Imidazole catalyzed silica synthesis: Progress toward understanding the role of histidine in (bio)silicification
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Elena N. Danilovtseva and Vadim V. Annenkov Limnological Institute of Siberian Branch of Russian Academy of Sciences, Irkutsk 664033, Russia
Carole C. Perrya) Biomolecular and Materials Interface Research Group, School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, United Kingdom (Received 26 September 2008; accepted 25 November 2008)
Histidine is an amino acid present in proteins involved in biosilica formation and often found in peptides identified during phage display studies but its role(s) and the extent of its involvement in the silica precipitation process is not fully understood. In this contribution we describe results from an in vitro silicification study conducted using poly-histidine (P-His) and a series of different molecular weight synthetic polymers containing the imidazole functionality (polyvinylimidazole, PVI) for comparison. We show that the presence of imidazole from PVI or P-His is able to catalyze silicic acid condensation; the effect being greater for P-His. The catalytic mechanism is proposed to involve the dual features of the imidazole group—its ability to form hydrogen bonds with silicic acid and electrostatic attraction toward oligomeric silicic acid species.
I. INTRODUCTION
The intricate and highly repeatable biosilica structures exhibited by marine organisms demonstrate the magnificence of construction control exerted by life. Studies of these organisms have revealed that organic biomolecules including proteins are associated with the growth of biosilica structures; however the role(s) of these biomolecules at the molecular level are not yet fully understood. Silicatein proteins extracted from sponges such as T. aurantia and S. domuncula have high sequence homology with cathepsin-L that has a hydrolytic function in the lysosome.1,2 A site-directed mutagenesis study of silicatein has shown that Ser-26 and His-165 residues present at the active site of silicatein are necessary to catalyze the hydrolysis of tetraethylorthosilicate (TEOS), a silica precursor used in vitro.3 The necessity for two functional groups in close proximity (app˚ )1,4 to each other for the catalyzed roximately 6.8 A hydrolysis of TEOS has support from an in vitro study using bifunctional small molecules.5 As a consequence of these studies, the hypothesis that hydrogen bond formation between the hydroxyl group of Ser-26 and the imidazole of His-165 is important in silicification was proposed and has support from an ab initio densitya)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0223
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 15 Mar 2015
functional theory study using a gas-phase model.6 This latter study has shown that the imidazole group of His-165 is free to rotate and hence able to form a hydrogen bond with the –OH of Ser-26. When a similar simulation was performed in the presence of monosilicic acid, the binding energy (BE) and formation energy (FE) values obtained showed that po
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