31P Solid-state NMR Research Articles

Active S2168 and inactive S21IRS pinholin interact differently with the lipid bilayer: A 31P and 2H solid state NMR study

Pinholins are a family of lytic membrane proteins responsible for the lysis of the cytosolic membrane in host cells of double stranded DNA bacteriophages. Protein-lipid interactions have been shown to influence membrane protein topology as well as its function. This work investigated the interactions of pinholin with the phospholipid bilayer while in active and inactive confirmations to elucidate the different interactions the two forms have with the bilayer. Pinholin incorporated into deuterated DMPC-d54 lipid bilayers, along with 31P and 2H solid state NMR (SS-NMR) spectroscopy were used to probe the protein-lipid interactions with the phosphorus head group at the surface of the bilayer while interactions with the 2H nuclei were used to study the hydrophobic core. A comparison of the 31P chemical shift anisotropy (CSA) values of the active S2168 pinholin and inactive S21IRS pinholin indicated stronger head group interactions for the pinholin in its active form when compared to that of the inactive form supporting the model of a partially externalized peripheral transmembrane domain (TMD) of the active S2168 instead of complete externalized TMD1 as suggested by Ahammad et al. JPC B 2019. The 2H quadrupolar splitting analysis showed a decrease in spectral width for both forms of the pinholin when compared to the empty bilayers at all temperatures. In this case the decrease in the spectral width of the inactive S21IRS form of the pinholin showed stronger interactions with the acyl chains of the bilayer. The presence of the inactive form's additional TMD within the membrane was supported by the loss of peak resolution observed in the 2H NMR spectra.

Thermal and Calorimetric Evaluations of Some Chemically Modified Carbohydrate-Based Substrates with Phosphorus-Containing Groups.

In the present article, we report on the chemical modifications of some carbohydrate-based substrates, such as potato starch, dextran, β-cyclodextrin, agar agar and tamarind, by reacting with diethylchlorophosphate (DECP), in dispersions in dichloromethane (DCM), in the presence of triethylamine (TEA) as the base. The modified substrates, after recovery and purification, were analyzed for their chemical constitutions, thermal stabilities and calorimetric properties using a variety of analytical techniques. These included: solid-state 31P NMR, inductively coupled plasma-optical emission spectroscopy (ICP-OES), thermogravimetric analysis (TGA) and pyrolysis combustion flow calorimetry (PCFC). The unmodified counterparts were also subjected to the same set of analyses with a view to serving as controls. Phosphorus analyses, primarily through ICP-OES on the recovered samples, showed different degrees of incorporation. Such observations were optionally verified through solid-state 31P NMR spectroscopy. The thermograms of the modified substrates were noticeably different from the unmodified counterparts, both in terms of the general profiles and the amounts of char residues produced. Such observations correlated well with the relevant parameters obtained through PCFC runs. Overall, the modified systems containing phosphorus were found to be less combustible than the parent substrates, and thus can be considered as promising matrices for environmentally benign fire-resistant coatings.

Effects of Polyunsaturated Fatty Acids and Metallation on the Antimicrobial Activity and Membrane-Disruptive Properties of Host-Defense Metallopeptide PISCIDIN 1

