Hydroxyapatite surface-induced peptide folding

Hybrid Hydroxyapatite-Metal Complex Materials Derived from Amino Acids and Nucleobases

Calcium phosphates (CaPs) and their substituted derivatives encompass a large number of compounds with a vast presence in nature that have aroused a great interest for decades. In particular, hydroxyapatite (HAp, Ca10(OH)2(PO4)6) is the most abundant CaP mineral and is significant in the biological world, at least in part due to being a major compound in bones and teeth. HAp exhibits excellent properties, such as safety, stability, hardness, biocompatibility, and osteoconductivity, among others. Even some of its drawbacks, such as its fragility, can be redirected thanks to another essential feature: its great versatility. This is based on the compound's tendency to undergo substitutions of its constituent ions and to incorporate or anchor new molecules on its surface and pores. Thus, its affinity for biomolecules makes it an optimal compound for multiple applications, mainly, but not only, in biological and biomedical fields. The present review provides a chemical and structural context to explain the affinity of HAp for biomolecules such as proteins and nucleic acids to generate hybrid materials. A size-dependent criterium of increasing complexity is applied, ranging from amino acids/nucleobases to the corresponding macromolecules. The incorporation of metal ions or metal complexes into these functionalized compounds is also discussed.

Interaction of Statherin-Derived Peptide with the Surface of Hydroxyapatite: Perspectives Based on Molecular Dynamics Simulations

INTRODUCTION

Statherin-derived peptide (StatpSpS) has shown promise against erosive tooth wear. To elucidate its interaction with the hydroxyapatite (HAP) surface, the mechanism related to adsorption of this peptide with HAP was investigated through nanosecond-long all-atom molecular dynamics simulations.

METHODS

StatpSpS was positioned parallel to the HAP surface in 2 orientations: 1 - neutral and negative residues facing the surface and 2 - positive residues facing the surface. A system containing StatpSpS without HAP was also simulated as control. In the case of systems with HAP, both partially restrained surface and unrestrained surface were constructed. Structural analysis, interaction pattern, and binding-free energy were calculated.

RESULTS

In the peptide system without the HAP, there were some conformational changes during the simulation. In the presence of the surface, only moderate changes were observed. Many residues exhibited short and stable distances to the surface, indicating strong interaction. Specially, the residues ASP1 and SER2 have an important role to anchor the peptide to the surface, with positively charged residues, mainly arginine, playing a major role in the further stabilization of the peptide in an extended conformation, with close contacts to the HAP surface.

CONCLUSION

The interaction between StatpSpS and HAP is strong, and the negative charged residues are important to the anchoring of the peptide in the surface, but after the initial placement the peptide rearranges itself to maximize the interactions between positive charged residues.

De Novo Design of a Versatile Peptide-Based Coating to Impart Targeted Functionality at the Surface of Native Polystyrene

Peptide sequence periodicity is a simple design tool that can be used to generate functional peptide-based surface coatings. De novo-designed peptide N₃-PEG-VK16 is characterized by a hydrophobic periodicity of two that avidly binds to native polystyrene priming its surface for subsequent targeted functionalization via chemical ligation. The peptidic portion of N₃-PEG-VK16 is responsible for surface binding, converting polystyrene's hydrophobic surface into a wettable and electrostatically charged environment that facilitates cell attachment. Native polystyrene surfaces are coated by simple peptide adsorption from an aqueous buffered solution, and the resulting primed surface is easily functionalized by cycloaddition chemistry. Herein, we show that ligating a vitronectin-derived peptide to primed polystyrene surfaces enables adhesion, expansion, long-term culture, and phenotype maintenance of human induced pluripotent stem cells. To demonstrate scope, we also show that additional functional ligands can be used, for example, nerve growth factor protein, to control neurite outgrowth.

On the stability of peptide secondary structures on the TiO2 (101) anatase surface: a computational insight

The biological activity of proteins is partly due to their secondary structures and conformational states. Peptide chains are rather flexible so that finding ways inducing protein folding in a well-defined state is of great importance. Among the different constraint techniques, the interaction of proteins with inorganic surfaces is a fruitful strategy to stabilize selected folded states. Surface-induced peptide folding can have potential applications in different biomedicine areas, but it can also be of fundamental interest in prebiotic chemistry since the biological activity of a peptide can turn-on when folded in a given state. In this work, periodic quantum mechanical simulations (including implicit solvation effects) at the PBE-D2* level have been carried out to study the adsorption and the stability of the secondary structures (α-helix and β-sheet) of polypeptides with different chemical composition (i.e., polyglycine, polyalanine, polyglutamic acid, polylysine, and polyarginine) on the TiO₂ (101) anatase surface. The computational cost is reduced by applying periodic boundary conditions to both the surface and the peptides, thus obtaining full periodic polypeptide/TiO₂ surface systems. At variance with polyglycine, the interaction of the other polypeptides with the surface takes place with the lateral chain functionalities, leaving the secondary structures almost undistorted. Results indicate that the preferred conformation upon adsorption is the α-helix over the β-sheet, with the exception of the polyglutamic acid. According to the calculated adsorption energies, the affinity trend of the polypeptides with the (101) anatase surface is: polyarginine ≈ polylysine > polyglutamic acid > polyglycine ≈ polyalanine, both when adsorbed in gas phase and in presence of the implicit water solvent, which is very similar to the trend for the single amino acids. A set of implications related to the areas of surface-induced peptide folding, biomedicine and prebiotic chemistry are finally discussed.

