Chapter 24 Protein Chromatography on Hydroxyapatite Columns

Surface functionalization of hydroxyapatite nanoparticles for biomedical applications

This review completely covers the various aspects of hydroxyapatite (HAp) nanoparticles and their role in different biological situations, and provides the surface and interface contents on (i) hydroxyapatite nanoparticles and their hybridization with organic molecules, (ii) surface designing of hydroxyapatite nanoparticles to provide their biocompatibility and photofunction, and (iii) coating technology of hydroxyapatite nanoparticles. In particular, we summarized how the HAp nanoparticles interact with the different ions and molecules and highlighted the potential for hybridization between HAp nanoparticles and organic molecules, which is driven by the interactions of the HAp nanoparticle surface ions with several functional groups of biological molecules. In addition, we highlighted the studies focusing on the interfacial interactions between the HAp nanoparticles and proteins for exploring the enhanced biocompatibility. Such studies focus on how these interactions affect the hydration layers and protein adsorption. However, the hydration layer state involves diverse molecular interactions that can alter the shape of the adsorbed proteins, thereby affecting cell adhesion and spreading on the surfaces. We also summarized the relationship between the surface properties of the HAp nanoparticles and the hydration layer. Furthermore, we spotlighted the cytocompatible photoluminescent probes that can be developed by designing HAp/organic nanohybrid structures. We then emphasized the importance of photofunctionalization in theranostics, which involves the integration of diagnostics and therapy based on the surface design of the HAp nanoparticles. Furthermore, the coating techniques using HAp nanoparticles and HAp nanoparticle/polymer composites were outlined for fusing base biomaterials with biological tissues. The advantages of HAp/biocompatible polymer composite coatings include the ability to effectively cover porous or irregularly shaped surfaces while controlling the thickness of the coating layer, and the addition of HAp nanoparticles to the polymer matrix improves the mechanical properties, increases the roughness, and forms the morphologies that mimic bone nanostructures. Therefore, the fundamental design of hydroxyapatite nanoparticles and their surfaces was suggested from various aspects for biomedical applications.

The combined effects of binder addition and different sintering methods on the mechanical properties of bovine hydroxyapatite

Hydroxyapatite (HA) from bovine bones has been used as a biomaterial in dentistry due to its biocompatibility and bioactivity. However, dense HA bioceramics still present inadequate properties for applications that require high mechanical performance, such as infrastructure. Microstructural reinforcements and control of ceramic processing steps are methods to improve these shortcomings. The present study assessed the effects of polyvinyl butyral (PVB) addition in combination with two sintering methodologies (2-step and conventional), on the mechanical properties of polycrystalline bovine HA bioceramics. The samples were divided into four groups (with 15 samples per group): conventional sintering with binder (HBC) and without binder (HWC) and 2-step sintering with (HB2) and without binder (HW2). HA was extracted from bovine bones, turned into nanoparticles in a ball mill, and subjected to uniaxial and isostatic pressing into discs, according to ISO 6872 standards. All groups were characterized by x-ray diffractometry (XRD), differential thermal analysis (DTA) and Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and relative density. Besides, mechanical analyses (biaxial flexural strength (BFS) and modulus of elasticity) were also performed. The characterization results demonstrated that adding agglutinants or the sintering method did not affect HA's chemical and structural characteristics. Even so, the HWC group showed the highest mechanical values for BFS and modulus of elasticity being 109.0 (98.0; 117.0) MPa and 105.17 ± 14.65 GPa, respectively. The HA ceramics submitted to conventional sintering and without the addition of binders achieved better mechanical properties than the other groups. The impacts of each variable were discussed and correlated to the final microstructures and mechanical properties.

Fabrication of initial trabecular bone-inspired three-dimensional structure with cell membrane nano fragments

The extracellular matrix of trabecular bone has a large surface exposed to the bone marrow and plays important roles such as hematopoietic stem cell niche formation and maintenance. In vitro reproduction of trabecular bone microenvironment would be valuable not only for developing a functional scaffold for bone marrow tissue engineering but also for understanding its biological functions. Herein, we analyzed and reproduced the initial stages of trabecular bone formation in mouse femur epiphysis. We identified that the trabecular bone formation progressed through the following steps: (i) partial rupture of hypertrophic chondrocytes; (ii) calcospherite formation on cell membrane nano fragments (CNFs) derived from the ruptured cells; and (iii) calcospherite growth and fusion to form the initial three-dimensional (3D) structure of trabecular bones. For reproducing the initial trabecular bone formation in vitro, we collected CNFs from cultured cells and used as nucleation sites for biomimetic calcospherite formation. Strikingly, almost the same 3D structure of the initial trabecular bone could be obtained in vitro by using additional CNFs as a binder to fuse biomimetic calcospherites.

Drug Molecular Immobilization and Photofunctionalization of Calcium Phosphates for Exploring Theranostic Functions

Theranostics (bifunction of therapeutics and diagnostics) has attracted increasing attention due to its efficiency that can reduce the physical and financial burden on patients. One of the promising materials for theranostics is calcium phosphate (CP) and it is biocompatible and can be functionalized not only with drug molecules but also with rare earth ions to show photoluminescence that is necessary for the diagnostic purpose. Such the CP-based hybrids are formed in vivo by interacting between functional groups of organic molecules and inorganic ions. It is of great importance to elucidate the interaction of CP with the photofunctional species and the drug molecules to clarify the relationship between the existing state and function. Well-designed photofunctional CPs will contribute to biomedical fields as highly-functional ormultifunctional theranostic materials at the nanoscales. In this review, we describe the hybridization between CPs and heterogeneous species, mainly focusing on europium(III) ion and methylene blue molecule as the representative photofunctional species for theranostics applications.