Piscidins are host-defense peptides that exhibit properties critical for the development of novel therapeutics. While the capability of piscidins to physically disrupt membranes is well accepted, their ability to bind Cu2+ and chemically damage membranes is less understood. Metallation lowers their charge, which could affect membrane binding. It also provides a redox center for the formation of radicals that can convert unsaturated fatty acids (UFAs) into membrane-destabilizing oxidized phospholipids (OxPLs). Such mechanistic possibility is important to examine since bacteria such as Vibrio species incorporate exogenous poly-UFA (PUFA) in their membranes to affect membrane properties and virulence. Here, we investigated how metallation and the UFA-content of membranes influence the mechanism of action and antimicrobial potency of P1, a particularly membrane-active piscidin. First, using dye leakage experiments with model membranes constituted of phosphoglycerol (PG) and phosphatidylcholine (PC), we quantified the permeabilization of P1/P1-Cu on membranes containing 10% of either 18:2 PUFA or Aldo-PC, its oxidized version. We found that the combined effect of metallating P1 and making Aldo-PC available yielded a ten-fold increase in membranolysis. Second, we used 31P solid-state NMR to characterize the partitioning behaviors of P1/P1-Cu in these membranes. Both peptides demonstrated stronger affinity for PG than PC. This effect was more pronounced for P1-Cu than P1 in the Aldo-PC-containing bilayers. Hence, differences in the membrane partitioning of P1 and P1-Cu in OxPL-containing membranes underlie their contrasted permeabilization effects. Third, we probed how the changing PUFA-content of bacterial cell membranes affects virulence upon P1/P1-Cu exposure. When V. cholerae bacteria were fed specific PUFAs (18:2; 20:4; 20:5; 22:6) and either peptide, peptide activity was enhanced with all FAs, except the 20:4. This demonstrates the importance of considering exogenous UFAs in antimicrobial treatments.

Remarkable self-degradation of Cu/SAPO-34 selective catalytic reduction catalysts during storage at ambient conditions

A model Cu/SAPO-34 SCR catalyst with all Cu species maintained as isolated Cu(II) ions is synthesized. Following lengthy storage on the shelf under ambient conditions, this catalyst completely and irreversibly deactivates upon any heat treatments above ∼100 °C. Via detailed characterizations with surface area/pore volume analysis, XRD, H2-TPR, and 27Al, 29Si and 31P solid-state NMR, as well as continuous wave and pulsed EPR studies, it is concluded that over the course of storage, the SCR active sites [Cu(OH)]+ and Brønsted acid sites ≡Si-O(H)-Al≡ are attacked by H2O molecules trapped in the SAPO-34 framework pores, and undergo hydrolysis to form copper hydroxide and terminal Al sites. Upon thermal treatment, these species interact with each other to form copper-aluminate-like species, leading to irreversible deactivation of this catalyst. This deactivation mechanism does not require or necessarily lead to extensive degradation (i.e., crystallinity loss) of the SAPO-34 support.

Determination of n-alkane partitioning within phosphatidylethanolamine Lα/HII phases

Alkanes are known to promote the fluid lamellar (Lα)-to-inverted hexagonal (HII) phase transition of different phospholipids. In this work, we studied the interaction of decane and tetradecane with self-assemblies formed of 1-palmitoyl-2-oleoyl-sn-glycero-phosphoethanolamine (POPE), using sequential 2H and 31P solid-state NMR spectroscopy. This technique allowed calculating the partitioning constant of the alkanes between the Lα and HII phases of POPE. Our results show that both alkanes are preferentially distributed in the HII phase compared to the Lα phase. In the HII phase, both alkanes display a very high conformational disorder, consistent with their location in the intercylinder voids. This preferential partitioning in the HII phase is more pronounced for tetradecane than for decane. This finding is proposed to be associated with a less energetically favored insertion (limited solubility) of tetradecane in the lamellar phase. This proposition is supported by the observation that tetradecane experiences more conformational freedom than its shorter analog in POPE bilayers, suggesting that it is located, on average, closer to the middle of the bilayers. It is also established that the increase in size of the intercylinder voids, by the addition of 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) in the HII cylinders, enhances the partitioning of decane in the HII phase compared to the Lα phase. These findings propose handles to modulate the balance of the relative Lα/HII phase stability.

Insights into OCP identification and quantification in the context of apatite biomineralization

Monitoring apatite formation throughin situRAMAN andex situssNMR spectroscopy.

Structural variability of pendant groups within the interlayer region of zirconium arene-phosph(on)ates: chemical and structural characterization of oxy- and methyl-linked 2-naphthyl phosphonates, and mixed oxy-linked derivatives.

