Ordered adsorption of coagulation factor XII on negatively charged polymer surfaces probed by sum frequency generation vibrational spectroscopy

Protein adsorption on blood-contacting surfaces: A thermodynamic perspective to guide the design of antithrombogenic polymer coatings

Blood-contacting medical devices often succumb to thrombosis, limiting their durability and safety in clinical applications. Thrombosis is fundamentally initiated by the nonspecific adsorption of proteins to the material surface, which is strongly governed by thermodynamic factors established by the nature of the interaction between the material surface, surrounding water molecules, and the protein itself. Along these lines, different surface materials (such as polymeric, metallic, ceramic, or composite) induce different entropic and enthalpic changes at the surface-protein interface, with material wettability significantly impacting this behavior. Consequently, protein adsorption on medical devices can be modulated by altering their wettability and surface energy. A plethora of polymeric coating modifications have been utilized for this purpose; hydrophobic modifications may promote or inhibit protein adsorption determined by van der Waals forces, while hydrophilic materials achieve this by mainly relying on hydrogen bonding, or unbalanced/balanced electrostatic interactions. This review offers a cohesive understanding of the thermodynamics governing these phenomena, to specifically aid in the design and selection of hemocompatible polymeric coatings for biomedical applications. STATEMENT OF SIGNIFICANCE: Blood-contacting medical devices often succumb to thrombosis, limiting their durability and safety in clinical applications. A plethora of polymeric coating modifications have been utilized for addressing this issue. This review offers a cohesive understanding of the thermodynamics governing these phenomena, to specifically aid in the design and selection of hemocompatible polymeric coatings for biomedical applications.

Strategies for surface coatings of implantable cardiac medical devices

Cardiac medical devices (CMDs) are required when the patient's cardiac capacity or activity is compromised. To guarantee its correct functionality, the building materials in the development of CMDs must focus on several fundamental properties such as strength, stiffness, rigidity, corrosion resistance, etc. The challenge is more significant because CMDs are generally built with at least one metallic and one polymeric part. However, not only the properties of the materials need to be taken into consideration. The biocompatibility of the materials represents one of the major causes of the success of CMDs in the short and long term. Otherwise, the material will lead to several problems of hemocompatibility (e.g., protein adsorption, platelet aggregation, thrombus formation, bacterial infection, and finally, the rejection of the CMDs). To enhance the hemocompatibility of selected materials, surface modification represents a suitable solution. The surface modification involves the attachment of chemical compounds or bioactive compounds to the surface of the material. These coatings interact with the blood and avoid hemocompatibility and infection issues. This work reviews two main topics: 1) the materials employed in developing CMDs and their key characteristics, and 2) the surface modifications reported in the literature, clinical trials, and those that have reached the market. With the aim of providing to the research community, considerations regarding the choice of materials for CMDs, together with the advantages and disadvantages of the surface modifications and the limitations of the studies performed.

Unveiling the main factors triggering the coagulation at the SiC-blood interface

Hemocompatibility is the most significant criterion for blood-contacting materials in successful in vivo applications. Prior to the clinical tests, in vitro analyses must be performed on the biomaterial surfaces in accordance with the ISO 10993-4 standards. Designing a bio-functional material requires engineering the surface structure and chemistry, which significantly influence the blood cell activity according to earlier studies. In this study, we elucidate the role of surface terminations and polymorphs of SiC single crystals in the initial stage of the contact coagulation. We present a detailed analysis of phase, roughness, surface potential, wettability, consequently, reveal their effect on cytotoxicity and hemocompatibility by employing live/dead stainings, live cell imaging, ELISA and Micro BCA protein assay. Our results showed that the surface potential and the wettability strongly depend on the crystallographic polymorph as well as the surface termination. We show, for the first time, the key role of SiC surface termination on platelet activation. This dependency is in good agreement with the results of our in vitro analysis and points out the prominence of cellular anisotropy. We anticipate that our experimental findings bridge the surface properties to the cellular activities, and therefore, pave the way for tailoring advanced hemocompatible surfaces.