Applications of Nano Hydroxyapatite as Adsorbents: A Review

Nano hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) has aroused widespread attention as a green and environmentally friendly adsorbent due to its outstanding ability in removing heavy metal ions, radio nuclides, organic pollutants and fluoride ions for wastewater treatment. The hexagonal crystal structure of HAp supports the adsorption mechanisms including ionic exchange reaction, surface complexation, the co-precipitation of new partially soluble phases and physical adsorption such as electrostatic interaction and hydrogen bonding. However, nano HAp has some drawbacks such as agglomeration and a significant pressure drop during filtration when used in powder form. Therefore, instead of using nano HAp alone, researchers have worked on modificationsand composites of nano HAp to overcome these issues and enhance the adsorption capacity. The modification of cationic doping and organic molecule grafting for nano HAp can promote the immobilization of ions and then increase adsorption capacity. Developing nano HAp composite with biopolymers such as gelatin, chitosan and chitin has proven to obtain a synergetic effect for improving the adsorption capacity of composites, in which nano HAp fixed and dispersed in polymers can playmuch more of a role for adsorption. This review summarizes the adsorption properties and adsorbent applications of nano HAp as well as the methods to enhance the adsorption capacity of nano HAp.

Spray dried hydroxyapatite-based supraparticles with uniform and controllable size and morphology

This work aims to prepare uniform spray dried hydroxyapatite-based (SD HAP-based) supraparticles with controllable morphology via micro-fluidic spray drying. Sodium polyacrylate (PAAS) and sodium chloride (NaCl) were used to prepare the precursor suspensions by regulating the inter-particle repulsive forces and electrostatic shielding effect, respectively. The particle size (D50) and zeta potential of the suspension were highly associated with the mass ratio of HAP to PAAS (m_H/m_P) and the NaCl concentration (C_NaCl), which further had significant effect on the permeability (k) of the droplet shell formed during spray drying and ultimately the supraparticle morphology. D50 ˂ 2 µm and absolute zeta potential ˃ 20 mV, obtained when m_H/m_P ˂ 100 under low C_NaCl, rendered ultralow k and consequently deformed supraparticles; Whereas D50 ˃ 2 µm and absolute zeta potential ˂ 20 mV, achieved by decreasing PAAS amount, i.e. m_H/m_P ≥ 100 or improving C_NaCl to efficiently screen surface net charge of HAP, high k and spherical supraparticles were thus preferentially formed.

Nano-hydroxyapatite improves intestinal absorption of acetazolamide (BCS Class IV drug)–but how?

We earlier reported that coating poorly water-soluble drugs with nano-hydroxyapatite (nano-HAP) improves bioavailability after oral administration. In the present study, we coated BCS Class IV drug acetazolamide (AZ) with nano-HAP (AZ/HAP formulation), and investigated its bioavailability and nano-HAP’s role in promoting it. We tested AZ bioavailability after a single oral dose of the AZ/HAP formulation in rats, followed by a series of in vitro, ex vivo and in vivo testing. The binding state of AZ and nano-HAP was analyzed by gel filtration chromatography. AZ permeability was studied using a Caco-2 cell monolayer assay kit, to test for tight junction penetration, then using an Ussing chamber mounted with intestinal epithelium, both with and without Peyer’s patch tissue, to examine the role of intracellular transport. Fluorescence-labeled nano-HAP particles were administered orally in rats to investigate their localization in the intestinal tract. The area under the blood concentration time-curve in rats was about 4 times higher in the AZ/HAP formulation group than in the untreated AZ group. Gel filtration analysis showed AZ and nano-HAP were not bound. The Caco-2 study showed equivalent AZ permeability for both groups, but without significant change in transepithelial electrical resistance (TEER), indicating that tight junctions were not penetrated. In the Ussing chamber study, no significant difference in AZ permeability between the two groups was observed for epithelium containing Peyer’s patch tissue, but for epithelium without Peyer’s patch tissue, at high concentration, significantly higher permeability in the AZ/HAP formulation group was observed. Fluorescent labeling showed nano-HAP particles were present in both intestinal villi and Peyer’s patch tissue 30 min after oral administration. Our results suggest that nano-HAP’s enhancement of drug permeability from the small intestine occurs not via tight junctions, but intracellularly, via the intestinal villi. Further study to elucidate the mechanism of this permeability enhancement is required.

Purification of Tomato Bushy Stunt Virus Particles by One-Step Hydroxyapatite Column Chromatography

The main aim of this work was to develop a time-saving and cost-effective purification method of infectious plant viral nanoparticles. Virions of Tomato bushy stunt virus (TBSV), which is a member of Tombusvirus genus, were purified by one-step Bio-gel HT Hydroxyapatite (HA) column chromatography. Extracts from Nicotiana benthamiana plants infected with TBSV were directly loaded onto the HA column and eluted by 10 mM sodium phosphate buffer (pH 6.8). A specificity of virions has been confirmed by immunoblotting and electron microscopy. Homogeneity of virions was tested by SDS-PAGE, where only 41 kDa polypeptide bands referring to the capsid protein of TBSV were detected by Coomassie staining. The biological infectious activity of a purified material was demonstrated by observing TBSV-specific symptoms observed in N. benthamiana plants at 7‒10 days of post-inoculation (dpi). Moreover, purified virions were used for immunization of the BALb/c mouse to raise primary antibodies against the TBSV virus. Our results show that in low concentrations of sodium phosphate buffer total proteins extracted from infected plants adsorb to HA sorbent, while viral particles do not adsorb to the HA matrix and flow throw column due to Ca2+ ions implicated in TBSV virions’ structure. This highly effective and simple virus purification protocol can also be used for the isolation of other plant virions.