Several new zirconium phosphonates incorporating the naphthalene ring and having the general formula Zr(O3PR)1(O3PR')1 [R and R' = -CH2C10H7, -OC10H7, -CH3, -OC2H5, -OH] have been synthesized. These materials were chemically characterized using thermal gravimetric analysis (percentage of organic content), infrared spectroscopy (presence of the desired organic functional groups), and solid-state 31P NMR (phosphorus environments), while the structural parameters were determined using X-ray powder diffraction (interlayer d spacings). The two new zirconium bis(phosphonates), Zr(O3PC10H7)2 and Zr(O3PCH2C10H7)2, were found to have d spacings of 19.6 and 20.0 Å, respectively. Three of the four zirconium mixed phosphonates examined are found to be single-phase structures with random mixtures of the organic moieties within the interlayer, and possess d spacings (14.3, 15.3, and 16.1 Å) that are between those of the two parent zirconium bis(phosphonates). The fourth is found to be a staged or segregated structure and possesses a d spacing that is approximately a sum of the two parent zirconium bis(phosphonates), with a d spacing of 28.2 Å. Solid-state 31P NMR of Zr(O3PCH2C10H7)2 revealed the presence of two isotropic resonances, which is interpreted in terms of two distinct, "locked-in" conformations of the -CH2C10H7 group. The experimental d spacings of the zirconium bis(phosphonates) correlate well with a simple predictive model based on the effective length and predominant conformation of the organic functional group.

Crystal structure and migration paths of alkaline ions in NaVPO4F.

A sample with a nominal composition 'NaVPO4F' is prepared by mechanochemically assisted solid-state synthesis using quenching. A detailed study of its crystal and local structure is conducted by means of XRD and FTIR and solid-state 31P NMR spectroscopies in comparison with Na3V2(PO4)2F3. It is shown that the as-prepared 'NaVPO4F' has a multiphase composition, including NaVPO4F as the main phase and Na3V2(PO4)2F3 and Na2.57V4P4O17F as the side products. The crystal structure of NaVPO4F is described in the monoclinic C2/c space group. It is characterized by negligible V3+/V4+ oxidation with the corresponding F-/O2- substitution and the presence of structural disordering. Using the Voronoi-Dirichlet partition (VDP) method, the Na+ and Li+ migration pathways in Tavorite-like NaVPO4F and closely related LiVPO4F (with the triclinic structure, P1[combining macron] S.G.) are analyzed. While the Na+ migration is suppressed in both cases, the Na+/Li+ ion exchange in NaVPO4F with the formation of monoclinic LiVPO4F could occur, but is difficult due to the sodium immobility rather than the instability of the lithium derivatives as was concluded from the DFT calculations.

Phosphorus speciation and fertiliser performance characteristics: A comparison of waste recovered struvites from global sources

Our immediate and future food security agenda requires sustainable phosphorus (P) management, and recovered P products such a struvite form part of the solution. Waste recovered struvite is generated from different waste streams and precipitation technologies. In this study, we compare P speciation and bioavailability of nine struvite products with traditional/granulated chemical fertilisers to investigate performance characteristics across soil types in the Irish ecoregion. Elemental-analysis/P-speciation of the struvites was examined by ICP-OES, 31P solid state NMR, PXRD and P-solubility tests. The fertiliser dose effect and bioavailability of the different struvite products were tested in 4-week and 3-month rhizotron experiments with perennial ryegrass (Lolium perenne). In addition to calculating plant-P offtake, bioavailability was assessed by Olsen-P and Diffusive-Gradients-in-Thin-Films (DGT). The struvites differed markedly in colour/appearance, texture, metal content and solubility. Struvites all had similar P-speciation, but minor impurity phases were detected. There was no significant difference between the struvite samples and triple-super-phosphate (TSP) with respect to either plant growth performance or P-bioavailability. The struvite products are produced from different wastewater streams/processes, yet possess a uniformity in overall fertilising performance within our experiments. Encouragingly, no appreciable agronomic difference in terms of yield was observed between the struvite and chemical fertilisers, which provides further evidence in support of struvite as a sustainable alternative to mined rock-phosphate derived products. Importantly the struvite samples analysed within this study contained both significantly fewer impurities than TSP and lower concentrations of toxic metals such as cadmium.

