Horst Hermel

Amyloid-β-sheet formation at the air-water interface.

An amyloid(1-40) solution rich in coil, turn, and alpha-helix, but poor in beta-sheet, develops monolayers with a high beta-sheet content when spread at the air-water interface. These monolayers are resistant to repeated compression-dilatation cycles and interaction with trifluoroethanol. The secondary structure motifs were detected by circular dichroism (CD) in solution and with infrared reflection-absorption spectroscopy (IRRAS) at the interface. Hydrophobic influences are discussed for the structure conversion in an effort to understand the completely unknown reason for the natural change of the normal prion protein cellular (PrP(C)) into the abnormal prion protein scrapie (PrP(Sc)).

Change and stabilization of the amyloid-β(1-40) secondary structure by fluorocompounds

The misfolding of the amyloid peptide, which is the result of a well-known alpha-to-beta transition, causes neurodegenerative disorder. Fluorinated alcohols have been described in the literature as potent solvents which can refold the beta-conformation. The present studies demonstrate the effectiveness of differently fluorinated alcohols for the beta-to-alpha refolding process on fibrillar aggregated amyloid beta(1-40). The regenerated helical structure is shown to be maintained in the absence of the fluoroalcohols, a behaviour which was found to contrast with immunoglobulin. We interpret this difference on the basis of the hydrophilic/hydrophobic domains in the amyloid sequence and present some speculations regarding the free-energy levels of the folded states of both proteins. The effect of the -CF(3) group on the observed conformational changes is interpreted as a result of alterations of the hydration shell of the peptides. Moreover, based on the results achieved with fluoroalcohols, we have used novel fluorinated amphiphiles possessing blood-compatibility properties and studied their effect on amyloid beta(1-40). First results point in the direction of a beta-to-alpha transition. Therefore, the use of fluorine groups in the development of new drugs is considered a new possibility requiring further investigation for the prevention of amyloidosis.

Band-shifting through polypeptide β-sheet structures in the cyanine UV-Vis spectrum

If oxa- or thiacarbocyanine is introduced into an aqueous poly-L-lysine (PL) solution in a concentration higher than that of aggregation, then a shift of the absorption band of the cyanine monomer (M) can be observed in the UV-Vis spectrum, provided that the PL has a beta-sheet conformation. Other polypeptide aggregates with a high beta-sheet content exhibit this effect as well, whereas for PL with an alpha-helix conformation no spectral shift is observed. The force-field optimized molecular models and the calculated interaction energies prove that the beta-sheet interacts significantly more intensively with the cyanine than the alpha-helix does. The quantum chemically calculated highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) energies of the cyanines and cyanine beta-sheet polypeptide complexes predict a M-shift to bathochromic frequencies in agreement with experimental findings. In the case of the measured M-shift to hypsochromic frequencies, the shift appears to be influenced by the presence of cyanine J-aggregates. The results open the way for a fast and simple method to identify polypeptide beta-sheet structures in biological and other systems containing polypeptides by using cyanine as a sensor.

beta-sheet recognition by time-resolved fluorescence spectroscopy.

A cyanine dye is used for the determination of β-sheet protein structures above the critical protein aggregation concentration through a bathochromic shift of the longwavelength absorption band. Simultaneously the fluorescence quantum yield of the dye increases strongly in the presence of β-sheet protein as compared to aqueous solution. Results from fluorescence lifetime experiments exhibit a short component (100 ps) of the dye in aqueous solution, which increases to about 1800 ps in protein-containing solutions. In contrast to absorption properties, the values of fluorescence lifetime and fluorescence intensity also depend on protein concentration and can be used not only to determine the protein structure but also to roughly estimate the concentration of the protein. Due to the protein-induced formation of H-aggregates, the absorption, the fluorescence intensity, and the amplitude of the fluorescence lifetime depend on time delay between sample preparation and measurement.