Minerals in biology and medicine

Natural minerals ('stone drugs') have been used in traditional Chinese medicines for over 2000 years, but there is potential for modern-day use of inorganic minerals to combat viral infections, antimicrobial resistance, and for other areas in need of new therapies and diagnostic aids. Metal and mineral surfaces on scales from milli-to nanometres, either natural or synthetic, are patterned or can be modified with hydrophilic/hydrophobic and ionic/covalent target-recognition sites. They introduce new strategies for medical applications. Such surfaces have novel properties compared to single metal centres. Moreover, 3D mineral particles (including hybrid organo-minerals) can have reactive cavities, and some minerals have dynamic movement of metal ions, anions, and other molecules within their structures. Minerals have a unique ability to interact with viruses, microbes and macro-biomolecules through multipoint ionic and/or non-covalent contacts, with potential for novel applications in therapy and biotechnology. Investigations of mineral deposits in biology, with their often inherent heterogeneity and tendency to become chemically-modified on isolation, are highly challenging, but new methods for their study, including in intact tissues, hold promise for future advances.

Separation of monoclonal antibody monomer-dimer mixtures by gradient elution with ceramic hydroxyapatite

Modeling the chromatographic separations of proteins at manufacturing scale is important since downstream processing costs are often dominant. At such scales, the columns are highly overloaded heightening the challenge of predicting performance. In this work, the separation of a monoclonal antibody monomer-dimer mixture is conducted by gradient elution chromatography with ceramic hydroxyapatite (CHT) columns Type I and Type II under overloaded conditions. Phosphate gradients are shown to be preferable over sodium chloride gradients since the latter result in undesirable pH transitions generated within the column itself. Using sodium phosphate gradients separation is obtained with both CHT types, achieving approximately 90% recovery at 99% monomer purity starting with a mixture containing 30% dimer at total protein loads up to 30 mg/mL. Because of its higher binding capacity, even higher loadings can be obtained with CHT Type I without monomer breakthrough. A hybrid model is developed to describe the separation. The model, based on an empirical description of two-component, competitive isotherms at low sodium phosphate concentration coupled with the stoichiometric displacement model at higher sodium phosphate concentrations, is in good agreement with the experiments using the linear driving force (LDF) approximation to describe adsorption/desorption kinetics. The same LDF rate coefficient predicts the separation at loadings between 0.8 and 30 mg/mL. The model developed in this work can be used as a general tool to optimize operating conditions, understand what factors limit performance, and compare different operating modes.

Calcium and hydroxyapatite binding site of human vitronectin provides insights to abnormal deposit formation

The human blood protein vitronectin (Vn) is a major component of the abnormal deposits associated with age-related macular degeneration, Alzheimer's disease, and many other age-related disorders. Its accumulation with lipids and hydroxyapatite (HAP) has been demonstrated, but the precise mechanism for deposit formation remains unknown. Using a combination of solution and solid-state NMR experiments, cosedimentation assays, differential scanning fluorimetry (DSF), and binding energy calculations, we demonstrate that Vn is capable of binding both soluble ionic calcium and crystalline HAP, with high affinity and chemical specificity. Calcium ions bind preferentially at an external site, at the top of the hemopexin-like (HX) domain, with a group of four Asp carboxylate groups. The same external site is also implicated in HAP binding. Moreover, Vn acquires thermal stability upon association with either calcium ions or crystalline HAP. The data point to a mechanism whereby Vn plays an active role in orchestrating calcified deposit formation. They provide a platform for understanding the pathogenesis of macular degeneration and other related degenerative disorders, and the normal functions of Vn, especially those related to bone resorption.

Mud in the blood: the role of protein-mineral complexes and extracellular vesicles in biomineralisation and calcification

Protein-mineral interaction is known to regulate biomineral stability and morphology. We hypothesise that fluid phases produce highly dynamic protein-mineral complexes involved in physiology and pathology of biomineralisation. Here, we specifically focus on calciprotein particles, complexes of vertebrate mineral-binding proteins and calcium phosphate present in the systemic circulation and abundant in extracellular fluids - hence the designation of the ensuing protein-mineral complexes as "mud in the blood". These complexes exist amongst other extracellular particles that we collectively refer to as "the particle zoo".

Technique for simple apatite coating on a dental resin composite with light-curing through a micro-rough apatite layer

Tooth root surfaces restored with dental resin composites exhibit inferior biocompatibility. The objective of this study was to develop a simple technique for coating apatite onto a resin composite to improve its surface biocompatibility. First, we fabricated a polymer film coated with a micro-rough apatite layer and pressed it (coating-side down) onto a viscous resin composite precursor. As a result of light-induced curing of the precursor through the overlaid film, the micro-rough apatite layer was integrated with the resin composite and, thus, transferred from the polymer film surface to the cured resin composite surface as a result of the difference in interfacial adhesion strength. The transferred apatite layer attached directly to the cured resin composite without any gaps at the microscopic level. The adhesion between the apatite layer and the cured resin composite was so strong that the layer was not peeled off even by a tape-detaching test. The flexural strength of the resulting apatite-coated resin composite was comparable to that of the clinically used resin composite while satisfying the ISO requirement for dental polymer-based restorative materials. Furthermore, the apatite-coated resin composite showed better cell compatibility than the uncoated resin composite. The present apatite coating technique is well suited for dental treatment because the coating is applied during a conventional light curing procedure through simple utilization of the apatite-coated polymer film in place of an uncoated film. The proposed technique represents a practical evolution in dental treatment using light-curing resin composites, although further in vitro and in vivo studies are needed.

