Abstract
Nucleation of minerals in the presence of additives is critical for achieving control over the formation of solids in biomineralization processes or during syntheses of advanced hybrid materials. Herein, we investigated the early stages of Fe(III) (oxy)(hydr)oxide formation with/without polyglutamic acid (pGlu) at low driving force for phase separation (pH 2.0 to 3.0). We employed an advanced pH-constant titration assay, X-ray diffraction, thermal analysis with mass spectrometry, Fourier Transform infrared spectroscopy, and scanning electron microscopy. Three stages were observed: initial binding, stabilization of Fe(III) pre-nucleation clusters (PNCs), and phase separation, yielding Fe(III) (oxy)(hydr)oxide. The data suggest that organic–inorganic interactions occurred via binding of olation Fe(III) PNC species. Fourier Transform Infrared Spectroscopy (FTIR) analyses revealed a plausible interaction motif and a conformational adaptation of the polypeptide. The stabilization of the aqueous Fe(III) system against nucleation by pGlu contrasts with the previously reported influence of poly-aspartic acid (pAsp). While this is difficult to explain based on classical nucleation theory, alternative notions such as the so-called PNC pathway provide a possible rationale. Developing a nucleation theory that successfully explains and predicts distinct influences for chemically similar additives like pAsp and pGlu is the Holy Grail toward advancing the knowledge of nucleation, early growth, and structure formation.
Highlights
IntroductionNucleation of inorganic minerals in the presence of (bio)macromolecules increasingly attracts research attention owing to the possibility of progression toward controlling the synthesis of advanced materials and the understanding of biomineralization processes
Nucleation of inorganic minerals in the presence ofmacromolecules increasingly attracts research attention owing to the possibility of progression toward controlling the synthesis of advanced materials and the understanding of biomineralization processes
We explored the early stages of Fe(III)(hydr)oxide formation in the presence of polyglutamic acid (pGlu) acid sodium salt with a different number of monomers in the backbone
Summary
Nucleation of inorganic minerals in the presence of (bio)macromolecules increasingly attracts research attention owing to the possibility of progression toward controlling the synthesis of advanced materials and the understanding of biomineralization processes. Iron (oxy)(hydr)oxides are important (bio)-minerals [1], and their nucleation and crystal growth are closely related to the presence of bio-macromolecules in natural systems [2,3,4]. The literature data on iron (oxy)(hydr)oxide nucleation is vast, and the formation of akagenéite, ferrihydrite, maghemite, and hematite has been extensively studied [6,7]. The influence of sodium oxalate on the formation of hematite mesocrystals was explained by a change in solution chemistry in the interfacial region of the solid [9]. It is important to study the changes that occur in the early, that is, the prenucleation stage, in a homogeneous solution, where Fe(III) prenucleation cluster (PNC) species and organic additives interact and may lead to the formation of distinct solids
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