Interfacial study of surface-modified ZrO2 nanoparticles with thioctic acid for the selective recovery of palladium and gold from electronic industrial wastewater

The stability and performance of surface-modified zirconia nanoparticles with thioctic acid to effectively and selectively capture Pd(II) and Au(III) ions from industrial electronic waste water were investigated. A particular interest laid in the robustness of the interface in highly acidic and oxidizing environments under real conditions. Surface modifications of commercial nano-ZrO2 were optimized from Brønsted acid-base surface reactions in aqueous solution, either by direct thioctic acid (TOA) grafting or by a two-step process: (i) alendronic acid (AA) grafting and (ii) amide coupling reaction between AA and TOA, giving inorganic-organic hybrid systems designated respectively as ZrO2-TOA and ZrO2-AA-TOA. All materials were thoroughly characterized and had a similar specific surface area (50 m2·g−1). They also demonstrated similar characteristics in terms of thermodynamic and kinetic adsorption data (Langmuir adsorption model and pseudo-second order model). The ATR-FTIR, DRIFT-FTIR and 31P solid state NMR data elucidated the bidentate bridging and tripodal bonding modes for TOA and AA coordination, respectively. In addition, the DRIFT technique was used to reveal the covalent amide bond between the NH2 group of the AA group and the COOH groups of the TOA. The adsorption capacities of ZrO2-TOA at Pd and Au of 44.6 mg/g and 6.3 mg/g respectively, showed that our systems had competitive adsorption capacities and selectivity compared to the literature. Nevertheless, ZrO2-TOA was too sensitive to hydrolysis in high acid concentration ([HCl] > 1 M) and the use of ZrO2-AA-TOA was then mandatory to overcome hydrolytic cleavage of M-OOC bonds in media with high acid concentration. The novel ZrO2-AA-TOA material demonstrated promising stability, efficiency and reusability in industrial applications for gold and palladium recovery.

Integrated solid-state NMR and molecular dynamics modeling determines membrane insertion of human \u03b2-defensin analog

Human β-defensins (hBD) play central roles in antimicrobial activities against various microorganisms and in immune-regulation. These peptides perturb phospholipid membranes for function, but it is not well understood how defensins approach, insert and finally disrupt membranes on the molecular level. Here we show that hBD-3 analogs interact with lipid bilayers through a conserved surface that is formed by two adjacent loops in the solution structure. By integrating a collection of 13C, 1H and 31P solid-state NMR methods with long-term molecular dynamic simulations, we reveal that membrane-binding rigidifies the peptide, enhances structural polymorphism, and promotes β-strand conformation. The peptide colocalizes with negatively charged lipids, confines the headgroup motion, and deforms membrane into smaller, ellipsoidal vesicles. This study designates the residue-specific, membrane-bound topology of hBD-3 analogs, serves as the basis for further elucidating the function-relevant structure and dynamics of other defensins, and facilitates the development of defensin-mimetic antibiotics, antifungals, and anti-inflammatories.

Antibacterial Copper-Doped Calcium Phosphate Glasses for Bone Tissue Regeneration.

Calcium phosphate glasses are a promising new generation of biomaterials that can simultaneously induce tissue regeneration and controlled release of therapeutic molecules. In this work, novel calcium phosphate glasses containing 0, 2, 4, and 6 mol % Cu2+ were synthesized via room temperature precipitation reaction in aqueous solution. The effect of Cu2+ addition on the glass properties and structure was investigated using thermal analysis, 31P solid-state MAS NMR, Raman spectroscopy, and X-ray diffraction. All glasses crystallize at temperature >500 °C and are mainly formed by Q1 groups. The release of P, Ca, and Cu in solution over time was monitored via inductively coupled plasma-optical emission spectroscopy. It was found that with increasing Cu content, the amount of P and Ca released decreases whereas the amount of Cu released increases. The effect of Cu2+ release on the antibacterial activity against S. aureus, a bacterial strain commonly found in postsurgery infections, has been investigated. The addition of copper has been shown to infer the glasses antibacterial properties. As expected, the antibacterial activity of the glasses increases with increasing Cu2+ content. Cytocompatibility was assessed by seeding human osteoblast-like osteosarcoma cells Saos-2 (HTB85) on the glass particles. A significant increase in cell number was observed in all the glasses investigated. The copper-doped calcium phosphate glasses have proven to be multifunctional, as they combine bone regenerative properties with antibacterial activity. Therefore, they have great potential as antibacterial bioresorbable materials for hard tissue regeneration.

