Aluminum binding by human apotransferrin.

<p>(A) UV absorption spectra (220–320 nm) of human apo-transferrin (1.28 mg/mL) in 25 mM sodium bicarbonate and 50 mM MOPS buffer (pH 7.4), titrated with increasing concentrations of Al(III) in the form of Al(NTA)₂. The appearance of a peak (absorbance maxima) at 240 nm was obtained with increasing concentrations of Al(III). A lesser peak between 285–300 nm and a trough between 250–285 nm were also present at the higher Al(III) concentrations. 1.28 mg/mL human apo-transferrin in 25 mM sodium bicarbonate and 50 mM MOPS buffer (pH 7.4) was used as reference. (B) The absorbance maxima at 240 nm was extracted (from A), corrected for variation in baseline/background at 320 nm and plotted against Al(III) concentration. Results are the mean (± SD) of three experiments. Dotted line shows the calculated point of transferrin saturation.</p

Transformation of Amorphous Calcium Carbonate in Air

In air, the mechanisms of amorphous calcium carbonate (ACC) transformation into crystalline polymorphs of CaCO₃ and whether the atomic ordering is attributable to a solid-state transformation or a dissolution and reprecipitation process are still under debate. While some studies observed a significant influence of relative humidity on ACC transformation, other studies suggested a dehydration process of ACC prior to crystallization. In the present study, we focus on the metastability of additive-free ACC in air and in particular on its interaction with relative humidity. Our findings indicate that the transformation of ACC into crystalline CaCO₃ is triggered only after the physisorption of a critical H₂O level. Consequently, ACC metastability was prolonged by retarding H₂O uptake and by keeping the physisorbed H₂O below the critical level, ACC remained in its metastable state. Therefore, the conceptual formation of a “thin film” of about four monolayers of physisorbed H₂O is considered to govern the transformation of ∼90 nm sized ACC particles via partial dissolution and reprecipitation. Furthermore, we observed simultaneous formation of calcite, vaterite, and aragonite from ACC, where distinct proportions correspond to different H₂O exposure conditions. Thus, polymorph formation from ACC depends also on physicochemical boundary conditions during transformation rather than on prestructural formation within ACC alone

Atomic lattice spacing of the Al-rich silica-containing nanoparticles.

<p>A histogram of lattice plane spacing measured (to the nearest 0.1 Å) from 49 individual crystallites (Al rich silica-containing nanoparticles; ASP) in 12 HAADF-STEM and BF-TEM images. Insert: shows measurement of the lattice spacing in a STEM image (from Figure 5B). Comparison of measured lattice spacing to standard X-ray diffraction data, taken from the International Crystal Diffraction Database (ICDD), produced no clear match to a specific phase (see text). </p

HAADF imaging of the Al-containing high Al-affinity silica polymer.

<p>Higher resolution high angle annular dark field (HAADF) imaging of the air-dried, 48-h aged, dilute Al(III) and the high Al affinity silica polymer containing-solution (8 µM Al(III), 320 µM Si, pH 7.2) by aberration corrected scanning transmission electron microscopy (SuperSTEM) confirmed the presence of an amorphous gel with ~ 4 nm particulates (A) that are crystalline in nature (B). (C & D) Show false colour elemental maps where the Al (<i>L</i>_2,3-edge) EELS elemental map has been colored green and the Si (<i>L</i>_2,3-edge) has been colored red and the two overlaid. From this it is clear that Al is confined to the particulates and there is little or no Al in the gel. (E) Shows a series of Al and Si <i>L</i>_<i>2,3</i>-edges obtained following a line-scan across a particle (inserted image), confirming little Al in the gel (black line spectra), higher levels in the particle and the presence of Si in the particle (spectra on the particle are colored red). </p

Comparison of Al binding affinity of the high-Al affinity silica polymer with commercial Ludox colloids.

<p>(A) Competitive binding of Al(III), from the DMHP-Al complex (8 µM, pH 7.2), by the high Al affinity silica polymer (HSP) and the commercial Ludox silica colloids was assessed at 274 nm by measuring the amount of free DMHP liberated in solution. These results suggest a relationship between Al binding affinity and particle size as SM30 (7 nm) had higher/greater affinity than LS30 (12 nm), which in turn had greater affinity than TM50 (22 nm). (B) To correct for differences in particle size, the Al binding curves for the Ludox silica colloids were re-plotted in terms of total surface area. The red line shows the best fit through the data points. From (A) it was estimated that our polymeric silica was 2.91 times smaller than the smallest Ludox colloid, SM30 (7 nm). This estimate for polymeric silica was confirmed in (C), upon re-plotting the Al binding curve in terms of surface area (squares). The red line shows the best fit line for the Ludox silica colloids (as shown in (B)). Results are the mean (± SD) of two experiments in triplicate.</p

Competition between high-Al affinity silica polymer and human apotransferrin for Al binding.

<p>Competition studies with human apotransferrin. The Al-transferrin complex (40 µM Al(III) and 1.28 mg/mL apo-transferrin) was titrated with the high Al affinity silica polymer (HSP, squares) and monomeric silica (circles) at pH 7.4 (25 mM sodium bicarbonate and 50 mM MOPS buffer). The concentration of Al-transferrin complex in solution was measured at its absorbance maxima at 240 nm and corrected for variation in baseline/background at 320 nm. To determine the amount of Al(III) displaced and bound by HSP or monomeric silica, the concentration of Al-transferrin complex in solution at the different silica concentrations (‘A’) was subtracted from the initial/starting concentration of the Al₂-transferrin complex in solution (A₀). Results are mean ± SD of three experiments. The dotted line indicates the onset of silica polymerization [14].</p

Control of Mg²⁺/Ca²⁺ Activity Ratio on the Formation of Crystalline Carbonate Minerals via an Amorphous Precursor

The formation of amorphous calcium carbonate (ACC) and its transformation to crystalline phases plays a key role in the formation of carbonate minerals on Earth’s surface environments. Nonetheless, the physicochemical parameters controlling the formation of crystalline CaCO₃ via an amorphous precursor are still under debate. In the present study we examine whether crystalline CaCO₃ formation occurs via an ACC precursor in the pH range from 7.8 to 8.8 and at initial Mg/Ca ratios from 1/3 to 1/8. The obtained results document that the transformation of Mg-rich ACC (Mg-ACC) to a crystalline phase is strictly controlled by the prevailing ratio of the Mg²⁺ to Ca²⁺ activity, <i>a</i>_Mg²⁺/<i>a</i>_Ca²⁺, of the reactive solution after Mg-ACC was synthesized: Mg-ACC transformed to (i) Mg-calcite at 5 ≤ <i>a</i>_Mg²⁺/<i>a</i>_Ca²⁺ ≤ 8 and to (ii) monohydrocalcite at 8 ≤ <i>a</i>_Mg²⁺/<i>a</i>_Ca²⁺ ≤ 12. Our findings suggest that the formation of the crystalline phase induces undersaturation of the reactive solution with respect to the ACC and triggers its dissolution. Thus, the metastability of Mg-ACC in the reactive solution is not determined by its Mg content but is related to the formation kinetics of the less soluble crystalline phase. The experimental results highlight the importance of prevailing physicochemical conditions of the reactive solution on Mg-ACC transformation pathways

MOESM1 of Scale-fragment formation impairing geothermal energy production: interacting H2S corrosion and CaCO3 crystal growth

Additional file 1. Additional figures and tables