‘Apples’ and ‘oranges’: comparing the structural aspects of biomineral- and ice-interaction proteins

Bioinspired Magnetic Nanochains for Medicine

Superparamagnetic iron oxide nanoparticles (SPIONs) have been widely used for medicine, both in therapy and diagnosis. Their guided assembly into anisotropic structures, such as nanochains, has recently opened new research avenues; for instance, targeted drug delivery. Interestingly, magnetic nanochains do occur in nature, and they are thought to be involved in the navigation and geographic orientation of a variety of animals and bacteria, although many open questions on their formation and functioning remain. In this review, we will analyze what is known about the natural formation of magnetic nanochains, as well as the synthetic protocols to produce them in the laboratory, to conclude with an overview of medical applications and an outlook on future opportunities in this exciting research field.

Chimeric Peptides Quickly Modify the Surface of Personalized 3D Printing Titanium Implants to Promote Osseointegration

Titanium (Ti) and titanium alloys have been widely used in the field of biomedicine. However, the unmatched biomechanics and poor bioactivities of conventional Ti implants usually lead to insufficient osseointegration. To tackle these challenges, it is critical to develop a novel Ti implant that meets the bioadaptive requirements for load-bearing critical bone defects. Notably, three-dimensional (3D)-printed Ti implants mimic the microstructure and mechanical properties of natural bones. Additionally, eco-friendly techniques based on inorganic-binding peptides have been applied to modify Ti surfaces. Herein, in our study, Ti surfaces were modified to reinforce osseointegration using chimeric peptides constructed by connecting W9, RP1P, and minTBP-1 directly or via (GP)4, respectively. PR1P is derived from the extracellular VEGF-binding domain of prominin-1, which increases the expression of VEGF and promotes the binding of VEGF to endothelial cells, thereby accelerating angiogenesis. W9 induces osteoblast differentiation in bone marrow mesenchymal stem cells and human mesenchymal stem cells to promote bone formation. Overall, chimeric peptides promote osseointegration by promoting angiogenesis and osteogenesis. Additionally, chimeric peptides with P3&4 were more effective than those with P1&2 in improving osseointegration, which might be ascribed to the capacity of P3&4 to provide a greater range for chimeric peptides to express their activity. This work successfully used chimeric peptides to modify 3D-Ti implant surfaces to improve osseointegration on the implant-bone surface.

Biomineralization process in hard tissues: The interaction complexity within protein and inorganic counterparts

Biomineralization can be considered as nature's strategy to produce and sustain biominerals, primarily via creation of hard tissues for protection and support. This review examines the biomineralization process within the hard tissues of the human body with special emphasis on the mechanisms and principles of bone and teeth mineralization. We describe the detailed role of proteins and inorganic ions in mediating the mineralization process. Furthermore, we highlight the various available models for studying bone physiology and mineralization starting from the historical static cell line-based methods to the most advanced 3D culture systems, elucidating the pros and cons of each one of these methods. With respect to the mineralization process in teeth, enamel and dentin mineralization is discussed in detail. The key role of intrinsically disordered proteins in modulating the process of mineralization in enamel and dentine is given attention. Finally, nanotechnological interventions in the area of bone and teeth mineralization, diseases and tissue regeneration is also discussed. STATEMENT OF SIGNIFICANCE: This article provides an overview of the biomineralization process within hard tissues of the human body, which encompasses the detailed mechanism innvolved in the formation of structures like teeth and bone. Moreover, we have discussed various available models used for studying biomineralization and also explored the nanotechnological applications in the field of bone regeneration and dentistry.

Polyelectrolyte-calcium complexes as a pre-precursor induce biomimetic mineralization of collagen

Polyelectrolytes such as polyaspartic acid (PAsp) are critical in biomimetic mineralization as stabilizers of amorphous calcium phosphate (ACP) precursors and as nucleation inhibitors similar to non-collagenous proteins (NCPs). Nevertheless, the application of polyelectrolyte-calcium complexes as a pre-precursor, such as PAsp-Ca complexes, in the mineralization of collagen is unexplored. Herein, we propose a polyelectrolyte-Ca complex pre-precursor (PCCP) process for collagen mineralization. By combining three-dimensional (3D) STORM, potential measurements, and cryogenic transmission electron microscopy with molecular dynamics simulations, we show that liquid-like electropositive PAsp-Ca complexes along with free calcium ions infiltrate electronegative collagen fibrils. The PAsp-Ca complexes are immobilized within the fibrils via chelation and hydrogen bonds, and outward movement of free calcium ions is prevented while phosphate and hydroxide are recruited through electrostatic attractions. Afterwards, ACP instantly forms and gradually crystallizes. The PCCP process not only unites two distinct crystallization pathways (classical (free Ca/P ions) and non-classical (polyelectrolyte-Ca complexes)), but also provides a novel strategy for rapid biomimetic mineralization of collagen.

