Published: September 22, 2011 r2011 American Chemical Society 13154 dx.doi.org/10.1021/la202576v | Langmuir 2011, 27, 13154–13158 ARTICLE pubs.acs.org/Langmuir Influence of the Charge Relay Effect on the Silanol Condensation Reaction as a Model for Silica Biomineralization Tatsuya Kuno, †,‡ Takayuki Nonoyama, †,‡ Kiyoshi Hirao, †,‡ and Katsuya Kato* ,‡ † Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555 Japan ‡ Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya, Aichi 463-8560 Japan b S Supporting Information 1. INTRODUCTION In nature, biominerals, such as bones, shells, and teeth, have good mechanical properties and are composed of organic inor- ganic composites controlled at the nanometer scale. Moreover, these specific structural materials are formed under ambient conditions. 1 4 Thus, biomineralization to form biominerals in living organisms is a low-energy process and has been recently studied as an environmentally friendly process. In biomineraliza- tion, the inorganic crystalline phase and morphology are con- trolled by an organic substance; therefore, it is expected to be able to create novel functional inorganic materials. 5 12 It is well-known that functional groups of biomolecules (hydroxyl, carboxyl, amino groups, etc.) have catalytic activity for the biomineralization process. 12 15 For example, carboxyl groups can mineralize calcium phosphate and calcium carbonate, and amino groups induce silica mineralization. 5 7,10,16 More- over, biomolecules, such as enzymes, antibodies, and proteins, show synergetic effects by a specific sequence of functional groups in nature. The enzyme active site is the best example in the synergy of functional group combinations. 17 19 In biominer- alization, the glycine proline hydroxyproline sequence of col- lagen is also considered an advantageous sequential unit of osteogenesis. 20,21 This so-called “charge relay effect” by a com- bination of functional groups is favorable for forming biominerals in mineralization. From these concepts, the effect of the functional group combination of peptide on silica biomineralization has been studied. Silica biomineralization is observed in marine organisms, such as diatoms, sponges, and radiolarian, 22 24 and silica has been researched and evaluated as a tissue-engineering material. 25 Therefore, the dehydration reaction of trimethylethoxysilane is demonstrated using simple sequential peptides as the model of silica biomineralization. Because trimethylethoxysilane has only one ethoxyl group, it is easy to quantify the dehydration reaction progress. In general, trimethylethoxysilane is hydrolyzed to trimethylsilanol in an aqueous environment, and then, two trimethyl- silanols are dehydrated to hexamethyldisiloxane (Scheme 1a). In peptide sequence design, lysine (K) and histidine (H) were selected as the standard amino acids because the amino and imidazole groups have highly catalytic ability for silica mineralization. 26,27 In addition, to promote the charge relay effect, aspartic acid (D) having a carboxyl group side chain was incorporated into the peptide sequence (Scheme 1b). From these concepts, homopolypeptides (K 10 and H 10 ), block polypeptides (K 5 D 5 and H 5 D 5 ), and alternate polypeptides [(KD) 5 and (HD) 5 ] were designed and synthesized by combinatorial solid-phase peptide synthesis. The process of trimethylethoxysilane dehydration reactions by these peptides were measured by gas chromatography (GC). The results of these model systems not only support the fundamental theory of charge relay effects in biomolecules, such Received: July 7, 2011 Revised: September 19, 2011 ABSTRACT: The catalytic effect of various sequential peptides for silica biomineralization has been studied. In peptide se- quence design, lysine (K) and histidine (H) were selected as the standard amino acids and aspartic acid (D) was selected to promote the charge relay effects, such as in the enzyme active site. Therefore, homopolypeptides (K 10 and H 10 ), block poly- peptides (K 5 D 5 and H 5 D 5 ), and alternate polypeptides [(KD) 5 and (HD) 5 ] were designed, and the dehydration reaction ability of trimethylethoxysilane was investigated as a quantitative model of silica mineralization. The catalytic activity per basic residue of alternate polypeptide was the highest because of the charge relay effects at the surface of the peptide. In silica mineralization using tetraethoxysilane, spherical silica particles were obtained, and their size is related to the catalytic activities of the peptides in the model systems. From these results, the effect of the functional group combination by the peptide sequence design enables the control of the efficiency of mineralization and preparation of specific inorganic materials.