L. Dobiášová, R. Kužel, H. Šíchová
In last years, renaissance of rather old and traditional technique - X-ray powder diffraction can be observed. This was initiated by both the interest in design of new materials (in materials science, physics and chemistry, where it plays the role of a basic method), and also by fast development in instrumental techniques – X-ray optics and detection which enhanced its possibilities.
Powder diffraction pattern contains different kind of information . Peak positions and intensities are related to crystal (atomic) structure, i.e. the type and size of lattice cell and atomic positions and consequently it can be used for structure refinement and even structure determination in some cases. As a finger print of each individual phase, the diffraction pattern can be an ideal tool for phase analysis.
However, there is much more hidden in the pattern. Variations of lattice parameters and intensitites can detect lattice defects. This is related to the so-called real structure of material, the term which is also used for structural features in the scale of nanometers, i.e. grains or subgrains. The topics which is now of great interest because of intense research of nanomaterials. Powder diffraction analysis nowadays may include application of different diffraction geometries and analysis of peak positions, intensities and widths. This makes possible a complete PXRD analysis – phase analysis, structure refinement, stress, strain, crystallite size and texture analysis.
Can the technique be of any use for biologists? There have not been many applications yet. Main interest of biologists now seems to be directed to protein crystallography where even synchrotron single crystal diffraction may be insufficient. However, recently, an attempt to use powder diffraction for structure refinement of proteins has appeared [1, 2] too.
In present work, we have tried to perform more complete diffraction analysis of different egg-shells.
The biological function of the egg-shell is a chamber for embryonic development and from which the chick is able to emerge at the appropriate time. The requirements of the table egg industry are different. The industry sustains economic loss from cracked eggs and some of the cracking can be attributed to the deficiencies in the egg-shell structure. This is one of the reasons why the attention to eggshell is devoted [3-5].
The egg-shell consists of several mutually through-growing layers of CaCO3. The innermost layer – mamilary layer ( @100 μm) grows on the outer egg membrane and creates the base on which the palisade layer constitutes the thickest part (@ 200 μm) of the egg-shell. The top layer is the vertical layer (@ 5-8 μm) covered by the organic cuticle.
Different kinds of hen´s and bird´s egg-shells in the powder form or as a whole from both sides of the shell were examined by powder diffractometry and film back-reflection method. The powder patterns were evaluated by the fitting of diffraction profiles with the Pearson VII function. The lattice parameters, peak intensities and profile broadening were analysed. At the Bragg-Brentano setting (2Θ= 40o) the Cu radiation penetrates approximately into the 9 μm of the egg-shell, so the measurements from the inner and outer shell surface can give evidence of the mamilary and palisade layer, respectively.
The results obtained on egg-shells of very different origins shown no significant differences in lattice parameters that correspond well to the PDF-2 values. The patterns contained only basic phase CaCO3 (space group no. 167: R-3c) with a small addition of magnesium (0.3 wt. % , determined by atomic absorption). Diffraction patterns of powders obtained from all the eggs investigated correspond very well to the pattern of standard CaCO3. The correspondence is very good including intensities. The patterns obtained from egg-shell powders are also very similar to the standard pattern, regardless larger line broadening.
However, there are differences between powders and both sides of the shells. For inner shell surfaces, the intensities are only slightly different than in powders (including standard one) but there is significant line broadening indicating fluctuations of lattice spacings (the mean local strain of about 0.2 %). On the other hand, for outer shell surfaces, there is much smaller broadening of lines, similar to powders, but significant changes of intensities indicating the 00l textures of grains. This is also an evidence of presence of two basic layers, structurally very different – mamilary and palisade. The meaning of crystallographic texture has been emphasized [3, 4]. It was steted that the breaking strength of the eggshell is inversely related to the degree of calcite orientation and conversely, reduced strength in the eggshell from aged hens coincides with a high variability of texture .
As a general conclusion and amazing fact, we can say that any differences of XRD parameters between the eggs of very different origin are not significant. So that their microstructure and composition, as they can be seen by XRD, are the same.
This work was an attempt for non-traditional application of powder diffraction and it was shown that it may be helpful for biologists not only for phase analysis but also for the study of nanostructure of inorganic crystalline phases in biological objects which is closely related to the overall microstructure which is strongly influenced by proteins taking part in the egg creation. The eggshell matrix proteins influences the process of crystal growth by controlling size, shape and orientation of calcite crystals. The formation of avian eggs belongs to most rapid mineralization processes known.
The work has been initiated and supported only by private interests of the authors.
1. R. B. Von Dreele, “Combined Rietveld and Stereochemical Restraint Refinement of a Protein Crystal Structure,” Journal of Applied Crystallography 32, 1084-1089 (1999).
3. Y. Nys, J. Gautron, M. D. McKee, J. M. Garcia-Ruiz, M. T. Hincke, Biochemical and functional characterisation of eggshell matrix proteins in hens.World’s Poultry Science Journal, 57 (2001), 401-413.
4. R.M.G. Hamilton, The Microstructure of the Hen’s Egg Shell – A short review., Food Microstructure, vol 5 (1986), 99-110.
5. P. Hunton, Understanding the architecture of the egg shell, World’s Poultry Science Journal, vol. 51 (1995), 141-147.