Hydroxyapatite Surfaces Functionalized with a Self-Assembling Peptide: XPS, RAIRS and NEXAFS Study

Hydroxyapatite (HAP) coatings can improve the biocompatibility and bioactivity of titanium alloys, such as Ti6Al4V, commonly used as material for orthopedic prostheses. In this framework, we have studied the surface of HAP coatings enriched with Mg and either Si or Ti deposited by RF magnetron sputtering on Ti6Al4V. HAP coatings have been furtherly functionalized by adsorption of a self-assembling peptide (SAP) on the HAP surface, with the aim of increasing the material bioactivity. The selected SAP (peptide sequence AbuEAbuEAbuKAbuKAbuEAbuEAbuKAbuK) is a self-complementary oligopeptide able to generate extended ordered structures by self-assembling in watery solutions. Samples were prepared by incubation of the HAP coatings in SAP solutions and subsequently analyzed by X-Ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared (FTIR) and Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopies, in order to determine the amount of adsorbed peptide, the peptide stability and the structure of the peptide overlayer on the HAP coatings as a function of the HAP substrate and of the pH of the mother SAP solution. Experimental data yielded evidence of SAP adsorption on the HAP surface, and peptide overlayers showed ordered structure and molecular orientation. The thickness of the SAP overlayer depends on the composition of the HAP coating.

Structure-Activity Relationships of Hydroxyapatite-Binding Peptides

Elucidating the structure-activity relationships between biomolecules and hydroxyapatite (HAP) is essential to understand bone mineralization mechanisms, develop HAP-based implants, and design drug delivery vectors. Here, four peptides identified by phage display were selected as model HAP-binding peptides (HBPs) to examine the effects of primary amino acid sequence, phosphorylation of serine, presence of charged amino acid residues, and net charge of the peptide on (1) HAP-binding affinity, (2) secondary conformation, and (3) HAP nucleation and crystal growth. Binding affinities were determined by obtaining adsorption isotherms by mass depletion, and the conformations of the peptides in solution and bound states were observed by circular dichroism. Results showed that the magnitude of the net charge primarily controlled binding affinity, with little dependence on the other HBP features. The binding affinity and conformation results were in good agreement with our previous molecular dynamics simulation results, thus providing an excellent benchmark for the simulations. Transmission electron microscopy was used to explore the effect of these HBPs on calcium phosphate (Ca-PO₄) nucleation and growth. Results indicated that HBPs may inhibit nucleation of Ca-PO₄ nanoparticles and their phase transition to crystalline HAP, as well as control crystal growth rates in specific crystallographic directions, thus changing the classical needle-like morphology of inorganically grown HAP crystals to a biomimetic plate-like morphology.

Human osteoblasts grow transitional Si/N apatite in quickly osteointegrated Si3N4 cervical insert

Silicon nitride (Si₃N₄) ceramics possesses surface chemistry that accelerates bone repair, as previously established by in vitro experiments using both osteosarcoma and mesenchymal cells. The release of silicic acid and nitrogen compounds from the surface Si₃N₄ enhanced in vitro cellular activity. The results of this study demonstrate for the first time that the osseointegration behavior previously observed is operative with a peculiar chemistry within the human milieu. Si and N elements stimulated progenitor cell differentiation and osteoblastic activity, which ultimately resulted in accelerated bone ingrowth. At the molecular scale, insight into the effect of silicon and nitrogen ions released from the Si₃N₄ surface was obtained through combined histomorphometric analyses, Raman, Fourier-transform-infrared, and X-ray photoelectron spectroscopies. Identical analyses conducted on a polyetheretherketone (PEEK) spinal explant showed no chemical changes and a lower propensity for osteogenic activity. Silicon and nitrogen are key elements in stimulating cells to generate bony apatite with crystallographic imperfections, leading to enhanced bioactivity of Si₃N₄ biomedical devices.

STATEMENT OF SIGNIFICANCE

This research studies osseointegration processes comparing results from explanted PEEK and Si₃N₄ spinal spacers. Data show that the formation of hydroxyapatite on silicon nitride bio-ceramic surfaces happens with a peculiar mechanism inside the human body. Silicon and nitrogen were incorporated inside the bony tissue structure allowing the developing of off-stoichiometric bony apatite and stimulating progenitor cell differentiation/osteoblastic activity. Silicon and nitrogen ions released from the Si₃N₄ surface were detected through combined histologic analyses, Raman microspectroscopy, Fourier-transform-infrared, and X-ray photoelectron spectroscopies.

Binding patterns of homo-peptides on bare magnetic nanoparticles: insights into environmental dependence

Magnetic nanoparticles (MNP) are intensively investigated for applications in nanomedicine, catalysis and biotechnology, where their interaction with peptides and proteins plays an important role. However, the characterisation of the interaction of individual amino acids with MNP remains challenging. Here, we classify the affinity of 20 amino acid homo-hexamers to unmodified iron oxide nanoparticles using peptide arrays in a variety of conditions as a basis to identify and rationally design selectively binding peptides. The choice of buffer system is shown to strongly influence the availability of peptide binding sites on the MNP surface. We find that under certain buffer conditions peptides of different charges can bind the MNP and that the relative strength of the interactions can be modulated by changing the buffer. We further present a model for the competition between the buffer and the MNP's electrostatically binding to the adsorption sites. Thereby, we demonstrate that the charge distribution on the surface can be used to correlate the binding of positively and negatively charged peptides to the MNP. This analysis enables us to engineer the binding of MNP on peptides and contribute to better understand the bio-nano interactions, a step towards the design of affinity tags for advanced biomaterials.