Ice Nucleation on a Corrugated Surface

Heterogeneous ice nucleation is a key process in many environmental and technical fields and is of particular importance in modeling atmospheric behavior and the Earth's climate. Despite an improved understanding of how water binds at solid surfaces, no clear picture has emerged to describe how 3D ice grows from the first water layer, nor what makes a particular surface efficient at nucleating bulk ice. This study reports how water at a corrugated, hydrophilic/hydrophobic surface restructures from a complex 2D network, optimized to match the solid surface, to grow into a continuous ice film. Unlike the water networks formed on plane surfaces, the corrugated Cu(511) surface stabilizes a buckled hexagonal wetting layer containing both hydrogen acceptor and donor sites. First layer water is able to relax into an "icelike" arrangement as further water is deposited, creating an array of donor and acceptor sites with the correct spacing and corrugation to stabilize second layer ice and allow continued commensurate multilayer ice growth. Comparison to previous studies of flat surfaces indicates nanoscale corrugation strongly favors ice nucleation, implying surface corrugation will be an important aspect of the surface morphology on other natural or engineered surfaces.

How Can Additives Control the Early Stages of Mineralisation?

The interactions between additives and mineral precursors and intermediates are at the heart of additive-controlled crystallisation, which is of high importance for various fields. In this commentary, we reflect on potential modes of additive control according to classical nucleation theory on one hand, and from the viewpoint of the so-called pre-nucleation cluster pathway on the other. This includes a brief review of the corresponding literature. While the roles of additives are discussed generally, i.e., without specific chemical or structural details, corresponding properties are outlined where possible. Altogether, our discussion illustrates that “non-classical” nucleation pathways promise an improved understanding of additive-controlled scenarios, which could be utilised in targeted applications in various fields, ranging from scale inhibition to materials chemistry.

SCPP Genes and Their Relatives in Gar: Rapid Expansion of Mineralization Genes in Osteichthyans

Gar is an actinopterygian that has bone, dentin, enameloid, and ganoin (enamel) in teeth and/or scales. Mineralization of these tissues involves genes encoding various secretory calcium-binding phosphoproteins (SCPPs) in osteichthyans, but no SCPP genes have been identified in chondrichthyans to date. In the gar genome, we identified 38 SCPP genes, seven of which encode "acidic-residue-rich" proteins and 31 encode "Pro/Gln (P/Q) rich" proteins. These gar SCPP genes constitute the largest known repertoire, including many newly identified P/Q-rich genes expressed in teeth and/or scales. Among gar SCPP genes, six acidic and three P/Q-rich genes were identified as orthologs of sarcopterygian genes. The sarcopterygian orthologs of most of these acidic genes are involved in bone and/or dentin formation, and sarcopterygian orthologs of all three P/Q-rich genes participate in enamel formation. The finding of these genes in gar suggests that an elaborate SCPP gene-based genetic system for tissue mineralization was already present in stem osteichthyans. While SCPP genes have been thought to originate from ancient SPARCL1, SPARCL1L1 appears to be more closely related to these genes, because it established a structure similar to acidic SCPP genes probably in stem gnathostomes, perhaps at about the same time with the origin of tissue mineralization. Assuming enamel evolved in stem osteichthyans, all P/Q-rich SCPP genes likely arose within the osteichthyan lineage. Furthermore, the absence of acidic SCPP genes in chondrichthyans might be explained by the secondary loss of earliest acidic genes. It appears that many SCPP genes expanded rapidly in stem osteichthyans and in basal actinopterygians.

Role of Macromolecular Assembly of Enamel Matrix Proteins in Enamel Formation

Unlike other mineralized tissues, mature dental enamel is primarily (> 95% by weight) composed of apatitic crystals and has a unique hierarchical structure. Due to its high mineral content and organized structure, enamel has exceptional functional properties and is the hardest substance in the human body. Enamel formation (amelogenesis) is the result of highly orchestrated extracellular processes that regulate the nucleation, growth, and organization of forming mineral crystals. However, major aspects of the mechanism of enamel formation are not well-understood, although substantial evidence suggests that protein-protein and protein-mineral interactions play crucial roles in this process. The purpose of this review is a critical evaluation of the present state of knowledge regarding the potential role of the assembly of enamel matrix proteins in the regulation of crystal growth and the structural organization of the resulting enamel tissue. This review primarily focuses on the structure and function of amelogenin, the predominant enamel matrix protein. This review also provides a brief description of novel in vitro approaches that have used synthetic macromolecules ( i.e., surfactants and polymers) to regulate the formation of hierarchical inorganic (composite) structures in a fashion analogous to that believed to take place in biological systems, such as enamel. Accordingly, this review illustrates the potential for developing bio-inspired approaches to mineralized tissue repair and regeneration. In conclusion, the authors present a hypothesis, based on the evidence presented, that the full-length amelogenin uniquely regulates proper enamel formation through a process of cooperative mineralization, and not as a pre-formed matrix.