Improved packing of preparative biochromatography columns by mechanical vibration

The bioprocessing industry relies on packed-bed column chromatography as its primary separation process to attain the required high product purities and fulfill the strict requirements from regulatory bodies. Conventional column packing methods rely on flow packing and/or mechanical compression. In this work, the application of ultrasound and mechanical vibration during packing was studied with respect to packing density and homogeneity. We investigated two widely used biochromatography media, incompressible ceramic hydroxyapatite, and compressible polymethacrylate-based particles, packed in a laboratory-scale column with an inner diameter of 50 mm. It was shown that ultrasonic irradiation led to reduced particle segregation during sedimentation of a homogenized slurry of polymethacrylate particles. However, the application of ultrasound did not lead to an improved microstructure of already packed columns due to the low volumetric energy input (~152 W/L) caused by high acoustic reflection losses. In contrast, the application of pneumatic mechanical vibration led to considerable improvements. Flow-decoupled axial linear vibration was most suitable at a volumetric force output of ~1,190 N/L. In the case of the ceramic hydroxyapatite particles, a 13% further decrease of the packing height was achieved and the reduced height equivalent to a theoretical plate (rHETP) was decreased by 44%. For the polymethacrylate particles, a 18% further packing consolidation was achieved and the rHETP was reduced by 25%. Hence, it was shown that applying mechanical vibration resulted in more efficiently packed columns. The application of vibration furthermore is potentially suitable for in situ elimination of flow channels near the column wall.

Injectable biomaterials for delivery of interleukin-1 receptor antagonist: Toward improving its therapeutic effect

Interleukin-1 receptor antagonist (IL-1Ra) is a naturally occurring anti-inflammatory cytokine that inhibits IL-1 activity and has been proposed to treat a wide variety of systemic and local inflammatory pathologies for multiple decades. However, the short half-life and high concentration required to inhibit IL-1 activity has limited its use in clinical applications. Many strategies have been developed with the goal of improving the therapeutic efficacy of IL-1Ra for a variety of pathologies, including fusing IL-1Ra to protein/peptide/polymer partners, releasing IL-1Ra from injectable polymer or mineral particles, and release of IL-1Ra from injectable coacervates and gels. This literature review examines injectable biomaterials engineered to improve IL-1Ra delivery, both locally and systemically, to increase its efficacy and ease of use in clinic. STATEMENT OF SIGNIFICANCE: Interleukin-1 receptor antagonist (IL-1Ra) is a therapeutic protein with the potential to treat numerous inflammatory conditions and diseases. However, its short biological half-life and high therapeutic concentration may limit its utility in all but a few clinical scenarios. In this review, we present the biomaterial based delivery strategies which have been explored to deliver IL-1Ra to improve its efficacy and applicability to treat inflammation.

Sorption of divalent heavy metal ions onto functionalized biogenic hydroxyapatite with caffeic acid and 3,4-dihydroxybenzoic acid

The sorption ability of biogenic hydroxyapatite (BHAP) towards heavy metal ions (Pb, Cu, Ni, Cd, and Zn) is compared with functionalized BHAP powders with caffeic acid (CA) and 3,4-dihydroxybenzoic acid (3,4-DHBA). The functionalization of the BHAP with either CA or 3,4-DHBA is indicated by the appearance of the colored powders due to the formation of the interfacial charge transfer (ICT) complexes. The detailed characterization of as-prepared and functionalized BHAP samples was performed using transmission electron microscopy, reflection spectroscopy, thermogravimetric analysis and determination of zeta potential. All three sorbents clearly displayed preferential sorption of Pb ions when the total concentration of multi-component equimolar solutions of heavy metal ions is high. It should be emphasized that the sorption capacity of functionalized BHAP with either CA or 3,4-BHAP was found to be higher, up to 60%, compared to as-prepared BHAP without the decrease of selectivity towards Pb ions.

Chromatographic purification of recombinant human erythropoietin

Recombinant human erythropoietin is a valuable therapeutic protein used in the treatment of several serious diseases. It exists in different isoforms and is produced by genetically modified mammalian cells such as Chinese hamster ovary or human embryonic kidney cells. As for other biopharmaceutical drugs, a key factor for its successful industrial production is to achieve a high degree of purity and to decrease the content of critical impurities to trace amounts. This goal is achieved in the separation sequence which substantial part is formed by chromatographic steps. Therefore, downstream processing forms an essential part of production costs. This review presents the overview of published separation sequences and, analyzes the use of different types of chromatographic media such as affinity, ion-exchange, reversed-phase, hydrophobic interaction, multimodal, and size-exclusion chromatography adsorbents. Their application is discussed with regard to their place in the purification stages generally denoted as capture, intermediate purification and polishing.

Aqueous-phase catalytic hydroxylation of phenol with H2O2 by using a copper incorporated apatite nanocatalyst

Copper incorporated apatite (Cu-apatite) nanomaterial was prepared by a co-precipitation method. The obtained material was characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) and Raman spectroscopy, scanning electron microscopy (SEM, STEM) and nitrogen adsorption–desorption. The as-prepared Cu-apatite was used to catalyze phenol hydroxylation with hydrogen peroxide as an oxidant. The influencing parameters including reaction time, temperature, H₂O₂/phenol ratio and catalyst mass have been investigated. Under the optimized conditions, the Cu-apatite catalyst gave a phenol conversion of 64% with 95% selectivity to dihydroxybenzenes. More importantly, the results of catalyst recycling indicated that the same catalytic performance has been obtained after four cycles with a slight loss of activity and selectivity.

Copper incorporated apatite (Cu-apatite) nanomaterial was prepared by a co-precipitation method.

Retention Mechanism of Proteins in Hydroxyapatite Chromatography - Multimodal Interaction Based Protein Separations: A Model Study

BACKGROUND

Retention mechanism of proteins in hydroxyapatite chromatography (HAC) was investigated by linear gradient elution experiments (LGE).

