2100, Physical Electronics). TOF-SIMS spectra and images of the protein on the glass plates were obtained, and this "mutual information", as defined by information theory, was employed to analyze the TOF-SIMS spectra of proteins. Fragment ions from protein A and IgG were distinguished by the mutual, reinforcing information and specific fragment ions to each protein were selected to obtain the TOF-SIMS image of the protein. It is evident from the TOF-SIMS images of each protein that protein A was immobilized on the substrate homogeneously and that the reaction between the immobilized protein A and IgG is not localized in this condition. Chemical images of the proteins by TOF-SIMS will contribute to a better understanding of the reaction on the biosensor surface, and thus will help the development of more sophisticated biosensors. In addition, the requisite chemical conditions as well as the interaction between the biosensor surface and the immobilized proteins were investigated by TOF-SIMS by means of sets of reinforcing, mutually supportive information.

Adsorbed layers of oriented fibronectin: A strategy to control cell-surface interactions

Fibronectin (Fn) is a matrix protein known to induce cell attachment and spreading through its cell binding site and related synergy sites. Fn-coated surfaces are therefore useful in tissue engineering and other cell contacting applications, but a problem with many immobilization strategies is a random distribution of molecular orientations. We sought to control Fn orientation, and thus enhance the availability of its cell binding site, by immobilizing Fn via a carboxymethyl dextran layer onto which are chemically attached monoclonal antibodies specific to a region near to Fn's C terminus (and thus away from the cell binding site). Using optical waveguide lightmode spectroscopy, we show the presence of chemically coupled antibodies to yield a considerably denser and thicker Fn layer, consistent with a more vertically aligned protein. Human umbilical vein endothelial cells spread significantly faster, and in a more spherically symmetric way, on an oriented Fn layer (i.e., in the presence of immobilized monoclonal antibodies) as compared with a control Fn layer (i.e., in the absence of bound antibodies). However, we observe human umbilical vein endothelial cell spreading on the oriented Fn layer to be similar to that on a Fn layer in the absence of a carboxymethyl dextran layer, suggesting that although orienting Fn is a promising strategy, coupling strategies using linkers other than dextran may be needed.

Fibronectin adsorption on surface-activated poly(dimethylsiloxane) and its effect on cellular function

This article reports that surface modification of poly(dimethylsiloxane) (PDMS) influences fibronectin (Fn) adsorption and enhances cell attachment. Controlled adsorption of Fn on chemically activated polymer substrates is known to influence cellular function. Thin films of PDMS were spun cast on silicon wafers to obtain homogeneous and molecularly smooth surfaces. The films were made hydrophilic by exposure to ultraviolet ozone activation (PDMS*). The films then were characterized by contact angle goniometry, ellipsometry, atomic force microscopy (AFM), Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy. Contact angle measurements indicated higher hydrophobicity of the nonactivated PDMS substrates than PDMS*. AFM scans of the substrates indicated higher surface roughness of PDMS* (Ra = 0.55 nm) than PDMS (Ra = 0.25 nm). Although Fn surface density (Gamma) was slightly higher on PDMS than on PDMS*, due to hydrophobic interactions between substrate and Fn, cell function was greatly enhanced on the Fn-coated PDMS* (PDMS*-Fn) than on PDMS (PDMS-Fn). Higher attachment of MC3T3-E1 osteoblast-like cells was observed on PDMS*-Fn than on PDMS-Fn. Moreover, cell spreading and cytoskeleton organization after 72 h was clearly favored on the Fn-coated PDMS* surfaces.