Modification of Titanium Substrates with Chimeric Peptides Comprising Antimicrobial and Titanium-Binding Motifs Connected by Linkers To Inhibit Biofilm Formation

Bacterial adhesion and biofilm formation are the primary causes of implant-associated infection, which is difficult to eliminate and may induce failure in dental implants. Chimeric peptides with both binding and antimicrobial motifs may provide a promising alternative to inhibit biofilm formation on titanium surfaces. In this study, chimeric peptides were designed by connecting an antimicrobial motif (JH8194: KRLFRRWQWRMKKY) with a binding motif (minTBP-1: RKLPDA) directly or via flexible/rigid linkers to modify Ti surfaces. We evaluated the binding behavior of peptides using quartz crystal microbalance (QCM) and atomic force microscopy (AFM) techniques and investigated the effect of the modification of titanium surfaces with these peptides on the bioactivity of Streptococcus gordonii (S. gordonii) and Streptococcus sanguis (S. sanguis). Compared with the flexible linker (GGGGS), the rigid linker (PAPAP) significantly increased the adsorption of the chimeric peptide on titanium surfaces (p < 0.05). Concentration-dependent adsorption is consistent with a single Langmuir model, whereas time-dependent adsorption is in line with a two-domain Langmuir model. Additionally, the chimeric peptide with the rigid linker exhibited more effective antimicrobial ability than the peptide with the flexible linker. This finding was ascribed to the ability of the rigid linker to separate functional domains and reduce their interference to the maximum extent. Consequently, the performance of chimeric peptides with specific titanium-binding motifs and antimicrobial motifs against bacteria can be optimized by the proper selection of linkers. This rational design of chimeric peptides provides a promising alternative to inhibit the formation of biofilms on titanium surfaces with the potential to prevent peri-implantitis and peri-implant mucositis.

Formation of calcium phosphate nanostructures under the influence of self-assembling hybrid elastin-like-statherin recombinamers

The self-assembly properties of elastin-like-statherin recombinamers have great influence on calcium phosphate mineralization.

Biomimetic synthesis of struvite with biogenic morphology and implication for pathological biomineralization

Recent studies have found that certain urinary proteins can efficiently inhibit stone formation. These discoveries are significant for developing effective therapies for stone disease, but the inhibition mechanism of crystallization remains elusive. In the present study, polyaspartic acid (PASP) was employed as a model peptide to investigate the effect of urinary proteins on the crystallization and morphological evolution of struvite. The results demonstrate that selective adsorption/binding of PASP onto the {010} and {101} faces of struvite crystals results in arrowhead-shaped morphology, which further evolves into X-shaped and unusual tabular structures with time. Noticeably, these morphologies are reminiscent of biogenic struvite morphology. Concentration-dependent experiments show that PASP can inhibit struvite growth and the inhibitory capacity increases with increasing PASP concentration, whereas aspartic acid monomers do not show a significant effect. Considering that PASP is a structural and functional analogue of the subdomains of aspartic acid-rich proteins, our results reveal that aspartic acid-rich proteins play a key role in regulating biogenic struvite morphology, and aspartic acid residues contribute to the inhibitory capacity of urinary proteins. The potential implications of PASP for developing therapeutic agents for urinary stone disease is also discussed.

Calcium carbonate crystal growth beneath Langmuir monolayers of acidic β-hairpin peptides

Four amphiphilic peptides with designed hairpin structure were synthesized and their monolayers were employed as model systems to study biologically inspired calcium carbonate crystallization. Langmuir monolayers of hairpin peptides were investigated by surface pressure area isotherms, surface potential isotherms, Brewster angle microscopy (BAM), atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy. A β-hairpin conformation was found for all peptides at the air-water interface although their packing arrangements seem to be different. Crystallization of calcium carbonate under these peptide monolayers was investigated at different surface pressures and growth times both by in situ optical microscopy, BAM and ex situ investigations such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). An amorphous calcium carbonate precursor was found at the initial crystallization stage. The crystallization process occurred in three stages. It starts from the nucleation of amorphous particles being a kinetically controlled process. Crystal nuclei subsequently aggregate to large particles and vaterite crystals start to form inside the amorphous layer, with the monolayer fluidity exerting an important role. The third process includes the re-crystallization of vaterite to calcite, which is thermodynamically controlled by monolayer structural factors including the monolayer flexibility and packing arrangement of the polar headgroups. Thus, the kinetic factors, monolayer fluidity and flexibility as well as structure factors govern the crystal morphology and polymorph distribution simultaneously and synergistically.

