@article{PuppeLeueSommeretal.2022,
  author    = {Puppe, Daniel and Leue, Martin and Sommer, Michael and Schaller, J{\"o}rg and Kaczorek, Danuta},
  title     = {Auto-fluorescence in phytoliths},
  series = {Frontiers in Environmental Science},
  volume    = {10},
  journal   = {Frontiers in Environmental Science},
  publisher = {Frontiers Media},
  address   = {Lausanne},
  issn      = {2296-665X},
  doi       = {10.3389/fenvs.2022.915947},
  pages     = {14},
  year      = {2022},
  abstract  = {The detection of auto-fluorescence in phytogenic, hydrated amorphous silica depositions (phytoliths) has been found to be a promising approach to verify if phytoliths were burnt or not, especially in archaeological contexts. However, it is unknown so far at what temperature and how auto-fluorescence is induced in phytoliths. We used fluorescence microscopy, scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX), and Fourier transform infrared spectroscopy to analyze auto-fluorescence in modern phytoliths extracted from plant samples or in intact leaves of winter wheat. Leaves and extracted phytoliths were heated at different temperatures up to 600 degrees C. The aims of our experiments were i) to find out what temperature is needed to induce auto-fluorescence in phytoliths, ii) to detect temperature-dependent changes in the molecular structure of phytoliths related to auto-fluorescence, and iii) to derive a mechanistic understanding of auto-fluorescence in phytoliths. We found organic compounds associated with phytoliths to cause auto-fluorescence in phytoliths treated at temperatures below approx. 400 degrees C. In phytoliths treated at higher temperatures, i.e., 450 and 600 degrees C, phytolith auto-fluorescence was mainly caused by molecular changes of phytolith silica. Based on our results we propose that auto-fluorescence in phytoliths is caused by clusterization-triggered emissions, which are caused by overlapping electron clouds forming non-conventional chromophores. In phytoliths heated at temperatures above about 400 degrees C dihydroxylation and the formation of siloxanes result in oxygen clusters that serve as non-conventional chromophores in fluorescence events. Furthermore, SEM-EDX analyses revealed that extractable phytoliths were dominated by lumen phytoliths (62\%) compared to cell wall phytoliths (38\%). Our findings might be not only relevant in archaeological phytolith-based examinations, but also for studies on the temperature-dependent release of silicon from phytoliths and the potential of long-term carbon sequestration in phytoliths.},
  language  = {en}
}
@article{PopovaHundertmarkSeckleretal.2011,
  author    = {Popova, Antoaneta V. and Hundertmark, Michaela and Seckler, Robert and Hincha, Dirk K.},
  title     = {Structural transitions in the intrinsically disordered plant dehydration stress protein LEA7 upon drying are modulated by the presence of membranes},
  series = {Biochimica et biophysica acta : Biomembranes},
  volume    = {1808},
  journal   = {Biochimica et biophysica acta : Biomembranes},
  number    = {7},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0005-2736},
  doi       = {10.1016/j.bbamem.2011.03.009},
  pages     = {1879 -- 1887},
  year      = {2011},
  abstract  = {Dehydration stress-related late embryogenesis abundant (LEA) proteins have been found in plants, invertebrates and bacteria. Most LEA proteins are unstructured in solution, but some fold into amphipathic a-helices during drying. The Pfam LEA_4 (Group 3) protein LEA7 from the higher plant Arabidopsis thaliana was predicted to be 87\% alpha-helical, while CD spectroscopy showed it to be largely unstructured in solution and only 35\% alpha-helical in the dry state. However, the dry protein contained 15\% beta-sheets. FTIR spectroscopy revealed the (beta-sheets to be largely due to aggregation. beta-Sheet content was reduced and alpha-helix content increased when LEA7 was dried in the presence of liposomes with secondary structure apparently influenced by lipid composition. Secondary structure was also affected by the presence of membranes in the fully hydrated state. A temperature-induced increase in the flexibility of the dry protein was also only observed in the presence of membranes. Functional interactions of LEA7 with membranes in the dry state were indicated by its influence on the thermotropic phase transitions of the lipids and interactions with the lipid headgroup phosphates.},
  language  = {en}
}
@article{HundertmarkPopovaRauschetal.2012,
  author    = {Hundertmark, Michaela and Popova, Antoaneta V. and Rausch, Saskia and Seckler, Robert and Hincha, Dirk K.},
  title     = {Influence of drying on the secondary structure of intrinsically disordered and globular proteins},
  series = {Biochemical and biophysical research communications},
  volume    = {417},
  journal   = {Biochemical and biophysical research communications},
  number    = {1},
  publisher = {Elsevier},
  address   = {San Diego},
  issn      = {0006-291X},
  doi       = {10.1016/j.bbrc.2011.11.067},
  pages     = {122 -- 128},
  year      = {2012},
  abstract  = {Circular dichroism (CD) spectroscopy of five Arabidopsis late embryogenesis abundant (LEA) proteins constituting the plant specific families LEA_5 and LEA_6 showed that they are intrinsically disordered in solution and partially fold during drying. Structural predictions were comparable to these results for hydrated LEA_6, but not for LEA_5 proteins. FTIR spectroscopy showed that verbascose, but not sucrose, strongly affected the structure of the dry proteins. The four investigated globular proteins were only mildly affected by drying in the absence, but strongly in the presence of sugars. These data highlight the larger structural flexibility of disordered compared to globular proteins and the impact of sugars on the structure of both disordered and globular proteins during drying.},
  language  = {en}
}