Facet selectivity in gold binding peptides: exploiting interfacial water structure

Peptide sequences that can discriminate between gold facets under aqueous conditions offer a promising route to control the growth and organisation of biomimetically-synthesised gold nanoparticles. Knowledge of the interplay between sequence, conformations and interfacial properties is essential for predictable manipulation of these biointerfaces, but the structural connections between a given peptide sequence and its binding affinity remain unclear, impeding practical advances in the field. These structural insights, at atomic-scale resolution, are not easily accessed with experimental approaches, but can be delivered via molecular simulation. A current unmet challenge lies in forging links between predicted adsorption free energies derived from enhanced sampling simulations with the conformational ensemble of the peptide and the water structure at the surface. To meet this challenge, here we use an in situ combination of Replica Exchange with Solute Tempering with Metadynamics simulations to predict the adsorption free energy of a gold-binding peptide sequence, AuBP1, at the aqueous Au(111), Au(100)(1 × 1) and Au(100)(5 × 1) interfaces. We find adsorption to the Au(111) surface is stronger than to Au(100), irrespective of the reconstruction status of the latter. Our predicted free energies agree with experiment, and correlate with trends in interfacial water structuring. For gold, surface hydration is predicted as a chief determining factor in peptide-surface recognition. Our findings can be used to suggest how shaped seed-nanocrystals of Au, in partnership with AuBP1, could be used to control AuNP nanoparticle morphology.

Surface-induced changes in the conformation and glucan production of glucosyltransferase adsorbed on saliva-coated hydroxyapatite

Glucosyltransferases (Gtfs) from S. mutans play critical roles in the development of virulent oral biofilms associated with dental caries disease. Gtfs adsorbed to the tooth surface produce glucans that promote local microbial colonization and provide an insoluble exopolysaccharides (EPS) matrix that facilitates biofilm initiation. Moreover, agents that inhibit the enzymatic activity of Gtfs in solution often have reduced or no effects on surface-adsorbed Gtfs. This study elucidated the mechanisms responsible for the differences in functionality that GtfB exhibits in solution vs surface-adsorbed. Upon adsorption to planar fused-quartz substrates, GtfB displayed a 37% loss of helices and 36% increase of β-sheets, as determined by circular dichroism (CD) spectroscopy, and surface-induced conformational changes were more severe on substrates modified with CH3- and NH2-terminated self-assembled monolayers. GtfB also underwent substantial conformation changes when adsorbing to hydroxyapatite (HA) microspheres, likely due to electrostatic interactions between negatively charged GtfB and positively charged HA crystal faces. Conformational changes were lessened when HA surfaces were coated with saliva (sHA) prior to GtfB adsorption. Furthermore, GtfB remained highly active on sHA, as determined by in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, producing glucans that were structurally different than GtfB in solution and known to increase the accumulation and virulence of biofilms. Our data provide the first insight into the structural underpinnings governing Gtf conformation and enzymatic function that occur on tooth surfaces in vivo, which may lead to designing potent new inhibitors and improved strategies to combat the formation of pathogenic oral biofilms.

Improving the description of interactions between Ca2+ and protein carboxylate groups, including γ-carboxyglutamic acid: revised CHARMM22* parameters

We show that the CHARMM22* force-field over-binds the interaction between aqueous carboxylates and Ca²⁺, and introduce a modification that can recover experimentally-determined binding free energies for these systems.

Redox-responsive nanoreservoirs based on collagen end-capped mesoporous hydroxyapatite nanoparticles for targeted drug delivery

Schematic illustration of a redox-responsive system based on collagen-capped MHAp for cell-targeted drug delivery.

Circular dichroism analysis of cyclic β-helical peptides adsorbed on planar fused quartz

Conformational changes of three cyclic β-helical peptides upon adsorption onto planar fused-quartz substrates were detected and analyzed by far-ultraviolet (UV) circular dichroism (CD) spectroscopy. In trifluoroethanol (TFE), hydrophobic peptides, Leu β and Val β, form left- and right-handed helices, respectively, and water-soluble peptide WS β forms a left-handed helix. Upon adsorption, CD spectra showed a mixture of folded and unfolded conformations for Leu β and Val β and predominantly unfolded conformations for WS β. X-ray photoelectron spectroscopy (XPS) provided insight about the molecular mechanisms governing the conformational changes, revealing that ca. 40% of backbone amides in Leu β and Val β were interacting with the hydrophilic substrate, while only ca. 15% of the amines/amides in WS β showed similar interactions. In their folded β-helical conformations, Leu β and Val β present only hydrophobic groups to their surroundings; hydrophilic surface groups can only interact with backbone amides if the peptides change their conformation. Conversely, as a β helix, WS β presents hydrophilic side chains to its surroundings that could, in principle, interact with hydrophilic surface groups, with the peptide retaining its folded structure. Instead, the observed unfolded surface conformation for WS β and the relatively small percentage of surface-bound amides (15 versus 40% for Leu β and Val β) suggest that hydrophilic surface groups induce unfolding. Upon this surface-induced unfolding, WS β interacts with the surface preferentially via hydrophilic side chains rather than backbone amides. In contrast, the unfolded β-hairpin-like form of WS β does not irreversibly adsorb on fused quartz from water, highlighting that solvation effects can be more important than initial conformation in governing peptide adsorption. Both label-free methods demonstrated in this work are, in general, applicable to structural analysis of a broad range of biomolecules adsorbed on transparent planar substrates, the surface properties of which could be customized.