Role of osteopontin in modulation of hydroxyapatite formation

The presence of osteopontin (OPN) at high levels in both mineralized tissues such as bone and ectopic calcifications such as atherosclerotic plaque presents a conundrum: is OPN a promoter or inhibitor of hydroxyapatite (HA) formation? In vitro studies show that OPN adsorbs tightly to HA and is a potent inhibitor of crystal growth. Although the mechanism of the OPN-HA interaction is not fully understood, it is probably electrostatic in nature. Phosphorylation enhances OPN's ability to adsorb to and inhibit the growth of HA crystals, although other anionic groups also contribute to these properties. Recent findings suggest that OPN is an intrinsically unordered protein and that its lack of folded structure facilitates the protein's adsorption by allowing multiple binding geometries and the sequential formation of ionic bonds with Ca(2+) ions of the crystal surface. By analogy with other biominerals, it is likely that adsorption of OPN to HA results in "pinning" of growth steps. The abundance of OPN at sites of ectopic calcification reflects upregulation of the protein in response to crystal formation or even in response to elevated phosphate levels. Therefore, it appears that OPN is one of a group of proteins that function to prevent crystal formation in soft tissues. The role of OPN in bone mineralization, if any, is less clear. However, it is possible that it modulates HA formation, either by preventing crystal growth in "inappropriate" areas such as the osteoid seam or by regulating crystal growth habit (size and shape).

In vitro models of collagen biomineralization

Over the last several years, significant progress has been made toward understanding the mechanisms involved in the mineralization of hard collagenous tissues, such as bone and dentin. Particularly notable are the identification of transient mineral phases that are precursors to carbonated hydroxyapatite, the identification and characterization of non-collagenous proteins that are involved in controlling mineralization, and significant improvements in our understanding of the structure of collagen. These advances not only represent a paradigm shift in the way collagen mineralization is viewed and understood, but have also brought new challenges to light. In this review, we discuss how recent in vitro models have addressed critical questions regarding the role of the non-collagenous proteins in controlling mineralization, the nature of the interactions between amorphous calcium phosphate and collagen during the early stages of mineralization, and the role of collagen in the mineralization process. We discuss the significance of these findings in expanding our understanding of collagen biomineralization, while addressing some of the limitations that are inherent to in vitro systems.

Cooperative modulation of mineral growth by prismatic-associated Asprich Sequences and Mg(II)

Cooperative effects of magnesium ions and acidic polypeptides originating from a family of proteins known as Asprich (mollusk Atrina rigida) were studied. In our previous studies, these two acidic polypeptides were found to be effective in controlling the morphology of the calcium carbonate mineral, the main inorganic constituent of prismatic layer of the mollusk shell. Since these Asprich sequences are believed to contain a putative magnesium binding domain, the morphology-controlling effects were further investigated with the addition of magnesium ions. The mineral morphology was dramatically changed by the combined influence of each polypeptides and the magnesium ions, substantiating the recognized importance of magnesium in the formation of calcium carbonate-based biominerals.

Primary structure and phosphorylation of dentin matrix protein 1 (DMP1) and dentin phosphophoryn (DPP) uniquely determine their role in biomineralization

The SIBLING (small integrin-binding ligand N-linked glycoproteins) family is the major group of noncollagenous proteins in bone and dentin. These extremely acidic and highly phosphorylated extracellular proteins play critical roles in the formation of collagenous mineralized tissues. Whereas the lack of individual SIBLINGs causes significant mineralization defects in vivo, none of them led to a complete cessation of mineralization suggesting that these proteins have overlapping functions. To assess whether different SIBLINGs regulate biomineralization in a similar manner and how phosphorylation impacts their activity, we studied the effects of two SIBLINGs, dentin matrix protein 1 (DMP1) and dentin phosphophoryn (DPP), on mineral morphology and organization in vitro. Our results demonstrate distinct differences in the effects of these proteins on mineralization. We show that phosphorylation has a profound effect on the regulation of mineralization by both proteins. Specifically, both phosphorylated proteins facilitated organized mineralization of collagen fibrils and phosphorylated DMP1-induced formation of organized mineral bundles in the absence of collagen. In summary, these results indicate that the primary structure and phosphorylation uniquely determine functions of individual SIBLINGs in regulation of mineral morphology and organization.

Biominerals--hierarchical nanocomposites: the example of bone

Many organisms incorporate inorganic solids in their tissues to enhance their functional, primarily mechanical, properties. These mineralized tissues, also called biominerals, are unique organo-mineral nanocomposites, organized at several hierarchical levels, from nano- to macroscale. Unlike man-made composite materials, which often are simple physical blends of their components, the organic and inorganic phases in biominerals interface at the molecular level. Although these tissues are made of relatively weak components under ambient conditions, their hierarchical structural organization and intimate interactions between different elements lead to superior mechanical properties. Understanding basic principles of formation, structure, and functional properties of these tissues might lead to novel bioinspired strategies for material design and better treatments for diseases of the mineralized tissues. This review focuses on general principles of structural organization, formation, and functional properties of biominerals on the example the bone tissues.