Modeling and dynamical analysis of the full-length structure of factor XII with zinc

Zinc (II), the second most abundant transition metal in blood, binds to the initiator of the contact pathway, factor XII (FXII). This binding induces conformational changes in the structure of FXII eventually leading to its activation. Despite many in vitro and in vivo studies on zinc-mediated activation of FXII, its molecular mechanism remains elusive mainly due to absence of a full-length structural model of FXII. To this end, this study investigated the role of zinc in the structure and dynamics of the full-length structure FXII that was obtained through molecular modeling. We have used four structural templates covering more than 70% of the FXII sequence and the remaining interconnecting regions were built by loop modeling. The resulting full-length structure of FXII contained disordered regions, but in comparison to the AlphaFold (AF) prediction, our full-length model represented a more realistic structure because of the disordered regions which were modeled to yield a more compact full-length structure in our model than the AF structure. Other than the disordered regions, our model and AF prediction were highly similar. The resulting full-length FXII structure was used to generate different systems representing the zinc-bound form (holo). Further to assess the contribution of the disulfide bridges, we also analyzed the apo and holo FXII structures with oxidized or reduced cysteine side-chains. Simulations suggested zinc binding conferred rigidity to the structure, particularly to the light chain of FXII. Zinc binding alone was sufficient to limit the backbone flexibility while 15 disulfide bonds, which were scattered throughout the structure, made a less significant contribution to the backbone rigidity. Altogether our results provide insights into the first realistic full-length structure of FXII focusing on the impact of structural zinc and disulfide bridges in the dynamics of this structure.

Probing Orientations and Conformations of Peptides and Proteins at Buried Interfaces

Molecular structures of peptides/proteins at interfaces determine their interfacial properties, which play important roles in many applications. It is difficult to probe interfacial peptide/protein structures because of the lack of appropriate tools. Sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to elucidate molecular structures of peptides/proteins at buried solid/liquid and liquid/liquid interfaces. SFG has been successfully applied to study molecular interactions between model cell membranes and antimicrobial peptides/membrane proteins, surface-immobilized peptides/enzymes, and physically adsorbed peptides/proteins on polymers and 2D materials. A variety of other analytical techniques and computational simulations provide supporting information to SFG studies, leading to more complete understanding of structure-function relationships of interfacial peptides/proteins. With the advance of SFG techniques and data analysis methods, along with newly developed supplemental tools and simulation methodology, SFG research on interfacial peptides/proteins will further impact research in fields like chemistry, biology, biophysics, engineering, and beyond.

Strong Surface Hydration and Salt Resistant Mechanism of a New Nonfouling Zwitterionic Polymer Based on Protein Stabilizer TMAO

Zwitterionic polymers exhibit excellent nonfouling performance due to their strong surface hydrations. However, salt molecules may severely reduce the surface hydrations of typical zwitterionic polymers, making the application of these polymers in real biological and marine environments challenging. Recently, a new zwitterionic polymer brush based on the protein stabilizer trimethylamine N-oxide (TMAO) was developed as an outstanding nonfouling material. Using surface-sensitive sum frequency generation (SFG) vibrational spectroscopy, we investigated the surface hydration of TMAO polymer brushes (pTMAO) and the effects of salts and proteins on such surface hydration. It was discovered that exposure to highly concentrated salt solutions such as seawater only moderately reduced surface hydration. This superior resistance to salt effects compared to other zwitterionic polymers is due to the shorter distance between the positively and negatively charged groups, thus a smaller dipole in pTMAO and strong hydration around TMAO zwitterion. This results in strong bonding interactions between the O^- in pTMAO and water, and weaker interaction between O^- and metal cations due to the strong repulsion from the N⁺ and hydration water. Computer simulations at quantum and atomistic scales were performed to support SFG analyses. In addition to the salt effect, it was discovered that exposure to proteins in seawater exerted minimal influence on the pTMAO surface hydration, indicating complete exclusion of protein attachment. The excellent nonfouling performance of pTMAO originates from its extremely strong surface hydration that exhibits effective resistance to disruptions induced by salts and proteins.

Unsaturated Lipid Accelerates Formation of Oligomeric β-Sheet Structure of GP41 Fusion Peptide in Model Cell Membrane

Membrane fusion of the viral and host cell membranes is the initial step of virus infection and is catalyzed by fusion peptides. Although the β-sheet structure of fusion peptides has been proposed to be the most important fusion-active conformation, it is still very challenging to experimentally identify different types of β-sheet structures at the cell membrane surface in situ and in real time. In this work, we demonstrate that the interface-sensitive amide II spectral signals of protein backbones, generated by the sum frequency generation vibrational spectroscopy, provide a sensitive probe for directly capturing the formation of oligomeric β-sheet structure of fusion peptides. Using human immunodeficiency virus (HIV) glycoprotein GP41 fusing peptide (FP23) as the model, we find that formation speed of oligomeric β-sheet structure depends on lipid unsaturation. The unsaturated lipid such as POPG can accelerate formation of oligomeric β-sheet structure of FP23. The β-sheet structure is more deeply inserted into the hydrophobic region of the POPG bilayer than the α-helical segment. This work will pave the way for future researches on capturing intermediate structures during membrane fusion processes and revealing the fusion mechanism.

