CRYSTAL PROTEIN INTERACTIONS - CONTROLLED ANISOTROPIC CHANGES IN CRYSTAL MICROTEXTURE

A unique set of proteins extracted from a variety of invertebrate calc itic mineralized tissues is able to selectively interact in vitro with certain crystal faces and not others. This was previously demonstrate d by observing changes in morphology of crystals grown in the presence of proteins as compared to those grown in the absence of proteins. Fo llowing interaction, the proteins are overgrown by the crystal and are subsequently occluded within the crystal itself. Here we address the fundamental question of whether or not the proteins also alter the cry stal texture in an anisotropic manner. For this purpose we used high-r esolution synchrotron X-ray diffraction to monitor changes in coherenc e length and angular spread. We studied the interactions of proteins e xtracted from the mineralized skeletal hard parts of sea urchins and m ollusks, with crystals of two calcium dicarboxylic acids, calcium fuma rate and calcium malonate, as well as the polymorph of CaCO3 calcite. For the calcium dicarboxylate crystals, we did demonstrate that the co herence lengths are reduced in the directions perpendicular to the pla nes onto which the proteins preferentially adsorb. In contrast the cal cite crystals grown in the presence of the proteins exhibited an incre ase in angular spread compared to the controls, but no anisotropic eff ect in coherence length was detected. A biologically produced calcite crystal, on the other hand, showed a preferential reduction in coheren ce length in the direction of the c axis. Clearly in the case of calci te, the processes controlling crystal texture in the biological enviro nment are more sophisticated than those in vitro. The detection of a r eduction in coherence length in the directions perpendicular to the pl anes onto which the proteins preferentially adsorbed represents one of very few direct demonstrations that an additive that is able to selec tively alter crystal morphology also affects crystal texture in an ani sotropically specific manner. An understanding of this phenomenon may, in the future, improve our ability to control crystal texture in synt hetic materials.