MATERIALS AND METHODS

Several mobile phase (buffer) solution strategies and solutes were evaluated in order to probe the relative contributions of two adsorption sites of hydroxyapatite (HA) particles, C-site due to Ca (metal affinity) and P-site due to PO4 (cation-exchange). When P-site was blocked, two basic proteins, lysozyme (Lys) and ribonuclease A(RNase), were not retained whereas cytochrome C(Cyt C) and lactoferrin (LF) were retained and also retention of acidic proteins became stronger as the repulsion due to P-site was eliminated. The number of the binding site B values determined from LGE also increased, which also showed reduction of repulsion forces.

CONCLUSION

The selectivity (retention) of four basic proteins (RNase, Lys, Cyt C, LF) in HAC was different from that in ion-exchange chromatography. Moreover, it was possible to tune the selectivity by using NaCl gradient.

Elevated extracellular calcium ions promote proliferation and migration of mesenchymal stem cells via increasing osteopontin expression

Supplementation of mesenchymal stem cells (MSCs) at sites of bone resorption is required for bone homeostasis because of the non-proliferation and short lifespan properties of the osteoblasts. Calcium ions (Ca2+) are released from the bone surfaces during osteoclast-mediated bone resorption. However, how elevated extracellular Ca2+ concentrations would alter MSCs behavior in the proximal sites of bone resorption is largely unknown. In this study, we investigated the effect of extracellular Ca2+ on MSCs phenotype depending on Ca2+ concentrations. We found that the elevated extracellular Ca2+ promoted cell proliferation and matrix mineralization of MSCs. In addition, MSCs induced the expression and secretion of osteopontin (OPN), which enhanced MSCs migration under the elevated extracellular Ca2+ conditions. We developed in vitro osteoclast-mediated bone resorption conditions using mouse calvaria bone slices and demonstrated Ca2+ is released from bone resorption surfaces. We also showed that the MSCs phenotype, including cell proliferation and migration, changed when the cells were treated with a bone resorption-conditioned medium. These findings suggest that the dynamic changes in Ca2+ concentrations in the microenvironments of bone remodeling surfaces modulate MSCs phenotype and thereby contribute to bone regeneration.

One Ion to Rule Them All: Combined Antibacterial, Osteoinductive and Anticancer Properties of Selenite-Incorporated Hydroxyapatite

Although hydroxyapatite (HAp) has been doped with dozens of different ions, the quest for an ion imparting a combination of properties conducive to bone healing is still ongoing. Because of its protean potency and the similarity in size and shape to the phosphate tetrahedron, selenite ion presents a natural ionic substitute in HAp. The incorporation of selenite into synthetic HAp using two different methods - co-precipitation and ion-exchange sorption - was studied for its effect on crystal properties and on a triad of biological responses: antibacterial, anticancer and osteoinductive. Co-precipitation yielded HAp with higher selenite contents than sorption and the stoichiometry of HAp richest in selenite was represented as Ca_9.75(PO₄)_5.75(SeO₃)_0.25(OH)_1.75. Crystallinity of HAp decreased in direct proportion with the amount of selenite incorporated. Because of their lower selenite content, HAp powders prepared by ion-exchange exhibited a consistently higher crystallinity compared to the co-precipitated ones. Annealing partially recovered the crystallinity, yet the difference in crystallinity between powders prepared by co-precipitation and by ion-exchange remained, suggesting that the amorphization is mainly due to structural incorporation of selenite, not its effect on the crystal growth kinetics. The addition of selenite changed the morphology of HAp nanoparticles from acicular to rounded and affected the crystal lattice parameters in different ways depending on whether the powders were annealed or not. As for the annealed powders, the incorporation of selenite contracted the lattice in both a and c crystallographic directions. In the agar diffusion assay, the effectiveness of HAp was more dependent on the presence or absence of selenite in it than on its concentration and was highest against E. coli and S. aureus, moderately high against S. enteritidis and ineffective against P. aeruginosa. In liquid inoculation tests, on the other hand, the antibacterial activity of HAp was directly proportional to the amount of selenite contained in it. The viability of K7M2 osteosarcoma cells decreased in direct proportion with the amount of selenite in HAp and was significantly different from the untreated control and from pure HAp at contents equal to or higher than 1.9 wt.%. In contrast, no reduction was observed in the viability of primary fibroblasts treated with HAp incorporating different amounts of selenite ions, suggesting their potentially selective anticancer activity: lethal for the cancer cells and harmless for the healthy cells. Finally, mRNA expression of bone gamma-carboxyglutamate protein (BGLAP3) was higher in differentiated MC3T3-E1 osteoblastic cells treated with selenite-incorporated HAp particles than in cells treated with pure HAp. The osteoinductive effect was due to an overall higher metabolic activity of cells treated with the particles and not due to increased proliferation. In such a way, a triad of antibacterial, osteoinductive and anticancer activities was attributed to selenite-incorporated HAp.

Plasmon induced enhanced photocatalytic activity of gold loaded hydroxyapatite nanoparticles for methylene blue degradation under visible light

A facile surfactant free wet-precipitation process was employed to prepare hydroxyapatite (HAp) nanoparticles.

Role of Buffers in Protein Formulations

Buffers comprise an integral component of protein formulations. Not only do they function to regulate shifts in pH, they also can stabilize proteins by a variety of mechanisms. The ability of buffers to stabilize therapeutic proteins whether in liquid formulations, frozen solutions, or the solid state is highlighted in this review. Addition of buffers can result in increased conformational stability of proteins, whether by ligand binding or by an excluded solute mechanism. In addition, they can alter the colloidal stability of proteins and modulate interfacial damage. Buffers can also lead to destabilization of proteins, and the stability of buffers themselves is presented. Furthermore, the potential safety and toxicity issues of buffers are discussed, with a special emphasis on the influence of buffers on the perceived pain upon injection. Finally, the interaction of buffers with other excipients is examined.

