Biological Control on Calcite Crystallization by Polysaccharides Mingjun Yang,* ,†,‡ S. L. Svane Stipp, † and John Harding ‡ NanoScience Centre, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 Copenhagen, Denmark, and Department of Engineering Materials, UniVersity of Sheffield, Sheffield, U.K. ReceiVed May 15, 2008; ReVised Manuscript ReceiVed July 7, 2008 ABSTRACT: Polysaccharides control the growth of calcite in coccoliths by adsorbing preferentially onto particular surfaces of the calcite crystal. We chose four units from the coccolith associated polysaccharide (CAP) that is known to promote formation of vicinal faces in Emiliania huxleyi, namely, galactose, mannose, xylose, and rhamnose, and used molecular dynamics simulations to calculate the absorption of oligosaccharide units onto a number of calcite surfaces. The simulations show a wide range of adsorption energies, which depend on the combination of organic molecule and surface. Oligosaccharides on polar surfaces with surplus negative charge have the strongest adsorption, while those on polar surfaces with surplus positive charge have the weakest. Acute stepped vicinal surfaces have stronger adsorption than planar surfaces, while obtuse stepped surfaces have weaker adsorption than the planar surfaces. On the basis of these simulations, the behavior of two saccharides on the calcite {1 0 1 j 4} surface was observed experimentally with atomic force microscopy (AFM) and shown to be consistent with the simulations. This helps explain why the polysaccharides involved in biomineralization have the chemical composition that they do and also suggests criteria for new molecules to control calcite crystal growth. Introduction The dramatic difference between calcite crystals grown in pure solution and those grown by biomineralization in nature has attracted much interest. Synthetic calcite crystals grown in the absence of additives form perfect rhombohedra with only the stable {1 0 1 j 4} faces. On the other hand, calcite crystals grown in biological systems exhibit unusual crystal surfaces and, moreover, the physical shape of the crystal may be quite different from that predicted from a Wulff construction obtained from those surfaces (with the relevant surface or interfacial energies used in the calculation). For example, the unicellular algae, Emiliania huxleyi, forms a body cover of calcified platelets called coccoliths. These coccoliths consist of interlock- ing calcite single crystals, each of which has a complex shape. 1 All coccoliths of a species have the same highly complicated and strictly defined structure and in different parts of the disk, different crystal morphologies exist. Complex organic molecules, coccolith-associated polysaccharides (CAPs), are generally believed to control the biomineralization process. 2 The mechanisms that control biomineralization are not understood in detail. However, it has been generally agreed that the planes on which biomolecules are preferentially adsorbed become expressed as stable crystal surfaces because crystal growth is inhibited on these planes. Recently it was suggested that crystal shape is controlled by step-specific interactions between biomolecules and individual step edges on pre-existing crystal surfaces and changes in the elementary step shape generate a similarly modified bulk crystal shape. 3 Therefore, the key factor in the biological control of crystallization is the local interaction of biomolecules with the crystal surfaces. The role of CAPs in the development of coccoliths has been studied for more than two decades. The CAP extracted from E. huxleyi has been shown to inhibit CaCO 3 precipitation in in Vitro experiments. Atomic force microscopy (AFM) has been used to investigate the local interactions of CAPs with the calcite surface during dissolution, precipitation, and dynamic equilib- rium. The AFM experiments demonstrate that CAPs interact preferentially with surface sites defined by acute, rather than obtuse, angles and it blocks acute sites during dissolution and growth. 4 The polysaccharides from coccoliths have also been used to study the influence on the crystallization of calcium oxalate monohydrate crystals, and the results indicate that the inhibitory effect proceeds through a monolayer type of adsorp- tion onto the crystal surface. 5 The basic monosaccharide units that make up CAPs are mainly rhamnose/ribose, xylose, mannose, galactose, and others. 6 Structures for these units are presented in Figure 1. Molecular simulation is being increasingly used to study the crystal form of calcite in contact with water and the growth or inhibition of the mineral. Two stepped {1 0 1 j 4} surfaces of calcite (corresponding to the standard acute and obtuse steps) have been investigated by de Leeuw and colleagues, 7 and the results indicate both that water can stabilize calcite surfaces and that dissolution of calcite tends to occur preferentially from the obtuse step, in agreement with experimental observations. 8 The competitive adsorption among organic molecules with different functional groups on calcite surfaces has been studied with a combination of potential models and results suggest that some * To whom correspondence should be addressed. E-mail: mjyang@nano.ku.dk. Phone: +45 35 32 01 56. Fax: +45 35 32 02 14. † University of Copenhagen. ‡ University of Sheffield. Figure 1. Monosaccharide and trisaccharide units. CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 4066–4074 10.1021/cg800508t CCC: $40.75  2008 American Chemical Society Published on Web 09/12/2008