Exploring the Heterogeneity of Nanoparticles in Their Interactions with Plasma Coagulation Factor XII

Tuning the characteristics of nanoparticles (NPs) would be promising in improving their biocompatibilities, regarding biosafety and nanodrug considerations. Due to the high priority of the artificial NPs in contacting the circulatory system, understanding their interactions with plasma zymogens is of great importance. Four kinds of NPs, including 5 nm gold NPs (GNP-5), 5 and 20 nm silver NPs (SNP-5, SNP-20), and 20 nm silica NPs (SiNP-20), were investigated for their interactions with the coagulation factor XII (FXII). GNP-5 adsorbed FXII in a standing-up mode, and exhibited high binding affinity for the heavy chain of the protein without altering its secondary structure or inducing its activation. In contrast to GNP-5, FXII adsorption on the other tested NPs was in a lying-down mode, and their interactions with FXII induced its conformational changes, thus causing the evident zymogen cleavage. The structural alterations and activation of FXII induced by the NPs exhibited in specific surface area dependent manners, which were related with different NP cores and sizes. Additionally, the enzymatic activity of α-FXIIa was also influenced by NP incubation, and the alterations were dependent on the specific characters of the NPs as evidenced by the enzymatic inhibition effect of GNP-5 (noncompetitive) and SNP-5 (competitive), and enhanced enzymatic catalysis abilities of SNP-20 and SiNP-20. The interesting findings on the heterogeneity of NPs in their interactions with plasma FXII not only revealed the underlying mechanism for NP-triggered hematological responses, but also suggested the crucial role of tuning NP parameters in their potential bioapplication, like nanodrug design.

Factor XII and uPAR upregulate neutrophil functions to influence wound healing

Coagulation factor XII (FXII) deficiency is associated with decreased neutrophil migration, but the mechanisms remain uncharacterized. Here, we examine how FXII contributes to the inflammatory response. In 2 models of sterile inflammation, FXII-deficient mice (F12-/-) had fewer neutrophils recruited than WT mice. We discovered that neutrophils produced a pool of FXII that is functionally distinct from hepatic-derived FXII and contributes to neutrophil trafficking at sites of inflammation. FXII signals in neutrophils through urokinase plasminogen activator receptor-mediated (uPAR-mediated) Akt2 phosphorylation at S474 (pAktS474). Downstream of pAkt2S474, FXII stimulation of neutrophils upregulated surface expression of αMβ2 integrin, increased intracellular calcium, and promoted extracellular DNA release. The sum of these activities contributed to neutrophil cell adhesion, migration, and release of neutrophil extracellular traps in a process called NETosis. Decreased neutrophil signaling in F12-/- mice resulted in less inflammation and faster wound healing. Targeting hepatic F12 with siRNA did not affect neutrophil migration, whereas WT BM transplanted into F12-/- hosts was sufficient to correct the neutrophil migration defect in F12-/- mice and restore wound inflammation. Importantly, these activities were a zymogen FXII function and independent of FXIIa and contact activation, highlighting that FXII has a sophisticated role in vivo that has not been previously appreciated.

The pathology of the foreign body reaction against biomaterials

The healing process after implantation of biomaterials involves the interaction of many contributing factors. Besides their in vivo functionality, biomaterials also require characteristics that allow their integration into the designated tissue without eliciting an overshooting foreign body reaction (FBR). The targeted design of biomaterials with these features, thus, needs understanding of the molecular mechanisms of the FBR. Much effort has been put into research on the interaction of engineered materials and the host tissue. This elucidated many aspects of the five FBR phases, that is protein adsorption, acute inflammation, chronic inflammation, foreign body giant cell formation, and fibrous capsule formation. However, in practice, it is still difficult to predict the response against a newly designed biomaterial purely based on the knowledge of its physical-chemical surface features. This insufficient knowledge leads to a high number of factors potentially influencing the FBR, which have to be analyzed in complex animal experiments including appropriate data-based sample sizes. This review is focused on the current knowledge on the general mechanisms of the FBR against biomaterials and the influence of biomaterial surface topography and chemical and physical features on the quality and quantity of the reaction. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 927-940, 2017.

Calcium-modified microporous starch with potent hemostatic efficiency and excellent degradability for hemorrhage control

CaMS has been successfully developed to control hemorrhaging, and its hemostatic efficacy together with degradable properties were evaluated.

Size-dependent effects of nanoparticles on enzymes in the blood coagulation cascade

Nanoparticles (NPs) are increasingly used in diagnostic and drug delivery. After entering the bloodstream, a protein corona will form around NPs. The size and curvature of NPs is one of the major characteristics affecting the composition of bound protein in the corona. Key initiators of the intrinsic pathway of blood coagulation, the contact activation complex, (Kallikrein, Factor XII, and high molecular weight Kininogen) have previously been identified on NPs surfaces. We show that the functional impact of carboxyl-modified polystyrene NPs on these initiators of the intrinsic pathway is size dependent. NPs with high curvature affect the enzymatic activity differently from NPs with low curvature. The size dependency is evident in full blood plasma as well as in solutions of single coagulation factors. NPs induce significant alteration of the enzymatic activity in a size-dependent manner, and enzyme kinetics studies show a critical role for NPs surface area and curvature.