Probing the molecular mechanisms of quartz-binding peptides

Understanding the mechanisms of biomineralization and the realization of biology-inspired inorganic materials formation largely depends on our ability to manipulate peptide/solid interfacial interactions. Material interfaces and biointerfaces are critical sites for bioinorganic synthesis, surface diffusion, and molecular recognition. Recently adapted biocombinatorial techniques permit the isolation of peptides recognizing inorganic solids that are used as molecular building blocks, for example, as synthesizers, linkers, and assemblers. Despite their ubiquitous utility in nanotechnology, biotechnology, and medicine, the fundamental mechanisms of molecular recognition of engineered peptides binding to inorganic surfaces remain largely unknown. To explore propensity rules connecting sequence, structure, and function that play key roles in peptide/solid interactions, we combine two different approaches: a statistical analysis that searches for highly enriched motifs among de novo designed peptides, and, atomistic simulations of three experimentally validated peptides. The two strong and one weak quartz-binding peptides were chosen for the simulations at the quartz (100) surface under aqueous conditions. Solution-based peptide structures were analyzed by circular dichroism measurements. Small and hydrophobic residues, such as Pro, play a key role at the interface by making close contact with the solid and hindering formation of intrapeptide hydrogen bonds. The high binding affinity of a peptide may be driven by a combination of favorable enthalpic and entropic effects, that is, a strong binder may possess a large number of possible binding configurations, many of which having relatively high binding energies. The results signify the role of the local molecular environment among the critical residues that participate in solid binding. The work herein describes molecular conformations inherent in material-specific peptides and provides fundamental insight into the atomistic understanding of peptide/solid interfaces.

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.

From biominerals to biomaterials: the role of biomolecule–mineral interactions

Interactions between inorganic materials and biomolecules at the molecular level, although complex, are commonplace. Examples include biominerals, which are, in most cases, facilitated by and in contact with biomolecules; implantable biomaterials; and food and drug handling. The effectiveness of these functional materials is dependent on the interfacial properties, i.e. the extent of molecular level ‘association’ with biomolecules. The present article gives information on biomolecule–inorganic material interactions and illustrates our current understanding using selected examples. The examples include (i) mechanism of biointegration: the role of surface chemistry and protein adsorption, (ii) towards improved aluminium-containing materials, and (iii) understanding the bioinorganic interface: experiment and modelling. A wide range of experimental techniques (microscopic, spectroscopic, particle sizing, thermal methods and solution methods) are used by the research group to study interactions between (bio)molecules and molecular and colloidal species that are coupled with computational simulation studies to gain as much information as possible on the molecular-scale interactions. Our goal is to uncover the mechanisms underpinning any interactions and to identify ‘rules’ or ‘guiding principles’ that could be used to explain and hence predict behaviour for a wide range of (bio)molecule–mineral systems.

Molecular biomimetics: nanotechnology and bionanotechnology using genetically engineered peptides

Nature provides inspiration for designing materials and systems that derive their functions from highly organized structures. Biological hard tissues are hybrid materials having inorganics within a complex organic matrix, the molecular scaffold controlling the inorganic structures. Biocomposites incorporate both biomacromolecules such as proteins, lipids and polysaccharides, and inorganic materials, such as hydroxyapatite, silica, magnetite and calcite. The ordered organization of hierarchical structures in organisms begins via the molecular recognition of inorganics by proteins that control interactions and is followed by the highly efficient self-assembly across scales. Following the molecular biological principle, proteins could also be used in controlling materials formation in practical engineering via self-assembled, hybrid, functional materials structures. In molecular biomimetics, material-specific peptides could be the key in the molecular engineering of biology-inspired materials. With the recent developments of nanoscale engineering in physical sciences and the advances in molecular biology, we now combine genetic tools with synthetic nanoscale constructs to create a novel methodology. We first genetically select and/or design peptides with specific binding to functional solids, tailor their binding and assembly characteristics, develop bifunctional peptide/protein genetic constructs with both material binding and biological activity, and use these as molecular synthesizers, erectors and assemblers. Here, we give an overview of solid-binding peptides as novel molecular agents coupling bio- and nanotechnology.