Sol–Gel Treatments to Flame Retard PA11/Flax Composites

This work investigates the efficiency of sol–gel treatments to flame retard flax fabric/PA11 composites. Different sol–gel treatments applied to the flax fabrics were prepared using TEOS in combination with phosphorus and/or nitrogen containing co-precursors (DEPTES, APTES) or additives (OP1230, OP1311). When the nitrogen and the phosphorus co-precursors were used, two coating methods were studied: a ‘one-pot’ route and a successive layer deposition method. For the “one-pot” method, the three precursors (TEOS, DEPTES, and APTES) were mixed together in the same solutions whereas for the different layers deposition method, the three different treatments were deposited on the fibers successively, first the TEOS, then a mix of TEOS/DEPTES, and finally a mix of TEOS/APTES. After deposition, the sol–gel coatings were characterized using scanning electron microscope, electron probe microanalyzer, and 29Si and 31P solid-state NMR. When only TEOS or a mix of TEOS and DEPTES is used, homogeneous coatings are obtained presenting well-condensed Si units (mainly Q units). When APTES is added, the coatings are less homogenous and agglomerates are present. A lower condensation rate of the Si network is also noticed by solid-state NMR. When additives are used in combination with TEOS, the TEOS forms a homogenous and continuous film at the surface of the fibers, but the flame retardants are not well distributed and form aggregates. The flame retardant (FR) efficiency of the different treatments on flax fabrics was evaluated using horizontal flame spread test. The following ranking of the different systems is obtained: TEOS + Additives > TEOS > TEOS + DEPTES ~ TEOS + DEPTES + APTES > multilayers. All the sol–gel coatings improve the flame retardant properties of the flax fabric, except the multilayer treatment. Based on these results, the three most efficient sol–gels were selected to prepare sol–gel-modified flax/PA11 composites. The composite modified with only TEOS showed the best FR properties. Surprisingly, the composite modified with the phosphorus-based flame retardant (AlPi) did not exhibit improved FR properties. This effect was attributed to the fact that the amount of the FR additive deposited on the fabrics was too low.

Effect of phosphate additives on the hydration process of magnesium silicate cements

The role of phosphate additives on the hydration process of magnesium silicate cement pastes was investigated through a multi-technique approach. A MgO/SiO2 mixture was hydrated for 28 days either in the absence or in the presence of sodium hexametaphosphate, trimetaphosphate or orthophosphate. Information on the kinetics of the hydration reaction was acquired by monitoring the free water index by means of differential scanning calorimetry, while the hydration products were thoroughly investigated by X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and 29Si solid-state nuclear magnetic resonance spectroscopy. The overall results provide new insight into the effect of phosphates on the hydration reaction and on the structure of magnesium silicate hydrate cements. All additives showed a plasticizing effect and promoted the formation of the binding phase magnesium silicate hydrate (M–S–H), without significantly altering its structure. Sodium orthophosphate was found to be by far the best-performing additive, even better than sodium hexametaphosphate, which is commonly used in these cementitious formulations. For the first time, 31P solid-state NMR investigation allowed orthophosphate ion to be identified as the effective species.