Controlling whole blood activation and resultant clot properties by carboxyl and alkyl functional groups on material surfaces: a possible therapeutic approach for enhancing bone healing

Most research virtually ignores the important role of a blood clot in supporting bone healing.

Third-order effects in resonant sum-frequency-generation signals at electrified metal/liquid interfaces

Vibrational sum-frequency-generation (SFG) spectroscopy experiments at electrified interfaces involve incident laser radiation at frequencies in the IR and near-IR/visible regions as well as a static electric field on the surface. Here we show that mixing the three fields present on the surface can result in third-order effects in resonant SFG signals. This was achieved for closed packed self-assembled monolayers (SAMs) with molecular groups of high optical nonlinearity and surface potentials similar to those typically applied in cyclic voltammograms. Broadband SFG spectroscopy was applied to study a hydrophobic well-ordered araliphatic SAM on a Au(111) surface using a thin-layer analysis cell for spectro-electrochemical investigations in a 100 mM NaOH electrolyte solution. Resonant contributions were experimentally separated from non-resonant contributions of the Au substrate and theoretically analyzed using a fitting function including third-order terms. The resulting ratio of third-order to second-order susceptibilities was estimated to be [Formula: see text](10^-10) m/V.

Observing a model ion channel gating action in model cell membranes in real time in situ: membrane potential change induced alamethicin orientation change

Ion channels play crucial roles in transport and regulatory functions of living cells. Understanding the gating mechanisms of these channels is important to understanding and treating diseases that have been linked to ion channels. One potential model peptide for studying the mechanism of ion channel gating is alamethicin, which adopts a split α/3(10)-helix structure and responds to changes in electric potential. In this study, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), has been applied to characterize interactions between alamethicin (a model for larger channel proteins) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers in the presence of an electric potential across the membrane. The membrane potential difference was controlled by changing the pH of the solution in contact with the bilayer and was measured using fluorescence spectroscopy. The orientation angle of alamethicin in POPC lipid bilayers was then determined at different pH values using polarized SFG amide I spectra. Assuming that all molecules adopt the same orientation (a δ distribution), at pH = 6.7 the α-helix at the N-terminus and the 3(10)-helix at the C-terminus tilt at about 72° (θ(1)) and 50° (θ(2)) versus the surface normal, respectively. When pH increases to 11.9, θ(1) and θ(2) decrease to 56.5° and 45°, respectively. The δ distribution assumption was verified using a combination of SFG and ATR-FTIR measurements, which showed a quite narrow distribution in the angle of θ(1) for both pH conditions. This indicates that all alamethicin molecules at the surface adopt a nearly identical orientation in POPC lipid bilayers. The localized pH change in proximity to the bilayer modulates the membrane potential and thus induces a decrease in both the tilt and the bend angles of the two helices in alamethicin. This is the first reported application of SFG to the study of model ion channel gating mechanisms in model cell membranes.

Effect of dehydration on the interfacial water structure at a charged polymer surface: negligible χ(3) contribution to sum frequency generation signal

Interfacial water structure at charged surfaces plays a key role in many physical, chemical, biological, environmental, and industrial processes. Understanding the release of interfacial water from the charged solid surfaces during dehydration process may provide insights into the mechanism of protein folding and the nature of weak molecular interactions. In this work, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by quartz crystal microbalance (QCM) measurements, has been applied to study the interfacial water structure at polyelectrolyte covered surfaces. Poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) chains are grafted on solid surfaces to investigate the change of interfacial water structure with varying surface charge density induced by tuning the solution pH. At pH ≤ 7.1, SFG-VS intensity is linear to the loss of mass of interfacial water caused by the dehydration of PDMAEMA chains, and no reorientation of the strongly bonded water molecules is observed in the light of χ(ppp)/χ(ssp) ratio. χ((3)) contribution to SFG signal is deduced based on the combination of SFG and QCM results. It is the first direct experimental evidence to reveal that the χ((3)) has a negligible contribution to SFG signal of the interfacial water at a charged polymer surface.

