Chiral and Achiral Mechanisms of Regulation of Calcite Crystallization Mihoko Maruyama,*,† Katsuo Tsukamoto,† Gen Sazaki,‡,§,| Yoshihiro Nishimura,#,† and Peter G. Vekilov⊥
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 127–135
The Earth and Planetary Materials Science, Graduate School of Science, Tohoku UniVersity, 6-3, Aoba, Aramaki, Aoba, Sendai, 980-8578, Japan, Institute for Materials Research, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, Center for Interdisciplinary Research, Tohoku UniVersity, Aramaki, Aoba-ku, Sendai 980-8578, Japan, Department of Electrical Engineering, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, and Department of Chemical and Biomolecular Engineering and Department of Chemistry, UniVersity of Houston, Houston, Texas, 77204 ReceiVed December 12, 2007; ReVised Manuscript ReceiVed September 9, 2008
ABSTRACT: The regulation of calcite mineralization by chiral biological molecules is one of the fundamental unresolved issues at the interface between biological, geological, and physical sciences. Here we address the role of chirality of L-aspartic acid (LAsp), a model additive, in the regulation of the growth of calcite crystals. We apply phase-shift interferometry to nonintrusively monitor in-situ the morphology of the surface and quantify the velocity of propagation of the edges of the unfinished crystal layers, the steps, during crystallization. We show that L-Asp leads to several-fold increase in the step velocity, in all directions, at low supersaturations, and several-fold decrease in the step velocity at high supersaturations. L-Asp also introduces asymmetry in the velocity of steps related by mirror symmetry, however, of e10%. To explain the complex effects of L-Asp and, likely, of other biological regulators of calcite crystallization, we show that prior to incorporation into steps, calcium and carbonate ions adsorb on the terraces and diffuse toward the steps. L-Asp accelerates the surface diffusion toward the steps, an achiral process, likely because of rearrangement by L-Asp of the structure of the water coating the calcite crystal surface. Importantly, L-Asp delays by ∼20× the incorporation of calcite ions into the steps, the only chiral process in the calcite crystallization mechanism, likely by blocking a significant fraction of the kinks. We show that the low asymmetry between the two chiral directions is due to suppression of chirality by the symmetric surface supply fields of the steps. The results and analyses presented here suggest that the chiral effects of biogenerated molecules on crystallization may be too weak to determine the biological or nonbiological origin of minerals. Introduction Some of the steps on the surface of calcite crystals are chiral because of the chirality of the kinks along steps in different directions.1,2 This chirality is related to at least two fundamental questions of modern science. The first one is the homochirality of all biological molecules. While this is a crucial aspect of the understanding of the origin of life, the prebiotic mechanisms of chiral separation have yet not been clarified. There have been numerous studies on the origin of homochirality.3-6 Recent work6 has linked the selective adsorption of L- and D-amino acids on chirally related surfaces of calcite single crystals to the chirality of the kinks along the steps covering the respective segments. It was concluded that only one of these chiral surfaces may have been active in the prebiotic selection of chiral biomolecules.6 The second issue related to the chirality of calcite steps is how chiral biological molecules regulate the crystallization of calcite in living organisms.7 It was recently shown that chiral molecules, such as L-aspartic acid (L-Asp), affect the morphology of the growth steps on the surface of calcite by changing the length of mirror symmetric crystalline edges.8 Another study found that low concentrations of L-Asp and ∼15 other amino acids and oligopeptides accelerate the rate of crystal growth, * Corresponding author. E-mail:
[email protected]. † The Earth and Planetary Materials Science, Tohoku University. ‡ Institute for Materials Research, Tohoku University. § Center for Interdisciplinary Research, Tohoku University. | Osaka University. ⊥ University of Houston. # Present address: Lasertec Corporation, 4-10-4 Tsunashimahigashi, Kohokuku, Yokohama, 223-8551 Japan.
while high concentrations slow growth.9,10 Analyses of the chemical features of the employed additives suggested that the counterintuitive acceleration is due the rearrangement by the additives of the water structures, which are attached to the calcium and carbonate ions in solution and coat the surface of the calcite crystal.9 Since the destruction of the water structures has been shown to comprise the main component of the free energy barrier for incorporation on molecules from solution into crystals,11,12 rearranging of these structures may lead to faster crystal growth. The slowdown at high additive concentrations was assigned to classical “impurity effects.” Here we address several issues related to the regulation of calcite crystallization by chiral molecules. First, we show that the additives may affect the solubility of calcite and in this way control the rate of crystallization through the degree of supersaturation. Then we show that the effects of chiral additives on the kinetics of crystallization are not constrained to the line energy of the step, as claimed in previous work,8 but the rate constants of incorporation of the ions from the solution into the crystals are also affected. We demonstrate that the so-called surface diffusion mechanism operates in calcite crystal growth, i.e., solute ions, on their way toward the steps, first adsorb on the crystal surface and diffuse along this surface toward the steps. We examine how the stages of the crystallization mechanism (adsorption on the terraces between steps, surface diffusion, and incorporation from the surface into the steps13,14) are affected by chiral additives. Lastly, we compare the effects, which are identical for step directions related by mirror symmetry, to those which exhibit chirality and in this way distinguish between chiral and achiral regulation.