Study on Phase Evolution and Material Properties of Li10SiP2S12-XOx

Oxygen substitution was carried out on Li10SiP2S12 (LSiPS) which has the Li10GeP2S12 (LGPS) type crystalline structure. The general formula of Li10SiP2S12-xOx (LSiPSO) where 0 ≤ x ≤ 1.75 was set for solid state reaction to produce oxy-sulfide sites. The phase distribution and local structural units of LSiPSO were determined by powder X-ray diffraction (PXRD), and Raman, FT-IR,29Si and 31P Solid State NMR spectroscopies. The LGPS like LSiPSO phase increased while the impurity phase, β-Li3PS4 was reduced and formed Li3PO4 phase by O substitution. The optimized lithium ionic conductivity was found for x = 0.7 at 3.1 x 10-3 S/cm at 25℃. However, for the higher doped oxygen such as x > 0.9, a degradation of LSiPSO occurs and it is also expected to become the Li3PO4 phase which leads to a decrease in the ionic conductivity. The local structural units was determined by 31P and 29Si NMR and spectrum deconvolution to show sulfide, oxide and mixed oxy-sulfide local units. On the other hand, the doped O was found that only incorporating with phosphate centered tetrahedral unit such as PS4-xOx during no pure orthosilicate SiO4 4- units were observed, even at the highest x examined.

Molecular speciation of phosphorus in phosphogypsum waste by solid-state nuclear magnetic resonance spectroscopy

Phosphogypsum (PG), a waste by-product of the phosphate fertilizer industry as well as a point-source P contaminant, has caused serious environmental problems particularly in estuarine and coastal regions. However, in-depth understanding of P speciation in PG, which is critical for its restoration and management, remains largely unknown. Using solid-state 31P NMR spectroscopy, density functional theory calculations of the NMR parameters and NanoSIMS, we for the first time reported that P in PG ubiquitously exists as phosphate incorporated into gypsum and minor fluorapatite. The occasional presences of mineral phosphate phases mainly associated with Ca and Al were also detected. The molecular environment of the incorporated phosphate is HPO42− substituting for SO42− in the gypsum lattice with the H atom away from the H2O molecules and almost parallel to the a–c plane. A high spatial heterogeneity was observed for the distribution of this phosphate species in PG at the submicron scale. Upon heating, at least 64% of the incorporated phosphate could be converted to the easy-to-recover fluorapatite or amorphous calcium phosphate by thermal treatments at above 750 °C for 2–4 h. This information of P speciation transformation may pave a solid basement for the sustainable recovery of P from PG.

A Comparative Study on Interactions of Antimicrobial Peptides L- and D-phenylseptin with 1,2-dimyristoyl-sn-glycero-3-phosphocholine

L-phenylseptin (L-Phes) and D-phenylseptin (D-Phes) are amphibian antimicrobial peptides isolated from the skin secretion of Hypsiboas punctatus. In the N-termini, L-Phes and D-Phes contain three consecutive phenylalanine residues, l-Phe-l-Phe-l-Phe and l-Phe-d-Phe-l-Phe, respectively. They are known to exhibit antimicrobial activity against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Xanthomonas axonopodis pv. Glycines. However, their mechanism of action and the role of the D-amino acid residue have not been elucidated yet. In this study, the interactions of both peptides with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were investigated by means of quartz crystal microbalance, circular dichroism, vibrational circular dichroism, 31P solid-state NMR, and molecular dynamics simulation. Both peptides have similar binding constants to the DMPC lipid bilayers, in the order of 106 M−1, and form an α-helix structure in the DMPC lipid bilayers. Both the peptides induce similar changes in the dynamics of DMPC lipids. Thus, in spite of the difference in the conformations caused by the chirality at the N-terminus, the peptides showed similar behavior in the membrane-bound state, experimentally and computationally.

Judd-Ofelt analysis and luminescence studies on Dy3+ -doped different phosphate glasses for white light emitting material applications

This article reports detailed analysis of trivalent dysprosium (Dy3+) doped alkali and alkaline earth phosphate glasses prepared by melt quenching technique. From X-ray diffraction, Scanning electron microscope and 31P solid-state NMR spectral investigations, the structural properties have been evaluated. The optical absorption spectra of these Dy3+ doped phosphate glasses have been analyzed and discussed using Judd-Ofelt (J–O) theory and intensity parameters have been evaluated. Different radiative parameters such as radiative lifetimes (τR), branching ratios (βr) and emission cross sections (σP) have been calculated in all the alkali and alkaline earth phosphate glasses and compared. Emission from 4F9/2 state has been identified and the measured lifetimes have been obtained from the decay curves and compared with calculated lifetime values. The CIE chromaticity co-ordinates have been measured from the photoluminescence spectra.