Positive charge of chitosan retards blood coagulation on chitosan films

In this study, a series of chitosan films with different protonation degrees were prepared by deacidification with NaOH aqueous or ethanol solutions. The films were then used as a model to investigate the effects of the positive charge of chitosan on blood coagulation. The results showed that the positive charge of chitosan acted as a double-edged sword, in that it promoted erythrocyte adhesion, fibrinogen adsorption, and platelet adhesion and activation, but inhibited activation of the contact system. In contrast to prevailing views, we found that the positive charge of chitosan retarded thrombin generation and blood coagulation on these films. At least two reasons were responsible for this phenomenon. First, the positive charge inhibited the contact activation, and second, the positive charge could not significantly promote the activation of non-adherent platelets in the bulk phase during the early stage of coagulation. The present findings improve our understanding of the events leading to blood coagulation on chitosan films, which will be useful for the future development of novel chitosan-based hemostatic devices.

Amidolytic, procoagulant, and activation-suppressing proteins produced by contact activation of blood factor XII in buffer solution

The relative proportions of enzymes with amidolytic or procoagulant activity produced by contact activation of the blood zymogen factor XII (FXII, Hageman factor) in buffer solution depends on activator surface chemistry/energy. As a consequence, chromogenic assay of amidolytic activity (cleavage of amino acid bonds in s-2302 chromogen) does not correlate with the traditional plasma coagulation time assay for procoagulant activity (protease activity inducing clotting of blood plasma). Amidolytic activity did not vary significantly with activator particle surface energy, herein measured as water adhesion tension τ(o)=γ(lv)(o)cosθ(a) ; where γ(lv)(o) is pure buffer interfacial tension and θ(a) is the advancing contact angle. By contrast, procoagulant activity varied as a parabolic-like function of τ(o), high at both hydrophobic and hydrophilic extremes of activator surface energy and falling through a broad minimum within a 20<τ(o)<40 mJ/m(2) (55°<θ(a) < 75°) range, corroborating and expanding previously-published work. It is inferred from these functional assays that an unknown number of protein fragments are produced by contact activation of FXII (a.k.a. autoactivation) rather than just αFXIIa and βFXIIa as popularly believed. Autoactivation products produced by activator particles within the 20<τ(o)<40 mJ/m(2) (55°<θ(a) < 75°) surface-energy range suppresses production of procoagulant enzymes by activators selected from the hydrophobic or hydrophilic surface-energy extremes through as-yet unknown biophysical chemistry. Suppression proteins may be responsible for the experimentally-observed autoinhibition of the autoactivation reaction.

Immune responses to implants - a review of the implications for the design of immunomodulatory biomaterials

A key for long-term survival and function of biomaterials is that they do not elicit a detrimental immune response. As biomaterials can have profound impacts on the host immune response the concept emerged to design biomaterials that are able to trigger desired immunological outcomes and thus support the healing process. However, engineering such biomaterials requires an in-depth understanding of the host inflammatory and wound healing response to implanted materials. One focus of this review is to outline the up-to-date knowledge on immune responses to biomaterials. Understanding the complex interactions of host response and material implants reveals the need for and also the potential of "immunomodulating" biomaterials. Based on this knowledge, we discuss strategies of triggering appropriate immune responses by functional biomaterials and highlight recent approaches of biomaterials that mimic the physiological extracellular matrix and modify cellular immune responses.

Solvent effect and time-dependent behavior of C-terminus-cysteine-modified cecropin P1 chemically immobilized on a polymer surface

Sum frequency generation (SFG) vibrational spectroscopy has been applied to the investigation of peptide immobilization on a polymer surface as a function of time and peptide conformation. Surface immobilization of biological molecules is important in many applications such as biosensors, antimicrobial materials, biobased fuel cells, nanofabrication, and multifunctional materials. Using C-terminus-cysteine-modified cecropin P1 (CP1c) as a model, we investigated the time-dependent immobilization behavior in situ in real time. In addition, potassium phosphate buffer (PB) and mixtures of PB and trifluoroethanol were utilized to examine the effect of peptide secondary structure on CP1c immobilization to polystyrene maleimide (PS-MA). The orientation of immobilized CP1c on PS-MA was determined using polarized SFG spectra. It was found that the peptide solution concentration, solvent composition, and assembly state (monomer vs dimer) prior to immobilization all influence the orientation of CP1c on a PS-MA surface. The detailed relationship between the interfacial peptide orientation and these immobilization conditions is discussed.

Probing the spontaneous membrane insertion of a tail-anchored membrane protein by sum frequency generation spectroscopy

In addition to providing a semipermeable barrier that protects a cell from harmful stimuli, lipid membranes occupy a central role in hosting a variety of biological processes, including cellular communications and membrane protein functions. Most importantly, protein-membrane interactions are implicated in a variety of diseases and therefore many analytical techniques were developed to study the basis of these interactions and their influence on the molecular architecture of the cell membrane. In this study, sum frequency generation (SFG) vibrational spectroscopy is used to investigate the spontaneous membrane insertion process of cytochrome b(5) and its mutants. Experimental results show a significant difference in the membrane insertion and orientation properties of these proteins, which can be correlated with their functional differences. In particular, our results correlate the nonfunctional property of a mutant cytochrome b(5) with its inability to insert into the lipid bilayer. The approach reported in this study could be used as a potential rapid screening tool in measuring the topology of membrane proteins as well as interactions of biomolecules with lipid bilayers in situ.

Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecular-level understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecular-level information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.

Surface orientation of magainin 2: molecular dynamics simulation and sum frequency generation vibrational spectroscopic studies

We combined molecular dynamics based free energy calculations with sum frequency generation (SFG) spectroscopy to study the orientational distribution of solvated peptides near hydrophobic surfaces. Using a simplified atomistic model of the polystyrene (PS) surface, molecular dynamics simulations have been applied to compute the orientational probability of an α-helical peptide, magainin 2, with respect to the PS/water interface. Free energy calculations revealed that the preferred (horizontal) peptide orientation was driven by the favorable interactions between the hydrophobic PS surface and the hydrophobic residues on the helix, and additional simulations examined the importance of small aggregate formation. Concentration-dependent measurements obtained via SFG vibrational spectroscopy suggest that, at very low peptide concentrations, magainin molecules tend to lie down at the PS/solution interface, which correlates well with the simulation results. When the concentration is increased, peptides exhibit behavior not captured by MD simulations using single helical peptides. A combination of simulations and experiments was shown to yield more reliable results with molecular-level insights into interaction between peptides and polymer surfaces.

Orientation determination of interfacial beta-sheet structures in situ

Structural information such as orientations of interfacial proteins and peptides is important for understanding properties and functions of such biological molecules, which play crucial roles in biological applications and processes such as antimicrobial selectivity, membrane protein activity, biocompatibility, and biosensing performance. The alpha-helical and beta-sheet structures are the most widely encountered secondary structures in peptides and proteins. In this paper, for the first time, a method to quantify the orientation of the interfacial beta-sheet structure using a combined attenuated total reflectance Fourier transformation infrared spectroscopic (ATR-FTIR) and sum frequency generation (SFG) vibrational spectroscopic study was developed. As an illustration of the methodology, the orientation of tachyplesin I, a 17 amino acid peptide with an antiparallel beta-sheet, adsorbed to polymer surfaces as well as associated with a lipid bilayer was determined using the regular and chiral SFG spectra, together with polarized ATR-FTIR amide I signals. Both the tilt angle (theta) and the twist angle (psi) of the beta-sheet at interfaces are determined. The developed method in this paper can be used to obtain in situ structural information of beta-sheet components in complex molecules. The combination of this method and the existing methodology that is currently used to investigate alpha-helical structures will greatly broaden the application of optical spectroscopy in physical chemistry, biochemistry, biophysics, and structural biology.

Factor XII: what does it contribute to our understanding of the physiology and pathophysiology of hemostasis & thrombosis

Factor XII (FXII) is a coagulation protein that is essential for surface-activated blood coagulation tests but whose deficiency is not associated with bleeding. For over forty years, investigators in hemostasis have not considered FXII important because its deficiency is not associated with bleeding. It is because there is a dichotomy between abnormal laboratory assay findings due to FXII deficiency and clinical hemostasis that investigators sought explanations for physiologic hemostasis independent of FXII. FXII is a multidomain protein that contains two fibronectin binding consensual sequences, two epidermal growth factor regions, a kringle region, a proline-rich domain, and a catalytic domain that when proteolyzed turns into a plasma serine protease. Recent investigations with FXII deleted mice that are protected from thrombosis indicate that it contributes to the extent of developing thrombus in the intravascular compartment. These findings suggest that it has a role in thrombus formation without influencing hemostasis. Last, FXII has been newly appreciated to be a growth factor that may influence tissue injury repair and angiogenesis. These combined studies suggest that FXII may become a pharmacologic target to reduce arterial thrombosis risk and promote cell repair after injury, without influencing hemostasis.

Orientation difference of chemically immobilized and physically adsorbed biological molecules on polymers detected at the solid/liquid interfaces in situ

A surface sensitive second order nonlinear optical technique, sum frequency generation vibrational spectroscopy, was applied to study peptide orientation on polymer surfaces, supplemented by a linear vibrational spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy. Using the antimicrobial peptide Cecropin P1 as a model system, we have quantitatively demonstrated that chemically immobilized peptides on polymers adopt a more ordered orientation than less tightly bound physically adsorbed peptides. These differences were also observed in different chemical environments, for example, air versus water. Although numerous studies have reported a direct correlation between the choice of immobilization method and the performance of an attached biological molecule, the lack of direct biomolecular structure and orientation data has made it difficult to elucidate the relationship between structure, orientation, and function at a surface. In this work, we directly studied the effect of chemical immobilization method on biomolecular orientation/ordering, an important step for future studies of biomolecular activity. The methods for orientation analysis described within are also of relevance to understanding biosensors, biocompatibility, marine-antifouling, membrane protein functions, and antimicrobial peptide activities.