Valency-dependent affinity of bioactive hydroxyapatite-binding dendrons

Hydroxyapatite (HA)-coated surfaces are used widely as stationary phase for protein and enzyme purification, coatings for dental and orthopedic implants, and composite materials for tissue engineering substrates. More advanced applications are envisioned, but progress has been slowed by the limited ability to controllably functionalize the surface of HA with biomolecules in a translationally relevant manner. Herein we report the synthesis and characterization of a series of multivalent, HA-binding peptide bioconjugates with variable valency and tether length which afford the ability to precisely tune the desired binding behavior. The respective binding affinities of the multivalent constructs to HA surface were characterized by quartz crystal microbalance with dissipation monitoring (QCM-D) techniques, and the relationship between dendron structure and binding affinity was revealed. Tetravalent constructs of HA-binding peptides show a 100-fold enhancement in binding affinity compared to HA-binding peptide sequences reported previously. Both biotin and bone morphogenic protein-2 (BMP-2) derivative peptide were successfully linked to the focal point as initial demonstrations.

Hydroxylapatite nanoparticles: fabrication methods and medical applications

Hydroxylapatite (or hydroxyapatite, HAp) exhibits excellent biocompatibility with various kinds of cells and tissues, making it an ideal candidate for tissue engineering, orthopedic and dental applications. Nanosized materials offer improved performances compared with conventional materials due to their large surface-to-volume ratios. This review summarizes existing knowledge and recent progress in fabrication methods of nanosized (or nanostructured) HAp particles, as well as their recent applications in medical and dental fields. In section 1, we provide a brief overview of HAp and nanoparticles. In section 2, fabrication methods of HAp nanoparticles are described based on the particle formation mechanisms. Recent applications of HAp nanoparticles are summarized in section 3. The future perspectives in this active research area are given in section 4.

Detection of hydroxyapatite in calcified cardiovascular tissues

OBJECTIVE

The objective of this study is to develop a method for selective detection of the calcific (hydroxyapatite) component in human aortic smooth muscle cells in vitro and in calcified cardiovascular tissues ex vivo. This method uses a novel optical molecular imaging contrast dye, Cy-HABP-19, to target calcified cells and tissues.

METHODS

A peptide that mimics the binding affinity of osteocalcin was used to label hydroxyapatite in vitro and ex vivo. Morphological changes in vascular smooth muscle cells were evaluated at an early stage of the mineralization process induced by extrinsic stimuli, osteogenic factors and a magnetic suspension cell culture. Hydroxyapatite components were detected in monolayers of these cells in the presence of osteogenic factors and a magnetic suspension environment.

RESULTS

Atherosclerotic plaque contains multiple components including lipidic, fibrotic, thrombotic, and calcific materials. Using optical imaging and the Cy-HABP-19 molecular imaging probe, we demonstrated that hydroxyapatite components could be selectively distinguished from various calcium salts in human aortic smooth muscle cells in vitro and in calcified cardiovascular tissues, carotid endarterectomy samples and aortic valves, ex vivo.

CONCLUSION

Hydroxyapatite deposits in cardiovascular tissues were selectively detected in the early stage of the calcification process using our Cy-HABP-19 probe. This new probe makes it possible to study the earliest events associated with vascular hydroxyapatite deposition at the cellular and molecular levels. This target-selective molecular imaging probe approach holds high potential for revealing early pathophysiological changes, leading to progression, regression, or stabilization of cardiovascular diseases.

Synthesis and characterisation of large chlorapatite single-crystals with controlled morphology and surface roughness

This work describes the synthesis of chlorapatite single crystals using the molten salt method with CaCl(2) as a flux. By manipulating the processing conditions (amount of flux, firing time and temperature, and cooling rates) it is possible to manipulate the crystal morphology from microscopic fibres to large crystals (up to few millimetre long and ~100 μm thick). The crystal roughness can be controlled to achieve very flat surfaces by changing the melt composition "in situ" at high temperature. The Young modulus and hardness of the crystals are 110 ± 15 and 6.6 ± 1.5 GPa respectively as measured by nanoindentation. Crystal dissolution in Hanks solution starts around the defects. Several in vitro assays were performed; ClAp crystals with different size and shape are biocompatible. Cell apoptosis was very low at 5, 10, and 15 days (Caspase-3) for all the samples. Proliferation (MTT) showed to be influenced by surface roughness and size of the crystals.

Cl–OH ion-exchanging process in chlorapatite (Ca5(PO4)3Cl x (OH)1 − x ) – a deep insight

We have synthesized large chlorapatite [ClAp, Ca5(PO4)3Cl x (OH)1 − x , where x = 1] single crystals using the molten salt method. We have corroborated that the hexagonal symmetry P63/m describes the crystal structure best, even though the crystals are synthetic and stoichiometric. Moreover, we have performed several thermal treatments on these ClAp crystals, generating new single crystals in the apatite system [Ca5(PO4)3Cl x (OH)1 − x , where x ≤ 1], where the chloride anions (Cl−) were systematically substituted by hydroxyl anions (OH−). These new single crystals were methodically characterized by powder and single-crystal X-ray diffraction (SXRD), scanning electron microscopy (SEM), Fourier transform–IR spectroscopy (FT–IR), and energy-dispersive X-ray spectroscopy (EDS). We have discovered a previously unreported OH− inclusion site substituting the Cl− anion during the ion-exchanging process. Finally, we evaluated the atomic rearrangements of the other species involved in the structure. These movements are associated with ionic exchange, which can be justified from an energetic point of view. We also found a novel phase transformation at high temperature. When the crystals are heated over 1753 K the apatite system evolves to a less ordered monoclinic structure, in which the complete loss of the species in the anionic channel (Cl−, OH−) has been confirmed.