Effect of nanolayering of calcium salts of phosphoric acid ester monomers on the durability of resin-dentin bonds

To investigate the contribution of nanolayering on resin-dentin bond durability, two phosphoric acid ester resin monomers, 10-methacryloyloxy-decyl-dihydrogen-phosphate (10-MDP) or its analog, methacryloyloxy-penta-propyleneglycol-dihydrogen-phosphate (MDA), were examined for their affinity for mineralized dentin powder in a column chromatography setup. Hydroxyapatite (HA) powder was dispersed in experimental primers consisting of 10-MDP or MDA solvated in ethanol/water and examined with FTIR, (31)P MAS-NMR and XPS. Light-curable 10-MDP or MDA primers were used for bonding to dentin, and examined after 24h or one-year of water-aging by TEM for evidence of nanolayering, and for microtensile bond strength evaluation. Primer-bonded dentin was examined by thin-film XRD to identify short-range order peaks characteristic of nanolayering of resin monomer-Ca salts. Although 10-MDP had better affinity for mineralized dentin than MDA, both monomers completely eluted from the mineralized dentin powder column using ethanol-water as mobile phase, indicating that the adsorption processes were reversible. This finding was supported by chemoanalytic data. XRD of 10-MDP-bonded dentin showed three diffraction peaks hat were absent from MDA-bonded dentin. Nanolayering was identified by TEM in 10-MDP-bonded dentin, but not in MDA-bonded dentin. Significant drop in bond strength (in MPa) was observed for both groups after one-year of water-aging compared with 24-h: 10-MDP group from 48.3±6.3 to 37.4±4.6; MDA group from 50.7±5.0 to 35.7±3.8 (P<0.05), with no significant difference between the two groups at the same time-point. Because both functional monomer-primed, resin-bonded dentin exhibited similar bond strength decline after water-aging, presence of nanolayering is unlikely to contribute to the overall resin-dentin bond durability.

STATEMENT OF SIGNIFICANCE

The durability of resin-dentin bonds in 10-MDP containing self-etching adhesives has been anecdotally attributed to the presence of nanolayering of 10-MDP-calcium salts in the resin-dentin interface. Results of the present work indicate that such a claim cannot be justified. Complete elution of the phosphoric acid ester monomer from mineralized dentin powder in the column chromatography experiments using ethanol-water mobile phase to simulate the solvent mixture employed in most 10-MDP-containing dentin adhesives further challenges the previously proposed adhesion-decalcification concept that utilizes chemical bonding of phosphoric acid ester monomers to apatite as a bonding mechanism in 10-MDP containing dentin adhesives.

Calcium Phosphate as a Key Material for Socially Responsible Tissue Engineering

Socially responsible technologies are designed while taking into consideration the socioeconomic, geopolitical and environmental limitations of regions in which they will be implemented. In the medical context, this involves making therapeutic platforms more accessible and affordable to patients in poor regions of the world wherein a given disease is endemic. This often necessitates going against the reigning trend of making therapeutic nanoparticles ever more structurally complex and expensive. However, studies aimed at simplifying materials and formulations while maintaining the functionality and therapeutic response of their more complex counterparts seldom provoke a significant interest in the scientific community. In this review we demonstrate that such compositional simplifications are meaningful when it comes to the design of a solution for osteomyelitis, a disease that is in its natural, non-postoperative form particularly prevalent in the underdeveloped parts of the world wherein poverty, poor sanitary conditions, and chronically compromised defense lines of the immune system are the norm. We show that calcium phosphate nanoparticles, which are inexpensive to make, could be chemically designed to possess the same functionality as a hypothetic mixture additionally composed of: (a) a bone growth factor; (b) an antibiotic for prophylactic or anti-infective purposes; (c) a bisphosphonate as an antiresorptive compound; (d) a viral vector to enable the intracellular delivery of therapeutics; (e) a luminescent dye; (f) a radiographic component; (g) an imaging contrast agent; (h) a magnetic domain; and (i) polymers as viscous components enabling the injectability of the material and acting as carriers for the sustained release of a drug. In particular, calcium phosphates could: (a) produce tunable drug release profiles; (b) take the form of viscous and injectable, self-setting pastes; (c) be naturally osteo-inductive and inhibitory for osteoclastogenesis; (d) intracellularly deliver bioactive compounds; (e) accommodate an array of functional ions; (f) be processed into macroporous constructs for tissue engineering; and (g) be naturally antimicrobial. All in all, we see in calcium phosphates the presence of a protean nature whose therapeutic potentials have been barely tapped into.

Enhanced photocurrent from Photosystem I upon in vitro truncation of the antennae chlorophyll

Current effects on climate change and dwindling fossil fuel reserves require new materials and methods to convert solar energy into a viable clean energy source. Recent progress in the direct conversion of light into photocurrent has been well documented using Photosystem I. In plants, PSI consists of a core complex and multiple light-harvesting complexes, denoted LHCI and LHCII. Most of the methods for isolating PSI from plants involve a selective, detergent solubilization from thylakoids followed by sucrose gradient density centrifugation. These processes isolate one variant of PSI with a specific ratio of Chl:P700. In this study, we have developed a simple and potentially scalable method for isolating multiple PSI variants using Hydroxyapatite chromatography, which has been well documented in other Photosystem I isolation protocols. By varying the wash conditions, we show that it is possible to change the Chl:P700 ratios. These different PSI complexes were cast into a PSI-Nafion-osmium polymer film that enabled their photoactivity to be measured. Photocurrent increases nearly 400% between highly washed and untreated solutions based on equal chlorophyll content. Importantly, the mild washing conditions remove peripheral Chl and some LHCI without inhibiting the photochemical activity of PSI as suggested by SDS-PAGE analysis. This result could indicate that more P700 could be loaded per surface area for biohybrid devices. Compared with other PSI isolations, this protocol also allows isolation of multiple PSI variants without loss of photochemical activity.