Interactions of alamethicin with model cell membranes investigated using sum frequency generation vibrational spectroscopy in real time in situ

Structures of membrane-associated peptides and molecular interactions between peptides and cell membrane bilayers govern biological functions of these peptides. Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study such structures and interactions at the molecular level. In this research, SFG has been applied, supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), to characterize the interactions between alamethicin (a model for larger channel proteins) and different lipid bilayers in the absence of membrane potential. The orientation of alamethicin in lipid bilayers has been determined using SFG amide I spectra detected with different polarization combinations. It was found that alamethicin adopts a mixed alpha-helical and 3(10)-helical structure in fluid-phase lipid bilayers. The helix (mainly alpha-helix) at the N-terminus tilts at about 63 degrees versus the surface normal in a fluid-phase 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine-1,1,2,2-d4-N,N,N-trimethyl-d9 (d-DMPC)/1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer. The 3(10)-helix at the C-terminus (beyond the Pro14 residue) tilts at about 43 degrees versus the surface normal. This is the first time to apply SFG to study a 3(10)-helix experimentally. When interacting with a gel-phase lipid bilayer, alamethicin lies down on the gel-phase bilayer surface or aggregates or both, which does not have significant insertion into the lipid bilayer.

Synergistic effect of hydrophobic and anionic surface groups triggers blood coagulation in vitro

Biomaterial induced coagulation encompasses plasmatic and cellular processes. The functional loss of biomedical devices possibly resulting from these thrombotic reactions motivates the need for a better understanding of processes occurring at blood-biomaterial interfaces. Well defined model surfaces providing specific chemical-physical properties (self assembled monolayers (SAMs)) displaying hydrophobic or/and acidic terminal groups were used to uncover initial mechanisms of biomaterial induced coagulation. We investigated the influence of electrical charge and wettability on platelet- and contact activation, the two main actors of blood coagulation, which are often considered as separate mechanisms in biomaterials research. Our results show a dependence of contact activation on acidic surface groups and a correlation of platelet adhesion to surface hydrophobicity. Clot formation resulting from the interplay of blood platelets and contact activation was only found on surfaces combining both acidic and hydrophobic surface groups but not on monolayers displaying extreme hydrophobic/acidic properties.

Surface-specific interaction of the extracellular domain of protein L1 with nitrilotriacetic acid-terminated self-assembled monolayers

We report a study on the interaction of the extracellular domain of trans-membrane proteins N-cadherin and L1 with nitrilotriacetic acid (NTA)-terminated self-assembled monolayers (SAMs) grown on silver and gold surfaces. Quartz crystal microbalance (QCM) and reflection absorption infrared spectroscopy (RAIRS) measurements reveal that upon addition of protein to an NTA-SAM there is a subsequent change in the mass and average chemical structure inside the films formed on the metal substrates. By using vibrational sum frequency generation (VSFG) spectroscopy and making a comparison to SAMs prepared with n-alkanethiols, we find that the formed NTA-SAMs are terminated by ethanol molecules from solution. The ethanol signature vanishes after the addition of L1, which indicates that the L1 proteins can interact specifically with the NTA complex. Although the RAIRS spectra display signatures in the amide and fingerprint regions, the VSFG spectra display only a weak feature at 866 cm(-1), which possibly indicates that some of the abundant phenyl rings in the complex are ordered. Although cell biology experiments suggest the directional complexation of L1, the VSFG experiments suggest that the alpha-helices and beta-sheets of L1 lack any preferential ordering.

Thrombosis in flowing blood

In this issue of Blood, van der Meijden and colleagues report on the mechanisms by which collagen exposure in flow-dependent circulation contributes to thrombus formation.

Sum Frequency Generation Studies on Bioadhesion: Elucidating the Molecular Structure of Proteins at Interfaces

The study of bioadhesion is significant to applications in a variety of scientific fields. Techniques that are surface sensitive need to be utilized to examine these kinds of systems because bioadhesion occurs at the interface between two surfaces. Recently, Sum Frequency Generation (SFG) has been applied to investigate different bioadhesive processes because of its intrinsic surface specificity, excellent sensitivity and its ability to perform experiments in situ. SFG studies on the bioadhesion of fibrinogen, factor XII and mefp-3 on various surfaces will be discussed in this review.

