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Janko, Marek (2012): Structure and stability of biological materials – characterisation at the nanoscale. Dissertation, LMU München: Faculty of Geosciences
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Abstract

Mummies are witnesses of the past harbouring information about the lives and fates of our ancestors. By examining them, the conditions of living, dietary, lifestyle and cultural habits as well as maladies in ancient times can be revealed. Knowledge of these maladies can be used to ascertain the evolution of diseases and may be helpful in characterising and treating them today. Uncovering information from mummies, however, depends on the preservation of the mummy tissue. Once degradation sets in, the molecular structure of the tissue is changed, and much information is lost. Favourable environmental conditions can slow down the process of decay and, hence, preserve organic material for long periods of time. As discussed in this work, biological tissue, which has substructural arrangements that are advantageous for withstanding mechanical load, might also be particularly favourable for preservation after the organism’s death. To address the question concerning the degree of preservation and to retrieve additional information from ancient tissue, two quasi-non-invasive analysis techniques, atomic force microscopy and Raman spectroscopy, were used. With these methods, the submicron structure, chemical composition, and nanomechanical properties of small mummified tissue samples were determined. In preliminary tests on recent collagen, the main connective tissue protein of vertebrates, results showed that in addition to imaging by atomic force microscopy, Raman spectroscopy is able to verify the alignment of this protein. Based on this knowledge, the arrangement and degree of collagen preservation in mummified human skin was investigated. Samples extracted from a 5300-year-old glacier mummy, the Iceman, were analysed. Extremely well-preserved collagen fibrils, in which the micro, ultra, and molecular structure were largely unaltered, were found. These results were in contrast, to the collagen fibrils found in the dermis of the Zweeloo mummy, a bog body of a female dating to the Roman period (78–233 AD). The Zweeloo mummy collagen fibrils showed moderate decomposition likely due to the acidic environment in the bog. Therefore, mummification due to freeze-drying, as in the Iceman, seems to be particularly beneficial for tissue preservation. The Iceman collagen, moreover, was found to be slightly stiffer than recent collagen, indicating that dehydration due to freeze-drying changed the mechanical properties of the tissue. This change likely improves the resilience of the freeze-dried collagen, stiffens the skin, and in turn maintains the skin’s protective function that prevents the underlying tissue from decomposing. Finally, also the preservation of red blood cells in wound tissue samples from the Iceman was observed. Single and clustered red blood cells were found whose morphological and molecular characteristics were similar to those of recent red blood cells. The ancient corpuscles moreover featured the typical red blood cell structure that indicates the preservation of healthy cells in Iceman tissue. Because fibrin, a protein formed during blood coagulation, was also detected, it appears that the clustered cells resembled remnants of a blood clot. The structure of the blood clot, stabilised by fibrin, may have been a protective envelope, which prevented the red blood cells from decomposing. Nonetheless, Raman spectra of the cells provided first indications of slight red blood cell degradation. These investigations emphasise the fundamental importance of the substructure and molecular arrangement of tissues, indicating that a tissue’s overall function and stability correlate with its molecular properties, in particular, the degree of cross-linking and the arrangement of the tissue molecular constituents. Last but not least the results show that ancient tissue can be preserved and its molecular properties probed and addressed even after millennia.