Effect of molecular conformations on the adsorption behavior of gold-binding peptides

Despite extensive recent reports on combinatorially selected inorganic-binding peptides and their bionanotechnological utility as synthesizers and molecular linkers, there is still only limited knowledge about the molecular mechanisms of peptide binding to solid surfaces. There is, therefore, much work that needs to be carried out in terms of both the fundamentals of solid-binding kinetics of peptides and the effects of peptide primary and secondary structures on their recognition and binding to solid materials. Here we discuss the effects of constraints imposed on FliTrx-selected gold-binding peptide molecular structures upon their quantitative gold-binding affinity. We first selected two novel gold-binding peptide (AuBP) sequences using a FliTrx random peptide display library. These were, then, synthesized in two different forms: cyclic (c), reproducing the original FliTrx gold-binding sequence as displayed on bacterial cells, and linear (l) dodecapeptide gold-binding sequences. All four gold-binding peptides were then analyzed for their adsorption behavior using surface plasmon resonance spectroscopy. The peptides exhibit a range of binding affinities to and adsorption kinetics on gold surfaces, with the equilibrium constant, Keq, varying from 2.5x10(6) to 13.5x10(6) M(-1). Both circular dichroism and molecular mechanics/energy minimization studies reveal that each of the four peptides has various degrees of random coil and polyproline type II molecular conformations in solution. We found that AuBP1 retained its molecular conformation in both the c- and l-forms, and this is reflected in having similar adsorption behavior. On the other hand, the c- and l-forms of AuBP2 have different molecular structures, leading to differences in their gold-binding affinities.

Regulation of in vitro calcium phosphate mineralization by combinatorially selected hydroxyapatite-binding peptides

We report selection and characterization of hydroxyapatite-binding heptapeptides from a peptide-phage library and demonstrate the effects of two peptides, with different binding affinities and structural properties, on the mineralization of calcium phosphate mineral. In vitro mineralization studies carried out using one strong- and one weak-binding peptide, HABP1 and HABP2, respectively, revealed that the former exhibited a drastic outcome on mineralization kinetics and particle morphology. Strong-binding peptide yielded significantly larger crystals, as observed by electron microscopy, in comparison to those formed in the presence of a weak-binding peptide or in the negative control. Molecular structural studies carried out by circular dichroism revealed that HABP1 and HABP2 differed in their secondary structure and conformational stability. The results indicate that sequence, structure, and molecular stability strongly influence the mineralization activity of these peptides. The implication of the research is that the combinatorially selected short-sequence peptides may be used in the restoration or regeneration of hard tissues through their control over of the formation of calcium phosphate biominerals.

Adsorption behavior of linear and cyclic genetically engineered platinum binding peptides

Recently, phage and cell-surface display libraries have been adapted for genetically selecting short peptides for a variety of inorganic materials. Despite the enormous number of inorganic-binding peptides reported and their bionanotechnological utility as synthesizers and molecular linkers, there is still a limited understanding of molecular mechanisms of peptide recognition of and binding to solid materials. As part of our goal of genetically designing these peptides, understanding the binding kinetics and thermodynamics, and using the peptides as molecular erectors, in this report we discuss molecular structural constraints imposed upon the quantitative binding characteristics of peptides with an affinity for inorganics. Specifically, we use a high-affinity seven amino acid Pt-binding sequence, PTSTGQA, as we reported in earlier studies and build two constructs: one is a Cys-Cys constrained "loop" sequence (CPTSTGQAC) that mimics the domain used in the pIII tail sequence of the phage library construction, and the second is the linear form, a septapeptide, without the loop. Both sequences were analyzed for their adsorption behavior on Pt thin films by surface plasmon resonance (SPR) spectroscopy and for their conformational properties by circular dichroism (CD). We find that the cyclic peptide of the integral Pt-binding sequence possesses single or 1:1 Langmuir adsorption behavior and displays equilibrium and adsorption rate constants that are significantly larger than those obtained for the linear form. Conversely, the linear form exhibits biexponential Langmuir isotherm behavior with slower and weaker binding. Furthermore, the structure of the cyclic version was found to adopt a random coil molecular conformation, whereas the linear version adopts a polyproline type II conformation in equilibrium with the random coil. The 2,2,2-trifluoroethanol titration experiments indicate that TFE has a different effect on the secondary structures of the linear and cyclic versions of the Pt binding sequence. We conclude that the presence of the Cys-Cys restraint affects both the conformation and binding behavior of the integral Pt-binding septapeptide sequence and that the presence or absence of constraints could be used to tune the adsorption and structural features of inorganic binding peptide sequences.

Hydroxyapatite surface-induced peptide folding

Herein, we describe the design and surface-binding characterization of a de novo designed peptide, JAK1, which undergoes surface-induced folding at the hydroxyapatite (HA)-solution interface. JAK1 is designed to be unstructured in buffered saline solution, yet undergo HA-induced folding that is largely governed by the periodic positioning of gamma-carboxyglutamic acid (Gla) residues within the primary sequence of the peptide. Circular dichroism (CD) spectroscopy and analytical ultracentrifugation indicate that the peptide remains unfolded and monomeric in solution under normal physiological conditions; however, CD spectroscopy indicates that in the presence of hydroxyapatite, the peptide avidly binds to the mineral surface adopting a helical structure. Adsorption isotherms indicate nearly quantitative surface coverage and Kd = 310 nM for the peptide-surface binding event. X-ray photoelectron spectroscopy (XPS) coupled with the adsorption isotherm data suggests that JAK1 binds to HA, forming a self-limiting monolayer. This study demonstrates the feasibility of using HA surfaces to trigger the intramolecular folding of designed peptides and represents the initial stages of defining the design rules that allow HA-induced peptide folding.