10.1021/cg701219h CCC: $40.75 2009 American Chemical Society Published on Web 12/08/2008
128 Crystal Growth & Design, Vol. 9, No. 1, 2009
Maruyama et al.
As a chiral additive, we chose L-aspartic acid, an often used model additive, for which a significant amount of data has been accumulated.2,6,8 We monitored the surface morphology and determined the step densities and the velocities of step propagation by a custom built phase-shift interferometer (PSI).15 PSI has depth resolution better than 1 nm, comparable to that of AFM. PSI monitors the surface in a noncontact manner.15-17 An important advantage of PSI for the issues addressed here is its wide field of view, up to 200 µm, without compromising the depth resolution. This allows monitoring of the step patterns and determinations of the step velocities far from the step source, where consequences of small kinetic asymmetries are revealed, and at high supersaturations, providing deeper insights into some of the surface processes.
Table 1. The Effects of L-Aspartic Acid on the Thermodynamic Characteristics and Rate Constants of Calcite Crystal Growtha e 2CCO (mol/L) 3 e 2+ CCa (mol/L) e 2aCO (mol/L) 3 e 2+ aCa (mol/L) βstep (µm/s) λ (nm) βads (µm/s) Λ (µm) Λs/λ
in the absence of L-Asp
with 0.01 M L-Asp
0.005 0.0003 0.000054 0.00011 120 25 0.67 65000 0.05
0.005 0.0009 0.000054 0.00034 100 75 0.35 130000 0.3
a e C , concentration at equilibrium; ae, activity at equilibrium; βstep, step kinetic coefficient; λ, characteristic surface diffusion length; βads, kinetic coefficient for adsorption on the crystal surface; Λ, resistance for adsorption on the crystal surface.
Experimental Section Crystals and Solutions. Calcite substrates (1 × 1 × 2 mm) were cleaved from a large single crystal of Iceland spar grade crystal immediately before the start of an experiment. The substrate was fixed on a titanium holder with silicone glue and rinsed clean with ultrapure (18 MΩ) water. Supersaturated solutions of calcium carbonate were prepared by mixing stock solutions. The stock solutions (1 M NaHCO3, 0.1 M CaCl2, and 2.5 M NaCl) were prepared by dissolving, respectively, sodium bicarbonate (Wako Pure Chemical Industries Ltd.), calcium chloride (CaCl2 · 2H2O, Wako Pure Chemical Industries Ltd.), and sodium chloride (Wako Pure Chemical Industries Ltd.) in ultrapure water and were mixed in appropriate proportions to prepare solutions supersaturated with respect to CaCO3. L-Aspartic acid (Wako Pure Chemical Industries Ltd.) was introduced into the supersaturated solutions to a final concentration of 0.01 M. The ionic strength and pH of each growth solution were kept at, respectively, 0.100 ( 0.005 M and 8.50 ( 0.04, by adding 2.5 M NaCl, 0.5 M NaOH, and 0.5 M HCl. All crystallization experiments were carried out at room temperature stabilized to 22 ( 0.2 °C. Solution Speciation and Supersaturation. Solution supersaturation and calcium and carbonate activities were calculated from the concentrations of the respective species and accounting for the other solution components using the computer program PECS.18 The thermodynamic supersaturation σ is defined as
σ≡
(
)
aCa2+aCO23 ∆µ ) ln , K kBT sp
e e Ksp ) aCa 2+aCO23
(1)
where a are the activities of the respective ions, ae are the activities at equilibrium, and Ksp is the solubility product of calcite at the chosen pH. Since supersaturation was induced by increasing the activity of Ca2+ while keeping the carbonate concentration constant at constant ionic strength, then e σ ) ln(aCa2+/aCa 2+)
(2) -8.54
The value of Ksp has been reported in the range from 10 to 10-8.29 refs 2,18-20 even in calcium carbonate solutions without additives. To determine the equilibrium activity of Ca2+, aeCa2+, and to compensate for the inaccuracy of Ksp, we monitored the formation of etchpits, as aCa2+ was lowered by lowering the concentration of calcium ions, CCa2+. The activity at which etchpits due to impurity particles incorporated in e 2+ the surface layer of the crystal were observed was taken as aCa and used in the determinations of σ. The error in this determination, due to the existence of a critical undersaturation for etchpit formation,21,22 equals the value of this critical undersaturation, which, for another ionic system, was found to be