Formation of stable strontium-rich amorphous calcium phosphate: Possible effects on bone mineral

Bone, tooth enamel, and dentin accumulate Sr2+, a natural trace element in the human body. Sr2+ comes from dietary and environmental sources and is thought to play a key role in osteoporosis treatments. However, the underlying impacts of Sr2+on bone mineralization remain unclear and the use of synthetic apatites (which are structurally different from bone mineral) and non-physiological conditions have led to contradictory results. Here, we report on the formation of a new Sr2+-rich and stable amorphous calcium phosphate phase, Sr(ACP). Relying on a bioinspired pathway, a series of Sr2+ substituted hydroxyapatite (HA) that combines the major bone mineral features is depicted as model to investigate how this phase forms and Sr2+ affects bone. In addition, by means of a comprehensive investigation the biomineralization pathway of Sr2+ bearing HA is described showing that not more than 10 at% of Sr2+, i.e. a physiological limit incorporated in bone, can be incorporated into HA without phase segregation. A combination of 31P and 1H solid state NMR, energy electron loss spectromicroscopy, transmission electron microscopy, electron diffraction, and Raman spectroscopy shows that Sr2+ introduces disorder in the HA culminating with the unexpected Sr(ACP), which co-exists with the HA under physiological conditions. These results suggest that heterogeneous Sr2+ distribution in bone is associated with regions of low structural organization. Going further, such observations give clues from the physicochemical standpoint to understand the defects in bone formation induced by high Sr2+ doses. Statement of SignificanceUnderstanding the role played by Sr2+ has a relevant impact in physiological biomineralization and provides insights for its use as osteoporosis treatments. Previous studies inspired by the bone remodelling pathway led to the formation of biomimetic HA in terms of composition, structures and properties in water. Herein, by investigating different atomic percentage of Sr2+ related to Ca2+ in the synthesis, we demonstrate that 10% of Sr2+ is the critical loads into the biomimetic HA phase; similarly to bone. Unexpectedly, using higher amount leads to the formation of a stable Sr2+-rich amorphous calcium phosphate phase that may high-dose related pathologies. Our results provide further understanding of the different ways Sr2+ impacts bone.

Strontium and Zinc Substitution in β-Tricalcium Phosphate: An X-ray Diffraction, Solid State NMR and ATR-FTIR Study.

β-tricalcium phosphate (β-TCP) is one of the most common bioceramics, widely applied in bone cements and implants. Herein we synthesized β-TCP by solid state reaction in the presence of increasing amounts of two biologically active ions, namely strontium and zinc, in order to clarify the structural modifications induced by ionic substitution. The results of X-ray diffraction analysis indicate that zinc can substitute for calcium into a β-TCP structure up to about 10 at% inducing a reduction of the cell parameters, whereas the substitution occurs up to about 80 at% in the case of strontium, which provokes a linear increase of the lattice constants, and a slight modification into a more symmetric structure. Rietveld refinements and solid-state 31P NMR spectra demonstrate that the octahedral Ca(5) is the site of β-TCP preferred by the small zinc ion. ATR-FTIR results indicate that zinc substitution provokes a disorder of β-TCP structure. At variance with the behavior of zinc, strontium completely avoids Ca(5) site even at high concentration, whereas it exhibits a clear preference for Ca(4) site. The infrared absorption bands of β-TCP show a general shift towards lower wavenumbers on increasing strontium content. Particularly significant is the shift of the infrared symmetric stretching band at 943 cm−1 due to P(1), that is the phosphate more involved in Ca(4) coordination, which further supports the occupancy preference of strontium.