Does adsorption at hydroxyapatite surfaces induce peptide folding? Insights from large-scale B3LYP calculations

Large-scale periodic quantum mechanical calculations (509 atoms, 7852 atomic orbitals) based on the hybrid B3LYP functional focused on the peptide folding induced by the adsorption on the (001) and (010) hydroxyapatite (HA) surfaces give interesting insights on the role of specific interactions between surface sites and the peptide, which stabilize the helix conformation over the "native" random coil ones for in silico designed model peptides. The two peptides were derived from the 12-Gly oligomer, with one (P1, C-tGGKGGGGGGEGGN-t) and two (P2, C-tGGKGGKEGGEGGN-t) glutamic acid (E) and lysine (K) residue mutations. The most stable gas-phase "native" conformation for both peptides resulted in a random coil (RC) structure, with the helix (H) conformation being ≈100 kJ mol(-1) higher in free energy. The two peptide conformations interact with the HA (001) and (010) surfaces by C═O groups via Ca(2+) ions, by hydrogen bond between NH(2) groups and the basic PO(4)(3-) groups and by a relevant fraction due to dispersion forces. Peptide adsorption was studied on the dry (001) surface, the wet one envisaging 2 H(2)O per surface Ca(2+) and, on the latter, also considering the adsorption of microsolvated peptides with 4 H(2)O molecules located at sites responsible of the interaction with the surface. The P1 mutant does prefer to be adsorbed as a random coil by ≈160 kJ/mol, whereas the reverse is computed for P2, preferring the helix conformation by ≈50 kJ/mol. Adsorption as helix of both P1 and P2 mutants brings about proton transfer toward the HA surfaces with a large charge transfer component to the interaction energy.

Cementomimetics-constructing a cementum-like biomineralized microlayer via amelogenin-derived peptides

Cementum is the outer-, mineralized-tissue covering the tooth root and an essential part of the system of periodontal tissue that anchors the tooth to the bone. Periodontal disease results from the destructive behavior of the host elicited by an infectious biofilm adhering to the tooth root and left untreated, may lead to tooth loss. We describe a novel protocol for identifying peptide sequences from native proteins with the potential to repair damaged dental tissues by controlling hydroxyapatite biomineralization. Using amelogenin as a case study and a bioinformatics scoring matrix, we identified regions within amelogenin that are shared with a set of hydroxyapatite-binding peptides (HABPs) previously selected by phage display. One 22-amino acid long peptide regions referred to as amelogenin-derived peptide 5 (ADP5) was shown to facilitate cell-free formation of a cementum-like hydroxyapatite mineral layer on demineralized human root dentin that, in turn, supported attachment of periodontal ligament cells in vitro. Our findings have several implications in peptide-assisted mineral formation that mimic biomineralization. By further elaborating the mechanism for protein control over the biomineral formed, we afford new insights into the evolution of protein-mineral interactions. By exploiting small peptide domains of native proteins, our understanding of structure-function relationships of biomineralizing proteins can be extended and these peptides can be utilized to engineer mineral formation. Finally, the cementomimetic layer formed by ADP5 has the potential clinical application to repair diseased root surfaces so as to promote the regeneration of periodontal tissues and thereby reduce the morbidity associated with tooth loss.

Coordination chemistry of Ca sites at the surface of nanosized hydroxyapatite: interaction with H₂O and CO

The affinity towards water of a selection of well-defined, nanostructured hydroxyapatite (HA) samples was investigated by H(2)O vapour adsorption microcalorimetry and infrared (IR) spectroscopy. A large hydrophilicity of all investigated materials was confirmed. The surface features of hydrated HA were investigated on the as-synthesized samples pre-treated in mild conditions at T=303 K, whereas dehydrated HA features were characterized on samples activated at T=573 K. The relatively large hydrophilicity of the hydrated surface (-Δ(ads)H~100-50 kJ mol(-1)) was due to the interaction of water with the highly polarized H(2)O molecules strongly coordinated to the surface Ca(2+) cations. At the dehydrated surface, exposing coordinatively unsaturated (cus) Ca(2+) cations, H(2)O was still molecularly adsorbed but more strongly (-Δ(ads)H~120-90 kJ mol(-1)). The use of CO adsorption to quantify the Lewis acidic strength of HA surface sites revealed only a moderate strength of cus Ca(2+) cations, as confirmed by both microcalorimetric and IR spectroscopic measurements and ab initio calculations. This result implies that the large HA/H(2)O interaction energy is due to the interplay between cus Ca(2+) sites and nearby hydrophilic PO(4) groups, not revealed by the CO probe. The lower density of cus Ca(2+) cations at the 573 K activated HA surface with respect to the pristine one did not affect the whole hydrophilicity of the surface, as the polarizing effect of Ca sites is so strong to extend up to the fourth hydrated layer, as confirmed by both high-coverage microcalorimetric and IR spectroscopic data. No specific effects due to the investigated specimen preparation method and/or different morphology were observed.

Ab initio modelling of protein-biomaterial interactions: influence of amino acid polar side chains on adsorption at hydroxyapatite surfaces

The adsorption from the gas phase of five different amino acids (AAs), namely Gly, Ser, Lys, Gln and Glu, on three surface models of hexagonal hydroxyapatite (HA) has been studied at B3LYP level with Gaussian type basis set within a periodic approach. The AA adsorption was simulated on the (001) and (010) stoichiometric surfaces, the latter both in its pristine and water-reacted form. Low/high AA coverage has been studied by doubling the HA unit cell size. The AAs have been docked to the HA surfaces following the electrostatic complementarity between the electrostatic potentials of AA and the bare HA. Gly adsorbs as a zwitterion at the (001) surface, whereas at the (010) ones, the proton of the COOH group is transferred to the surface resulting in an HA(+)/Gly(-) ion pair. For the other AAs, the common COOH-CH-NH(2) moiety behaves like in Gly, while the specific side-chain functionalities adsorb at the HA surfaces by maximizing electrostatic and H-bond interactions. The interactions between the side chains and the HA surface impart a higher stability compared with the Gly case, with Glu being the strongest adsorbate owing to its high Ca affinity and H-bond donor propensity. For AAs of large size, the adsorption is more favourable in conditions of low coverage as repulsion between adjacent AAs is avoided. For all considered AAs, the strongest interaction is always established on the (010) faces rather than on the (001) one, therefore suggesting an easier growth along the c-direction of HA crystals from AA solutions.