Impact of Porosity and Electrolyte Composition on the Surface Charge of Hydroxyapatite Biomaterials

The success or failure of a material when implanted in the body is greatly determined by the surface properties of the material and the host tissue reactions. The very first event that takes place after implantation is the interaction of soluble ions, molecules and proteins from the biological environment with the material surface leading to the formation of an adsorbed protein layer that will later influence cell attachment. In this context, the particular topography and surface charge of a material become critical as they influence the nature of the proteins that will adsorb. However, very limited information is available on the surface charge of porous substrates. Only until very recently was the determination of the zeta potential on porous membranes accurately determined. The goal of this work was to implement the previous findings for the determination of the zeta potential of a series of porous hydroxyapatite (HA) substrates and to assess how porosity affects the measurements. In addition, studies using various electrolytes were also performed to prove how the specific affinity of certain ions for HA can further impact surface charge. The results showed that all materials exhibited very similar external surface charge (approximately -23 mV), consistent with their almost identical topographies. However, the presence of interconnected pores underneath the sample surface resulted in an additional internal zeta potential that varied with the porosity content. Measurements with different electrolytes confirmed the selectivity of divalent ions for HA underlying the importance of testing biomaterials using relevant electrolytes.

In Vitro and In Vivo Evaluation of Whitlockite Biocompatibility: Comparative Study with Hydroxyapatite and β-Tricalcium Phosphate

Biomimicking ceramics have been developed to induce efficient recovery of damaged hard tissues. Among them, calcium phosphate-based bioceramics have been the most widely used because of their similar composition with human hard tissue and excellent biocompatibilities. However, the incomplete understanding of entire inorganic phases in natural bone has limited the recreation of complete bone compositions. In this work, broad biomedical evaluation of whitlockite (WH: Ca18Mg2(HPO4)2(PO4)12), which is the secondary inorganic phase in bone, is conducted to better understand human hard tissue and to seek potential application as a biomaterial. Based on the recently developed gram-scale method for synthesizing WH nanoparticles, the properties of WH as a material for cellular scaffolding and bone implants are assessed and compared to those of hydroxyapatite (HAP: Ca10(PO4)6(OH)2) and β-tricalcium phosphate (β-TCP: β-Ca3(PO4)2). WH-reinforced composite scaffolds facilitate bone-specific differentiation compared to HAP-reinforced composite scaffolds. Additionally, WH implants induce similar or better bone regeneration in calvarial defects in a rat model compared to HAP and β-TCP implants, with intermediate resorbability. New findings of the properties of WH that distinguish it from HAP and β-TCP are significant in understanding human hard tissue, mimicking bone tissue at the nanoscale and designing functional bioceramics.

Physicochemical fabrication of calcium phosphate-based thin layers and nanospheres using laser processing in solutions

We achieved simple and rapid fabrication of calcium phosphate (CaP)-based thin layers and nanospheres by laser processing in supersaturated solutions.

The Role of Hydroxyl Channel in Defining Selected Physicochemical Peculiarities Exhibited by Hydroxyapatite

Mysteries surrounding the most important mineral for the vertebrate biology, hydroxyapatite, are many. Perhaps the Greek root of its name, απαταo, meaning 'to deceive' and given to its mineral form by the early gem collectors who confused it with more precious stones, is still applicable today, though in a different connotation, descriptive of a number of physicochemical peculiarities exhibited by it. Comparable to water as the epitome of peculiarities in the realm of liquids, hydroxyapatite can serve as a paradigm for peculiarities in the world of solids. Ten of the peculiar properties of hydroxyapatite are sketched in this review piece, ranging from (i) the crystal lattice flexibility to (ii) notorious surface layer instability to (iii) finite piezoelectricity, pyroelectricity and conductivity to protons to (iv) accelerated growth and improved osteoconductivity in the electromagnetic fields to (v) high nucleation rate at low supersaturations and low crystal growth rate at high supersaturations to (vi) higher bioactivity and resorbability of biological apatite compared to the synthetic ones, and beyond. An attempt has been made to explain this array of curious characteristics by referring to a particular element of the crystal structure of hydroxyapatite: the hydroxyl ion channel extending in the direction of the c-axis, through a crystallographic column created by the overlapping calcium ion triangles.

A scoring model for phosphopeptide site localization and its impact on the question of whether to use MSA

The production of structurally significant product ions during the dissociation of phosphopeptides is a key to the successful determination of phosphorylation sites. These diagnostic ions can be generated using the widely adopted MS/MS approach, MS3 (Data Dependent Neutral Loss - DDNL), or by multistage activation (MSA). The main purpose of this work is to introduce a false-localization rate (FLR) probabilistic model to enable unbiased phosphoproteomics studies. Briefly, our algorithm infers a probabilistic function from the distribution of the identified phosphopeptides' XCorr Delta scores (XD-Scores) in the current experiment. Our module infers p-values by relying on Gaussian mixture models and a logistic function. We demonstrate the usefulness of our probabilistic model by revisiting the "to MSA, or not to MSA" dilemma. For this, we use human leukemia-derived cells (K562) as a study model and enriched for phosphopeptides using the hydroxyapatite (HAP) chromatography. The aliquots were analyzed with and without MSA on an Orbitrap-XL. Our XD-Scoring analysis revealed that the MS/MS approach provides more identifications because of its faster scan rate, but that for the same given scan rate higher-confidence spectra can be achieved with MSA. Our software is integrated into the PatternLab for proteomics freely available for academic community at http://www.patternlabforproteomics.org. Biological significance Assigning statistical confidence to phosphorylation sites is necessary for proper phosphoproteomic assessment. Here we present a rigorous statistical model, based on Gaussian mixture models and a logistic function, which overcomes shortcomings of previous tools. The algorithm described herein is made readily available to the scientific community by integrating it into the widely adopted PatternLab for proteomics. This article is part of a Special Issue entitled: Computational Proteomics.

