Distribution of enamel crystallite orientation through an entire tooth crown studied using synchrotron X-ray diffraction.

The biomineralization of human dental enamel has resulted in a highly anisotropic and heterogeneous distribution of hydroxyapatite crystallites, which in combination with its high mineral content has resulted in one of the most durable and hardest tissues in the human body. In this study, we used position-sensitive synchrotron X-ray diffraction to quantify the spatial variation in the direction and magnitude of the preferred orientation of enamel crystallites across a whole tooth crown. Two-dimensional synchrotron X-ray diffraction images were collected with 300 µm spatial resolution over a series of six sequential tooth sections obtained from a single maxillary first premolar and were analyzed using Rietveld refinement. Both the magnitude and the direction of the crystallite orientation were found to have a high spatial heterogeneity. Areas of high crystallite alignment were directed perpendicular to the biting surfaces, which is thought to meet the functional requirements of mastication. The results may assist in our understanding of the structure-function relationship and of the evolutionary development of enamel.

Anharmonic behavior in the multisubunit protein apoferritin as revealed by quasi-elastic neutron scattering.

Quasi-elastic neutron scattering (QENS) has been used to study the deviation from Debye-law harmonic behavior in lyophilized and hydrated apoferritin, a naturally occurring, multisubunit protein. Whereas analysis of the measured mean squared displacement (msd) parameter reveals a hydration-dependent inflection above 240 K, characteristic of diffusive motion, a hydration-independent inflection is observed at 100 K. The mechanism responsible for this low-temperature anharmonic response is further investigated, via analysis of the elastic incoherent neutron scattering intensity, by applying models developed to describe side-group motion in glassy polymers. Our results suggest that the deviation from harmonic behavior is due to the onset of methyl group rotations which exhibit a broad distribution of activated processes ( E a,ave = 12.2 kJ.mol (-1), sigma = 5.0 kJ x mol (-1)). Our results are likened to those reported for other proteins.

2D mapping of texture and lattice parameters of dental enamel.

We have used synchrotron X-ray diffraction to study the texture and the change in lattice parameter as a function of position in a cross section of human dental enamel. Our study is the first to map changes in preferred orientation and lattice parameter as a function of position within enamel across a whole tooth section with such high resolution. Synchrotron X-ray diffraction with a micro-focused beam spot was used to collect two-dimensional (2D) diffraction images at 150 microm spatial resolution over the entire tooth crown. Contour maps of the texture and lattice parameter distribution of the hydroxyapatite phase were produced from Rietveld refinement of diffraction patterns generated by azimuthally sectioning and integrating the 2D images. The 002 Debye ring showed the largest variation in intensity. This variation is indicative of preferred orientation. Areas of high crystallite alignment on the tooth cusps match the expected biting surfaces. Additionally we found a large variation in lattice parameter when travelling from the enamel surface to the enamel-dentine junction. We believe this to be due to a change in the chemical composition within the tooth. The results provide a new insight on the texture and lattice parameter profiles within enamel.