Matrix Gla protein inhibits ectopic calcification by a direct interaction with hydroxyapatite crystals

Mice lacking the gene encoding matrix gla protein (MGP) exhibit massive mineral deposition in blood vessels and die soon after birth. We hypothesize that MGP prevents arterial calcification by adsorbing to growing hydroxyapatite (HA) crystals. To test this, we have used a combined experimental-computational approach. We synthesized peptides covering the entire sequence of human MGP, which contains three sites of serine phosphorylation and five sites of γ-carboxylation, and studied their effects on HA crystal growth using a constant-composition autotitration assay. In parallel studies, the interactions of these sequences with the {100} and {001} faces of HA were analyzed using atomistic molecular dynamics (MD) simulations. YGlapS (amino acids 1-14 of human MGP) and SK-Gla (MGP43-56) adsorbed rapidly to the {100} and {001} faces and strongly inhibited HA growth (IC(50) = 2.96 μg/mL and 4.96 μg/mL, respectively). QR-Gla (MGP29-42) adsorbed more slowly and was a moderate growth inhibitor, while the remaining three (nonpost-translationally modified) peptides had little or no effect in either analysis. Substitution of gla with glutamic acid reduced the adsorption and inhibition activities of SK-Gla and (to a lesser extent) QR-Gla but not YGlapS; substitution of phosphoserine with serine reduced the inhibitory potency of YGlapS. These studies suggest that MGP prevents arterial calcification by a direct interaction with HA crystals that involves both phosphate groups and gla residues of the protein. The strong correlation between simulated adsorption and measured growth inhibition indicates that MD provides a powerful tool to predict the effects of proteins and peptides on crystal formation.

Prediction of calcite morphology from computational and experimental studies of mutations of a de novo-designed peptide

Many organisms use macromolecules, often proteins or peptides, to control the growth of inorganic crystals into complex materials. The ability to model peptide-mineral interactions accurately could allow for the design of novel peptides to produce materials with desired properties. Here, we tested a computational algorithm developed to predict the structure of peptides on mineral surfaces. Using this algorithm, we analyzed energetic and structural differences between a 16-residue peptide (bap4) designed to interact with a calcite growth plane and single- and double-point mutations of the charged residues. Currently, no experimental method is available to resolve the structures of proteins on solid surfaces, which precludes benchmarking for computational models. Therefore, to test the models, we chemically synthesized each peptide and analyzed its effects on calcite crystal growth. Whereas bap4 affected the crystal growth by producing heavily stepped corners and edges, point mutants had variable influences on morphology. Calculated residue-specific binding energies correlated with experimental observations; point mutations of residues predicted to be crucial to surface interactions produced morphologies most similar to unmodified calcite. These results suggest that peptide conformation plays a role in mineral interactions and that the computational model supplies valid energetic and structural data that can provide information about expected crystal morphology.

Osteopontin functionalization of hydroxyapatite nanoparticles in a PDLLA matrix promotes bone formation

We studied the osteoconductive tissue response of hydroxyapatite (HA) nanoparticles functionalized with osteopontin (OPN) in a matrix of poly-D,L-lactic-acid (PDLLA). In a canine endosseus 0.75-mm gap implant model, we tested the osteointegrative impact of the OPN functionalized composite as an implant coating, and a non-functionalized composite was used as reference control. During the four weeks of observation, the OPN functionalized composite coating significantly increased the formation of new bone in the porosities of the implant, but no differences were observed in the gap. The study provides evidence of its potential use either alone or in combination with other osteoconductive compounds.

A multi-scaled hybrid orthopedic implant: bone ECM-shaped Sr-HA nanofibers on the microporous walls of a macroporous titanium scaffold

We report here, for the first time, a novel multi-scaled hybrid orthopedic implant material consisting of a macroporous Ti scaffold, whose macropores' walls have a microporous titania layer which is fully covered with nanofibers of Sr-doped hydroxyapatite (Sr-HA). The microporous titania layer is formed on and within the Ti scaffold by micro-arc oxidation, which firmly binds to the Ti substrate and contains Ca2+, Sr2+ and PO4(3-) ions. It is then hydrothermally treated to form Sr-HA nanofibers. During the hydrothermal treatment, Sr-HA nanoprisms nucleate from Ca0.5Sr0.5TiO3 pre-formed on the TiO2 and grow in length to nanofibers at the expense of Ca2+, Sr2+ and PO4(3-) ions that migrate from the TiO2. These Sr-HA nanofibers construct a network structure similar to the hierarchical organization of bone extracellular matrix (ECM), and the resulting nanofibrous surface displays a firm adhesion to substrate, superhydrophilicity and apatite-inducing ability. The induced apatite prefers to nucleate on the basal-faceted surfaces of Sr-HA nanofibers. The nanofiber-walled scaffold has a great potential for load-bearing orthotopic use.