Contact activation of blood-plasma coagulation

This opinion identifies inconsistencies in the generally-accepted surface biophysics involved in contact activation of blood-plasma coagulation, reviews recent experimental work aimed at resolving inconsistencies, and concludes that this standard paradigm requires substantial revision to accommodate new experimental observations. Foremost among these new findings is that surface-catalyzed conversion of the blood zymogen factor XII (FXII, Hageman factor) to the enzyme FXIIa (FXII [surface] --> FXIIa, a.k.a. autoactivation) is not specific for anionic surfaces, as proposed by the standard paradigm. Furthermore, it is found that surface activation is moderated by the protein composition of the fluid phase in which FXII autoactivation occurs by what appears to be a protein-adsorption-competition effect. Both of these findings argue against the standard view that contact activation of plasma coagulation is potentiated by the assembly of activation-complex proteins (FXII, FXI, prekallikrein, and high-molecular weight kininogen) directly onto activating surfaces (procoagulants) through specific protein/surface interactions. These new findings supplement the observation that adsorption behavior of FXII and FXIIa is not remarkably different from a wide variety of other blood proteins surveyed. Similarity in adsorption properties further undermines the idea that FXII and/or FXIIa are distinguished from other blood proteins by unusual adsorption properties resulting in chemically-specific interactions with activating anionic surfaces. IMPACT STATEMENT: This review shows that the consensus biochemical mechanism of contact activation of blood-plasma coagulation that has long served as a rationale for poor hemocompatibility is an inadequate basis for surface engineering of advanced cardiovascular biomaterials.

The elusive physiologic role of Factor XII

Physiologic hemostasis upon injury involves many plasma proteins in a well-regulated cascade of proteolytic reactions to form a clot. Deficiency of blood coagulation Factors VIII, IX, or XI is associated with hemophilia. Factor XII (FXII) autoactivates by contact with a variety of artificial or biologic negatively charged surfaces (contact activation), resulting in blood coagulation and activation of the inflammatory kallikrein-kinin and complement systems. However, surprisingly, individuals deficient in FXII rarely suffer from bleeding disorders. Most biologic surfaces that activate FXII become expressed in disease states. Investigators have long searched for physiologic activators of FXII and its role in vivo. In this issue of the JCI, Maas et al. show that misfolded protein aggregates produced during systemic amyloidosis allow for plasma FXIIa and prekallikrein activation and increased formation of kallikrein-C1 inhibitor complexes, without Factor XIa activation and coagulation (see the related article beginning on page 3208). This study describes a novel biologic surface for FXII activation and activity, which initiates inflammatory events independent of hemostasis.

Misfolded proteins activate factor XII in humans, leading to kallikrein formation without initiating coagulation

When blood is exposed to negatively charged surface materials such as glass, an enzymatic cascade known as the contact system becomes activated. This cascade is initiated by autoactivation of Factor XII and leads to both coagulation (via Factor XI) and an inflammatory response (via the kallikrein-kinin system). However, while Factor XII is important for coagulation in vitro, it is not important for physiological hemostasis, so the physiological role of the contact system remains elusive. Using patient blood samples and isolated proteins, we identified a novel class of Factor XII activators. Factor XII was activated by misfolded protein aggregates that formed by denaturation or by surface adsorption, which specifically led to the activation of the kallikrein-kinin system without inducing coagulation. Consistent with this, we found that Factor XII, but not Factor XI, was activated and kallikrein was formed in blood from patients with systemic amyloidosis, a disease marked by the accumulation and deposition of misfolded plasma proteins. These results show that the kallikrein-kinin system can be activated by Factor XII, in a process separate from the coagulation cascade, and point to a protective role for Factor XII following activation by misfolded protein aggregates.

Probing molecular-level surface structures of polyethersulfone/pluronic F127 blends using sum-frequency generation vibrational spectroscopy

We blended Pluronic F127 into polyethersulfone (PES) to improve surface properties of PES, which has been extensively used in biomaterial and other applications. The molecular surface structures of PES/Pluronic F127 blends have been investigated by sum-frequency generation (SFG) vibrational spectroscopy. The molecular orientation of surface functional groups of PES changed significantly when blended with a small amount of Pluornic F127. Pluronic F127 on the blend surface also exhibited different features upon contacting with water. The entanglement of PES chains with Pluronic F127 molecules rendered the blends with long-term surface stability in water in contrast to the situation where a layer of Pluronic F127 adsorbed on the PES surface. Atomic force microscopy (AFM) and quartz crystal microbalance (QCM) measurements were included to determine the relative amount of protein that adsorbed to the blend surfaces. The results showed a decreased protein adsorption amount with increasing Pluronic F127 bulk concentration. The correlations between polymer surface properties and detailed molecular structures obtained by SFG would provide insight into the designing and developing of biomedical polymers and functional membranes with improved fouling-resistant properties.