On the Role of Poly-Glutamic Acid in the Early Stages of Iron(III) (Oxy)(hydr)oxide Formation

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.

Role of Water during the Early Stages of Iron Oxyhydroxide Formation by a Bacterial Iron Nucleator

“…X-ray diffraction (XRD) data of the solid derived after nucleation from the experiment at pH 3 with 80 μg/mL Mms6 (Figure S4) reveal that the akageneite phase of iron (oxy)(hydr)oxides was formed, in agreement with those obtained in the chloriderich environment, and also in the presence of polyglutamic acid. 14 The titration curve for the reference Fe(III) system without peptides (black curve) initially increases linearly due to the hydrolysis reaction Fe 3+ + H 2 O ⇌ (Fe−OH) 2+ + H + , accompanied by the formation of Fe(III) solute olation PNCs. 10 With an increasing concentration of added Fe(III), c added (Fe 3+ ), the linear curve transits toward an upward-bent base consumption regime and asymptotically approaches a second, steeper, linear slope.…”

“…Recent studies of Fe(III) (oxy)(hydr)oxide nucleation imply that the process proceeds via nonclassical mechanisms, i.e., through the formation of highly dynamic olation prenucleation cluster (PNC) species, , which may interact with organic molecules and govern the nucleation process. − According to the nonclassical mechanism, the olation Fe(III) PNCs are thermodynamically stable and considered as solutes. During generation of these PNCs, − the dominant contribution to the free energy originates from the substantial entropy gain due to the release of coordinated water molecules. Later the reduction of the dynamics of PNC species occurs due to the transition from olation toward an oxolation hydrolysis regime, proceeding via liquid–liquid phase separation.…”

Counterintuitive Crystallization: Rate Effects in Calcium Phosphate Nucleation at Near-Physiological pH

“…Such experiments allow detailed study along the entire nucleation pathway and have led to the discovery of previously overlooked aspects in calcium carbonate nucleation . Investigations with additives and adaption to other chemical systems have shown how versatile this method is. − …”