Computational design of virus-like protein assemblies on carbon nanotube surfaces

There is a general need for the engineering of protein-like molecules that organize into geometrically specific superstructures on molecular surfaces, directing further functionalization to create richly textured, multilayered assemblies. Here we describe a computational approach whereby the surface properties and symmetry of a targeted surface define the sequence and superstructure of surface-organizing peptides. Computational design proceeds in a series of steps that encode both surface recognition and favorable intersubunit packing interactions. This procedure is exemplified in the design of peptides that assemble into a tubular structure surrounding single-walled carbon nanotubes (SWNTs). The geometrically defined, virus-like coating created by these peptides converts the smooth surfaces of SWNTs into highly textured assemblies with long-scale order, capable of directing the assembly of gold nanoparticles into helical arrays along the SWNT axis.

Binding of glycosaminoglycan saccharides to hydroxyapatite surfaces: A density functional theory study

Density functional theory calculations implemented by the SIESTA code are used to study the interactions of the saccharides N-acetylgalactosamine (GalNAc) and glucuronic acid (GlcA) with the (0001) and [Formula: see text] surfaces of the mineral hydroxyapatite (HAP). GalNAc and GlcA are the constituent monosaccharides of chondroitin, which is a glycosaminoglycan found in bone and cartilage, and whose interactions with HAP have been implicated as a controlling factor in the process of biomineralisation. Geometry optimisation calculations are used to identify low energy adsorption structures of the monosaccharides on the HAP surfaces, and to calculate the corresponding adsorption energies. The calculations show that GalNAc interacts with HAP principally through its hydroxy and acetyl amine functional groups, and deprotonated GlcA interacts principally through its hydroxy and carboxylate functional groups. The mode and strength of adsorption depends on the orientation of the saccharide with respect to the HAP surface, which has implications for the structural conformation of chondroitin chains in the presence of hydroxyapatite. Both monosaccharides bind more strongly to the [Formula: see text] surface than to the (0001) surface.

Enhanced stereoselectivity of a Cu(II) complex chiral auxiliary in the synthesis of Fmoc-L-γ-carboxyglutamic acid

L-γ-Carboxyglutamic acid (Gla) is an uncommon amino acid that binds avidly to mineral surfaces and metal ions. Herein, we report the synthesis of N-α-Fmoc-L-γ-carboxyglutamic acid γ,γ'-tert-butyl ester (Fmoc-Gla(O(t)Bu)(2)-OH), a suitably protected analogue for Fmoc-based solid-phase peptide synthesis. The residue was synthesized using a novel chiral Cu(II) complex, whose structure-based design was inspired by the blue copper protein rusticyanin. The five-coordinate complex is formed by Shiff base formation between glycine and the novel ligand (S)-2-(N-(2-methylthio)benzylprolyl)aminobenzophenone in the presence of copper. Michael addition of di-tert-butyl methylenemalonate to the α-carbon of the glycine portion of the complex occurs in a diastereoselective fashion. The resulting (S,S)-complex diastereomer can be easily purified by chromatography. Metal complex decomposition followed by Fmoc protection affords the enantiomerically pure amino acid. With the use of this novel chiral complex, the asymmetric synthesis of Fmoc-Gla(O(t)Bu)(2)-OH was completed in nine steps from thiosalicylic acid in 14.5% overall yield.

Soft landing of complex molecules on surfaces

Soft and reactive landing of mass-selected ions onto surfaces has become a topic of substantial interest due to its promising potential for the highly controlled preparation of materials. For example, there are possible applications in the production of peptide and protein microarrays for use in high-throughput screening, protein separation and conformational enrichment of peptides, redox protein characterization, thin-film production, and the preparation of catalysts through deposition of clusters and organometallic complexes. Soft landing overcomes many of the limitations associated with conventional thin-film production techniques and offers unprecedented selectivity and specificity of preparation of deposited species. This review discusses the fundamental aspects of soft and reactive landing of mass-selected ions on surfaces that pertain to applications of these techniques in biomaterials, molecular electronics, catalysis, and interfacial chemistry.

De novo design of peptide-calcite biomineralization systems

Many organisms produce complex, hierarchically structured, inorganic materials via protein-influenced crystal growth--a process known as biomineralization. Understanding this process would shed light on hard-tissue formation and guide efforts to develop biomaterials. We created and tested a computational method to design protein-biomineralization systems. The algorithm folds a protein from a fully extended structure and simultaneously optimizes the fold, orientation, and sequence of the protein adsorbed to a crystal surface. We used the algorithm to design peptides (16 residues) to modify calcite (CaCO(3)) crystallization. We chemically synthesized six peptides that were predicted to bind different states of a calcite growth plane. All six peptides dramatically affected calcite crystal growth (as observed by scanning electron microscopy), and the effects were dependent on the targeted state of the {001} growth plane. Additionally, we synthesized and assayed scrambled variants of all six designed peptides to distinguish cases where sequence composition determines the interactions versus cases where sequence order (and presumably structure) plays a role. Scrambled variants of negatively charged peptides also had dramatic effects on calcite crystallization; in contrast, scrambled variants of positively charged peptides had a variable effect on crystallization, ranging from dramatic to mild. Special emphasis is often placed on acidic protein residues in calcified tissue mineralization; the work presented here suggests an important role for basic residues as well. In particular, this work implicates a potential role for basic residues in sequence-order specificity for peptide-mineral interactions.

