Ragnvald H. Mathiesen

Existence of two crystal structures of the complex α-D-glucose . NaCl . H2O (2 :1:1) verified by three-beam diffraction experiments

Triplet phases from three-beam interference measurements have been used to resolve a case of apparent structural ambiguity. 13 crystals, all cut from one large single crystal of the complex α-d-glucose · NaCl · H2O (2:1:1) were subjected to physical measurements of triplet phases, which involved studies of a set of model-sensitive triplets being particularly sensitive to the differences in structure between the two models. From 8 to 43 triplets were examined for each crystal specimen. The estimated phases have been compared with the corresponding quantities calculated for the refined structure models. A statistical analysis of the results provides highly significant evidence that both structure models are present, but do not coexist in the same crystal. One possible explanation for this apparent anomaly is that a phase transition may be induced by the mechanical stress applied during cutting of the hard and nearly perfect crystals. Triplet phases for structure verification and discrimination between similar structure models, as demonstrated in the present work, appears to be a novel and potentially powerful application of three-beam interference measurements.

Physical estimation of triplet phases from two new proteins

Three-beam interference experiments have been performed with crystals of two glycosidases: guinea-fowl hexagonal lysozyme, MW 14.3 kDa, and C. thermocellum endoglucanase CelA, MW 40 kDa. In both cases triplet phases could be estimated. Experimental parameters and details of the procedure are presented along with some examples of the results. The average differences between the estimated phases and those calculated from the crystallographic refinements were 17.9 and 15.9 degrees, respectively. A brief discussion of alternative methods for physical phase acquisition is given, including possible strategies for the measurement and application of experimental phases in macromolecular crystallography.

Impact of buoyancy on the growth of equiaxed grains in Al–Cu

Abstract Equiaxed dendritic growth in grain refined Al–Cu (15, 20, 25 wt-%Cu) has been studied in-situ during directional solidification by means of high resolution synchrotron X-ray video microscopy. At these compositions, the α-Al grains have a lower density than the surrounding melt and experience buoyant forces which affect their growth rates and morphologies. A model has been derived, based on Scheil conditions to account for solute transport across the solid-liquid interface, and a spherical envelope approximation to the dendrite morphology in order to simplify both the interface geometry of the growing crystals and the Stokes drag exerted upon them during motion. The model was compared with experimental results to evaluate its present merits and to devise possible routes for further improvement and development.