A molecular view of peptoid-induced acceleration of calcite growth

Abstract

The extensive deposits of calcium carbonate (CaCO₃) generated by marine organisms constitute the largest and oldest carbon dioxide (CO₂) reservoir. These organisms utilize macromolecules like peptides and proteins to facilitate the nucleation and growth of carbonate minerals, serving as an effective method for CO₂ sequestration. However, the precise mechanisms behind this process remain elusive. In this study, we report the use of sequence-defined peptoids, a class of peptidomimetics, to achieve the accelerated calcite step growth kinetics with the molecular level mechanistic understanding. By designing peptoids with hydrophilic and hydrophobic blocks, we systematically investigated the acceleration in step growth rate of calcite crystals using in situ atomic force microscopy (AFM), varying peptoid sequences and concentrations, CaCO₃ supersaturations, and the ratio of Ca²⁺/ HCO₃^-. Mechanistic studies using NMR, three-dimensional fast force mapping (3D FFM), and isothermal titration calorimetry (ITC) were conducted to reveal the interactions of peptoids with Ca²⁺ and HCO₃^- ions in solution, as well as the effect of peptoids on solvation and energetics of calcite crystal surface. Our results indicate the multiple roles of peptoid in facilitating HCO₃^- deprotonation, Ca²⁺ desolvation, and the disruption of interfacial hydration layers of the calcite surface, which collectively contribute to a peptoid-induced acceleration of calcite growth. These findings provide guidelines for future design of sequence-specific biomimetic polymers as crystallization promoters, offering potential applications in environmental remediation (such as CO₂ sequestration), biomedical engineering, and energy storage where fast crystallization is preferred.

Keywords: CO2 sequestration; atomic force microscopy; biomimetic polymer; crystal growth.

Fig. 1.

(A) Sequences of peptoids employed to regulate calcite growth kinetics. (B–E) in situ AFM images showing the morphology of a calcite growth hillock in a supersaturated solution (σ = 0.226) for Pep_8,4 concentrations of (B) 0, (C) 31.6 nM, (D) 63.1 nM, and (E) 158.5 nM. (F) Measured step speeds as a function of concentration for all peptoids having four Npe groups (Nce_nNpe₄, where n = 4, 6, 8, 10, 12). (G) Measured step speeds as a function of concentration for all peptoids having eight Nce groups (Nce₈Npe_m, where m = 3, 4, 7). (H) Correlation between the optimal peptoid concentration and the values of n and m. (I) Correlation between the maximum enhancement ratio and the values of n and m.

Fig. 2.

(A and B) Acceleration of calcite obtuse (A) and acute (B) step rate as a function of Pep_8,4 concentration at different solution supersaturations (${\sigma }_{\mathit{calcite}}$ = 0.136, 0.226, 0.486, 0.799, 1.294), where v and v₀ denote step rates with and without peptoids, respectively. The curves are presented using a logarithmic scale for better clarity. (C and D) The relationship between step speeds and the concentration of Ca²⁺ for both obtuse (C) and acute (D) steps at different concentrations of Pep_8,4. (E and F) The relationship between step speeds and activity ratios for both obtuse (E) and acute (F) steps at a constant supersaturation and pH (${\sigma }_{\mathit{calcite}}$ = 0.226, pH = 8.25) at varied concentration of Pep_8,4. Error bars are one SD.

Fig. 3.

(A) Dependence of critical length of calcite steps on inverse supersaturation (1/σ) in the absence and presence of Pep_8,4 (63.1 nM). The bars represent SD. (B) Dependence of average terrace width on supersaturation (σ) in the absence and presence of Pep_8,4 (63.1 nM).

Fig. 4.

(A) High resolution AFM image of the (104) surface of calcite in a supersaturated solution (σ = 0.226). Vertical scale (i.e., black to white) is 0.1 nm. (B) and (C) In situ high resolution AFM image of the calcite step structure in a supersaturated solution (σ = 0.226) in the absence and presence of Pep_8,4 (63 nM). Vertical scale (i.e., black to white) is 0.4 nm. (D) Slices from force gradient maps normal to calcite surface in the absence and presence of Pep_8,4 (63.1 nM), with the hydration layers labeled with white dashes. (E) Average force gradient curves of the force maps in the absence and presence of Pep_8,4 (63.1 nM).

Fig. 5.

Peptoid–ion interactions revealed by solution-state NMR. (A) 2D ¹H-¹H NOESY (red and orange spectrum) and COSY (cyan spectrum) superimposed for ¹H NMR signal assignments, with the color coding showing the assignments with green being the backbone CH₂’s, yellow and dark blue the sidechain CH₂’s of phenyl groups, and pink and light blue the sidechain CH₂’s of carboxyl groups. (B) ¹H NMR spectra of 50 µM Pep_8,4, 50 µM Pep_8,4 + 5 mM CaCl₂ and 50 µM Pep_8,4 + 10 mM NaHCO₃. (C) Normalized signal intensity as a function of spin echo delay (2 × number of π pulses × single echo delay) for the phenyl protons, and (D) for β-protons of both phenyl and carboxyl groups. (E) ¹³C single-pulse NMR spectra of 10 mM NaHCO₃ (pH 8.8) with the addition of 1 to 100 µM Pep_8,4 from 1 mM stock solution (pH 8.8) reveal a subtle yet reproducible increase in the fraction of CO₃^2- up to 20 µM Pep_8,4, followed by a subsequent decrease at higher peptoid concentrations.