A density functional theory study of the interaction of collagen peptides with hydroxyapatite surfaces

Density functional theory calculations were applied to investigate the binding of four peptide strands, which are important in the collagen protein, to the bone and tooth mineral hydroxyapatite: amphiphilic PRO-HYP-GLY and HYP-PRO-GLY, and hydrophobic PRO-LYS-GLY and PRO-HYL-GLY. The particular peptide sequences are chosen for their different functional groups, containing (i) hydrophobic; (ii) uncharged polar; and (iii) charged polar side groups, thus allowing direct comparison of the general effect of these carboxylic acid and amine functional groups, as well as hydroxylation and charge, on their interactions with two major hydroxyapatite surfaces, (0001) and (0110). The calculated results are consistent with experiments, confirming that the terminal carboxyl groups and amine groups mainly contribute to the adsorption of the peptides to the hydroxyapatite surfaces and primarily to the (0110) surface rather than the dominant (0001) plane. Of the side groups in the tripeptide motifs representing the collagen protein, the -OH and positively charged -NH(3)(+) groups in particular bind strongly to the surfaces, and their presence should therefore promote hydroxyapatite growth.

Nanoparticle-induced folding and fibril formation of coiled-coil-based model peptides

Nanomedicine is a rapidly growing field that has the potential to deliver treatments for many illnesses. However, relatively little is known about the biological risks of nanoparticles. Some studies have shown that nanoparticles can have an impact on the aggregation properties of proteins, including fibril formation. Moreover, these studies also show that the capacity of nanoscale objects to induce or prevent misfolding of the proteins strongly depends on the primary structure of the protein. Herein, light is shed on the role of the peptide primary structure in directing nanoparticle-induced misfolding by means of two model peptides. The design of these peptides is based on the alpha-helical coiled-coil folding motif, but also includes features that enable them to respond to pH changes, thus allowing pH-dependent beta-sheet formation. Previous studies showed that the two peptides differ in the pH range required for beta-sheet folding. Time-dependent circular dichroism spectroscopy and transmission electron microscopy are used to characterize peptide folding and aggregate morphology in the presence of negatively charged gold nanoparticles (AuNPs). Both peptides are found to undergo nanoparticle-induced fibril formation. The determination of binding parameters by isothermal titration calorimetry further reveals that the different propensities of both peptides to form amyloid-like structures in the presence of AuNPs is primarily due to the binding stoichiometry to the AuNPs. Modification of one of the peptide sequences shows that AuNP-induced beta-sheet formation is related to the structural propensity of the primary structure and is not a generic feature of peptide sequences with a sufficiently high binding stoichiometry to the nanoparticles.

Secondary structure in de novo designed peptides induced by electrostatic interaction with a lipid bilayer membrane

We show that it is possible to induce a defined secondary structure in de novo designed peptides upon electrostatic attachment to negatively charged lipid bilayer vesicles without partitioning of the peptides into the membrane, and that the secondary structure can be varied via small changes in the primary amino acid sequence of the peptides. The peptides have a random-coil conformation in solution, and results from far-UV circular dichroism spectroscopy demonstrate that the structure induced by the interaction with silica nanoparticles is solely alpha-helical and also strongly pH-dependent. The present study shows that negatively charged vesicles, to which the peptides are electrostatically adsorbed via cationic amino acid residues, induce either alpha-helices or beta-sheets and that the conformation is dependent on both lipid composition and variations in peptide primary structure. The pH-dependence of the vesicle-induced peptide secondary structure is weak, which correlates well with small differences in the vesicles' electrophoretic mobility, and thus the surface charge, as the pH is varied.

Probing diameter-selective solubilisation of carbon nanotubes by reversible cyclic peptides using molecular dynamics simulations

Molecular dynamics simulations are used to explore the encapsulation behaviour of reversible cyclic peptides when adsorbed onto single-walled carbon nanotubes (CNTs) in aqueous solution. Our findings suggest that CNT encapsulation via cyclisation of a single peptide chain is relatively less likely, compared with encapsulation via two-chain complexes. These two-chain complexes comprise pairings of the motifs identified for single-chain adsorption. Our simulation data are compared with existing experimental findings [A. Ortiz-Acevedo et al., J. Am. Chem. Soc., 2005, 127, 9512], for relevant CNT diameters, and are found to be consistent with the experimental results. Our data help to explain the limited diameter selectivity reported by Ortiz-Acevedo et al. These findings should help in the optimisation and future design of peptides capable of enhanced selectivity for specific CNT diameters.

Modular peptides promote human mesenchymal stem cell differentiation on biomaterial surfaces

Molecular design strategies in biomedical applications often involve creating modular "fusion" proteins, in which distinct domains within a single molecule can perform multiple functions. We have synthesized a new class of modular peptides that include a biologically active sequence derived from the growth factor BMP-2 and a series of hydroxyapatite-binding sequences inspired by the N-terminal alpha-helix of osteocalcin. These modular peptides can bind in a sequence-dependent manner to the surface of "bone-like" hydroxyapatite coatings, which are nucleated and grown on a biodegradable polymer surface via a biomimetic process. The BMP-2-derived sequence of the modular peptides is biologically active, as measured by its ability to promote osteogenic differentiation of human mesenchymal stem cells. Our study indicates that the modular peptides described here are multifunctional, and the characteristics of this approach suggest that it can potentially be applied to a range of biomaterials for regenerative medicine applications.

Characterization of the dominant molecular step orientations on hydroxyapatite (100) surfaces

Hydroxyapatite (HAP) is the major inorganic component of bones and teeth. The characterization of HAP surfaces on the molecular level is important for achieving a fundamental understanding of bone remodeling and dental caries processes. On the microscopic level, hydroxyapatite growth and dissolution reactions mainly occur at steps. Therefore, this study focuses on individual molecular steps on HAP (100) facets under both static conditions and dynamic dissolution conditions using atomic force microscopy (AFM). We found that molecular steps parallel to the elongated axes of HAP crystals and those angled approximately 54 degrees against the elongated axis are not only energetically favorable but also kinetically dominant under dissolution conditions.