Structural Features That Distinguish Kinetically Distinct Biomineralization Polypeptides

AP7 and AP24 are mollusk shell proteins which are responsible for aragonite polymorph formation and stabilization within the nacre layer of the Pacific red abalone, Haliotis rufescens. It is known that the 30-AA N-terminal mineral modification domains of both proteins (AP7N, AP24N) possess identical multifunctional mineralization capabilities within in vitro assays but differ in terms of rate kinetics, with AP24N > AP7N. In this report, we identify previously unreported molecular features of AP24N and contrast the lowest energy polypeptide backbone structures of AP24N (planar configuration) with that of AP7N ("bent paper clip" configuration) using NMR data and simulated annealing molecular dynamics structure refinement. Like AP7N, we find that AP24N possesses an unfolded conformation, can sequester Ca(II) and other multivalent metal ions, can adsorb onto or within calcite crystals, and possesses anionic and cationic electrostatic "pocket" regions on its molecular surfaces. However, AP24N has some unique features: greater conformational responsiveness to Ca(II), the tendency to form a more planar backbone configuration, and longer anionic and hydrogen-bonding donor/acceptor sequence blocks. We conclude that the presence of unfolded polypeptide conformation, electrostatic surface pockets, and interactive sequence clustering endow both AP7N and AP24N with similar features that lead to comparable effects on crystal morphology and nucleation. However, AP24N possesses longer anionic and hydrogen-bonding sequence clusters and exhibits a tendency to adopt a more planar backbone configuration than AP7N does. We believe that these features facilitate peptide-mineral, peptide-ion, or water cluster interactions, thereby enhancing the mineralization kinetics of AP24N over AP7N.

Probing the conformational features of a phage display polypeptide sequence directed against single-walled carbon nanohorn surfaces

Single-walled carbon nanohorns (SWNHs) are interesting carbon nanostructures that have applications to science and technology. Using M13 phage display technology, polypeptides directed again SWNHs surfaces have been created for a number of nanotechnology and pharmaceutical purposes, yet the molecular mechanism of polypeptide sequence interaction and binding to SWNHs surfaces is not known. Recently, we identified a linear 12-AA M13 phage pIII sequence, NH-12-5-2 (DYFSSPYYEQLF), that binds with high affinity to SWNHs surfaces. To probe the structure of this pIII tail polypeptide further, we investigated the conformation of a model peptide representing the 12 AA NH-12-5-2 sequence. At neutral pH, the NH-12-5-2 model polypeptide is conformationally labile and exhibits two-state conformational exchange involving the D1-S5 N-terminal segment. Simultaneous with this conformational exchange process is the observation that the P6 residue exhibits imido ring conformational variation. In the presence of the structure-stabilizing solvent, TFE, or at pH 2.5, both the exchange process and Pro ring motion phenomena disappear, indicating that the structure of this peptide sequence can be stabilized by extrinsic factors. Interestingly, we observe NMR parameters (ROEs, (3)J coupling constants) for NH-12-5-2 in 90% v/v TFE that are consistent with the presence of a partial helical structure, similar to what was observed at low pH in our earlier CD experiments. We conclude that the NH-12-5-2 model polypeptide sequence possesses an inherent conformational instability that involves the D1-S5 sequence segment and the P6 residue but that this instability can be offset by extrinsic factors (e.g., charge neutralization, imido ring interconversion, and hydrophobic-hydrophobic interactions). These nonbonding interactions may play a role in the recognition and binding of this phage sequence region to SWNHs surfaces.

An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria

Magnetotactic bacteria are widespread aquatic microorganisms that use unique intracellular organelles to navigate along the Earth's magnetic field. These organelles, called magnetosomes, consist of membrane-enclosed magnetite crystals that are thought to help to direct bacterial swimming towards growth-favouring microoxic zones at the bottom of natural waters. Questions in the study of magnetosome formation include understanding the factors governing the size and redox-controlled synthesis of the nano-sized magnetosomes and their assembly into a regular chain in order to achieve the maximum possible magnetic moment, against the physical tendency of magnetosome agglomeration. A deeper understanding of these mechanisms is expected from studying the genes present in the identified chromosomal 'magnetosome island', for which the connection with magnetosome synthesis has become evident. Here we use gene deletion in Magnetospirillum gryphiswaldense to show that magnetosome alignment is coupled to the presence of the mamJ gene product. MamJ is an acidic protein associated with a novel filamentous structure, as revealed by fluorescence microscopy and cryo-electron tomography. We suggest a mechanism in which MamJ interacts with the magnetosome surface as well as with a cytoskeleton-like structure. According to our hypothesis, magnetosome architecture represents one of the highest structural levels achieved in prokaryotic cells.

