Phase transition of calcium carbonates induced by amino acids

1999 Fiscal Year Final Research Report Summary

• Project/Area Number: 10640466
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Petrology/Mineralogy/Science of ore deposit
• Research Institution: Yamaguchi University
• Principal Investigator: KAI Ayako Yamaguchi University, Faculty of Engineering, Assistant Professor, 工学部, 講師 (50253167)
• Co-Investigator(Kenkyū-buntansha): 甲斐 綾子 山口大学, 工学部, 講師 (50253167)
• Project Period (FY): 1998 – 1999
• Keywords: phase transition / calcite / vaterite / aragonite / amino acid / transition temperature / method of molecular orbitals / enthalpy

Research Abstract

Vaterite of unstable phase yielded dominantly under the coexistence of Asp with an acidic side-chain, Gly, Asn, Ser and Thr with a noncharged-polar side chain. Basic amino acids affected slightly on the vaterite formation, while nonpolar amino acids had no effect on it. Among the factors investigated, pH of the solution and the molecular weight of the amino acids were independent of the formation of vaterite but incorporation of amino acids was effective. Since the yield of vaterite correlated with the concentration of amino acids in the sample, the amino acids seem to stabilize the structure of vaterite and consequently inhibited to recrystallize to calcite of the thermally stable phase. SEM photographs also indicated that vaterite was prevented from growing in the initial stage of precipitation.
Frontier orbitals and electrostatic potential computed by ZINDO : INDO1 leaded to two models for covalent bonds between amino acids and CaCOィイD23ィエD2. One, which is common for nonpolar, noncharged-polar and acidic amino acids, is a bond between the nitrogen of α-amino group and an oxygen of the surface COィイD23ィエD2ィイD12-ィエD1 ion in CaCOィイD23ィエD2. Another for basic amino acids is a bond between an oxygen of α-carboxyl group and a carbon of the surface COィイD23ィエD2ィイD12-ィエD1 ion. Taking the Coulomb interaction into consideration, it is presumed that the reactivity is higher for the amino acids, and lower for the basic amino acids than the others. The presumption is consistent with the experimental results.
DSU curves of hybrid crystals containing vaterite revealed an exothermic reaction peak at about 450℃ corresponding to the transition from vaterite to calcite, while aragonitic samples showed an exothermic reaction peak at about 410℃, to the transition to calcite. The temperatures depended on the species of amino acids, and the correlation was found between the temperature of the transition of the transition from vaterite to calcite and the melting point of amino acids.