Identification of hydroxyapatite spherules provides new insight into subretinal pigment epithelial deposit formation in the aging eye

Accumulation of protein- and lipid-containing deposits external to the retinal pigment epithelium (RPE) is common in the aging eye, and has long been viewed as the hallmark of age-related macular degeneration (AMD). The cause for the accumulation and retention of molecules in the sub-RPE space, however, remains an enigma. Here, we present fluorescence microscopy and X-ray diffraction evidence for the formation of small (0.5-20 μm in diameter), hollow, hydroxyapatite (HAP) spherules in Bruch's membrane in human eyes. These spherules are distinct in form, placement, and staining from the well-known calcification of the elastin layer of the aging Bruch's membrane. Secondary ion mass spectrometry (SIMS) imaging confirmed the presence of calcium phosphate in the spherules and identified cholesterol enrichment in their core. Using HAP-selective fluorescent dyes, we show that all types of sub-RPE deposits in the macula, as well as in the periphery, contain numerous HAP spherules. Immunohistochemical labeling for proteins characteristic of sub-RPE deposits, such as complement factor H, vitronectin, and amyloid beta, revealed that HAP spherules were coated with these proteins. HAP spherules were also found outside the sub-RPE deposits, ready to bind proteins at the RPE/choroid interface. Based on these results, we propose a novel mechanism for the growth, and possibly even the formation, of sub-RPE deposits, namely, that the deposit growth and formation begin with the deposition of insoluble HAP shells around naturally occurring, cholesterol-containing extracellular lipid droplets at the RPE/choroid interface; proteins and lipids then attach to these shells, initiating or supporting the growth of sub-RPE deposits.

Virus purification and enrichment by hydroxyapatite chromatography on a chip

The spread of infectious diseases has become a global health concern. In order to diagnose infectious diseases quickly and accurately, next-generation DNA sequencing techniques for genetic analysis of infectious viruses have been developed rapidly. However, it takes a very long time to pretreat clinical samples for genetic analysis using next-generation sequencers. We have therefore developed a microfluidic chromatography chip that can purify and enrich viruses in a sample using hydroxyapatite particles packed in a micro-column. We demonstrated the purification of virus from a mixture of virus and FBS protein, and enrichment of the virus using this novel microfluidic chip.

Preparative purification of recombinant proteins: current status and future trends

Advances in fermentation technologies have resulted in the production of increased yields of proteins of economic, biopharmaceutical, and medicinal importance. Consequently, there is an absolute requirement for the development of rapid, cost-effective methodologies which facilitate the purification of such products in the absence of contaminants, such as superfluous proteins and endotoxins. Here, we provide a comprehensive overview of a selection of key purification methodologies currently being applied in both academic and industrial settings and discuss how innovative and effective protocols such as aqueous two-phase partitioning, membrane chromatography, and high-performance tangential flow filtration may be applied independently of or in conjunction with more traditional protocols for downstream processing applications.

Modeling biominerals formed by apatites and DNA

Different aspects of biominerals formed by apatite and DNA have been investigated using computer modeling tools. Firstly, the structure and stability of biominerals in which DNA molecules are embedded into hydroxyapatite and fluoroapatite nanopores have been examined by combining different molecular mechanics methods. After this, the early processes in the nucleation of hydroxyapatite at a DNA template have been investigated using molecular dynamics simulations. Results indicate that duplexes of DNA adopting a B double helix can be encapsulated inside nanopores of hydroxyapatite without undergoing significant distortions in the inter-strand hydrogen bonds and the intra-strand stacking. This ability of hydroxyapatite is practically independent of the DNA sequence, which has been attributed to the stabilizing role of the interactions between the calcium atoms of the mineral and the phosphate groups of the biomolecule. In contrast, the fluorine atoms of fluoroapatite induce pronounced structural distortions in the double helix when embedded in a pore of the same dimensions, resulting in the loss of its most relevant characteristics. On the other hand, molecular dynamics simulations have allowed us to observe the formation of calcium phosphate clusters at the surface of the B-DNA template. Electrostatic interactions between the phosphate groups of DNA and Ca(2+) have been found to essential for the formation of stable ion complexes, which were the starting point of calcium phosphate clusters by incorporating PO3(4) from the solution.

Single-step inline hydroxyapatite enrichment facilitates identification and quantitation of phosphopeptides from mass-limited proteomes with MudPIT

Herein we report the characterization and optimization of single-step inline enrichment of phosphopeptides directly from small amounts of whole cell and tissue lysates (100-500 μg) using a hydroxyapatite (HAP) microcolumn and Multidimensional Protein Identification Technology (MudPIT). In comparison to a triplicate HILIC-IMAC phosphopeptide enrichment study, ∼80% of the phosphopeptides identified using HAP-MudPIT were unique. Similarly, analysis of the consensus phosphorylation motifs between the two enrichment methods illustrates the complementarity of calcium- and iron-based enrichment methods and the higher sensitivity and selectivity of HAP-MudPIT for acidic motifs. We demonstrate how the identification of more multiply phosphorylated peptides from HAP-MudPIT can be used to quantify phosphorylation cooperativity. Through optimization of HAP-MudPIT on a whole cell lysate we routinely achieved identification and quantification of ca. 1000 phosphopeptides from a ∼1 h enrichment and 12 h MudPIT analysis on small quantities of material. Finally, we applied this optimized method to identify phosphorylation sites from a mass-limited mouse brain region, the amygdala (200-500 μg), identifying up to 4000 phosphopeptides per run.