Protein dynamics of bovine dentin phosphophoryn

Bovine dentin phosphophoryn (BDP), a protein rich in aspartyl (Asp) and o-phosphoseryl [Ser(P)] residues, is synthesized by odontoblasts and believed to be involved in matrix-mediated biomineralization of dentin. The elucidation of the structure-function relationship of phosphophoryn has been a challenge because of its high-molecular weight, high negative charge, repetitive sequence, and lability. We have used the dynamic behavior of the (1)H NMR signal at 600 MHz to provide insight into the molecular dynamics of phosphophoryn. Our results indicate that phosphophoryn is a molecule of uniformly high mobility, thus belonging to a recently identified class of intrinsically disordered proteins that are characterized by sequences of low complexity and rich in polar and charged residues. The significance of our results is that phosphophoryn, because of its uniform nature has the potential to be replaced by biomimetic synthetic peptide analogs that together with amorphous calcium phosphate may lead to the development of novel, nontoxic, apatite-based dental restorative materials.

Molecular characterization of the 30-AA N-terminal mineral interaction domain of the biomineralization protein AP7

The AP7 protein is one of several mollusk shell proteins which are responsible for aragonite polymorph formation and stabilization within the nacre layer of the Pacific red abalone, H. rufescens. Previously, we demonstrated that the 30-AA N-terminal domain of AP7, denoted as AP7-1, exists as an unfolded sequence and possesses the capability of inhibiting calcium carbonate crystal growth in vitro via growth step frustration or interruption. However, very little is known with regard to the interactive capabilities of this sequence with Ca(II) and with calcium carbonates. Using multidisciplinary techniques, we determine that the AP7-1 polypeptide interacts with Ca(II) ions at the -DD- sequence clusters, yet retains its unfolded, conformationally labile structure in the presence of Ca(II) ions. Further, NMR experiments reveal that the extended structured sequence blocks, -GNGM-, -SVRTQG-, and -ISYL, exhibit motional, chemical exchange, and/or backbone geometry perturbations in response to Ca(II) interactions with AP7-1. Solid-state NMR magic angle spinning studies verify that during the course of in vitro calcium carbonate crystal growth, AP7-1 becomes bound to calcite fragments and cannot be entirely displaced from the mineral fragments using competitive Ca(II) washing. Finally, using a scrambled sequence version of the AP7-1 polypeptide, we observe that sequence scrambling does not adversely affect the crystal growth inhibitory activity of AP7-1, suggesting that the amino acid composition of AP7-1 may be more critical to growth step inhibition than the linear ordering of amino acids.

Structural characterization of the N-terminal mineral modification domains from the molluscan crystal-modulating biomineralization proteins, AP7 and AP24

The AP7 and AP24 proteins represent a class of mineral-interaction polypeptides that are found in the aragonite-containing nacre layer of mollusk shell (H. rufescens). These proteins have been shown to preferentially interfere with calcium carbonate mineral growth in vitro. It is believed that both proteins play an important role in aragonite polymorph selection in the mollusk shell. Previously, we demonstrated the 1-30 amino acid (AA) N-terminal sequences of AP7 and AP24 represent mineral interaction/modification domains in both proteins, as evidenced by their ability to frustrate calcium carbonate crystal growth at step edge regions. In this present report, using free N-terminal, C(alpha)-amide "capped" synthetic polypeptides representing the 1-30 AA regions of AP7 (AP7-1 polypeptide) and AP24 (AP24-1 polypeptide) and NMR spectroscopy, we confirm that both N-terminal sequences possess putative Ca (II) interaction polyanionic sequence regions (2 x -DD- in AP7-1, -DDDED- in AP24-1) that are random coil-like in structure. However, with regard to the remaining sequences regions, each polypeptide features unique structural differences. AP7-1 possesses an extended beta-strand or polyproline type II-like structure within the A11-M10, S12-V13, and S28-I27 sequence regions, with the remaining sequence regions adopting a random-coil-like structure, a trait common to other polyelectrolyte mineral-associated polypeptide sequences. Conversely, AP24-1 possesses random coil-like structure within A1-S9 and Q14-N16 sequence regions, and evidence for turn-like, bend, or loop conformation within the G10-N13, Q17-N24, and M29-F30 sequence regions, similar to the structures identified within the putative elastomeric proteins Lustrin A and sea urchin spicule matrix proteins. The similarities and differences in AP7 and AP24 N-terminal domain structure are discussed with regard to joint AP7-AP24 protein modification of calcium carbonate growth.