RESUMO
Alzheimer's disease affects millions of human beings worldwide. The disease progression is characterized by the formation of plaques and neurofibrillary tangles in the brain, which are based on aggregation processes of the Aß peptide and tau protein. Today there is no cure and even no in vitro assay available for the identification of drug candidates, which provides direct information concerning the protein secondary structure label-free. Therefore, we developed an attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) sensor, which uses surface bound antibodies to immobilize a desired target protein. The secondary structure of the protein can be evaluated based on the secondary structure sensitive frequency of the amide I band. Direct information about the effect of a drug candidate on the secondary structure distribution of the total target protein fraction within the respective body fluid can be detected by a frequency shift of the amide I band. Thereby, the extent of the amide I shift is indicative for the compound efficiency. The functionality of this approach was demonstrated by the quantification of the effect of the drug candidate methylene blue on the pathogenic misfolded tau protein as extracted from cerebrospinal fluid (CSF). Methylene blue induces a shift from pathogenic folded ß-sheet dominated to the healthy monomeric state. A similar effect was observed for congo red on pathogenic Aß isoforms from CSF. In addition, the effect of berberine on synthetic Aß1-42 is studied. Berberine seems to decelerate the aggregation process of synthetic Aß1-42 peptides.
RESUMO
Protein immobilization studied by attenuated total reflection Fourier transform infrared (ATR-FT-IR) difference spectroscopy is an emerging field enabling the study of proteins at atomic detail. Gold or glass surfaces are frequently used for protein immobilization. Here, we present an alternative method for protein immobilization on germanium. Because of its high refractive index and broad spectral window germanium is the best material for ATR-FT-IR spectroscopy of thin layers. So far, this technique was mainly used for protein monolayers, which lead to a limited signal-to-noise ratio. Further, undesired protein-protein interactions can occur in a dense layer. Here, the germanium surface was functionalized with thiols and stepwise a dextran brush was generated. Each step was monitored by ATR-FT-IR spectroscopy. We compared a 70 kDa dextran with a 500 kDa dextran regarding the binding properties. All surfaces were characterized by atomic force microscopy, revealing thicknesses between 40 and 110 nm. To analyze the capability of our system we utilized N-Ras on mono-NTA (nitrilotriacetic acid) functionalized dextran, and the amount of immobilized Ras corresponded to several monolayers. The protein stability and loading capacity was further improved by means of tris-NTA for immobilization. Small-molecule-induced changes were revealed with an over 3 times higher signal-to-noise ratio compared to monolayers. This improvement may allow the observation of very small and so far hidden changes in proteins upon stimulus. Furthermore, we immobilized green fluorescent protein (GFP) and mCherry simultaneously enabling an analysis of the surface by fluorescence microscopy. The absence of a Förster resonance energy transfer (FRET) signal demonstrated a large protein-protein distance, indicating an even distribution of the protein within the dextran.