Abstract
Accumulation or aggregation of amyloidogenic proteins in the brain plays a central role in neurodegenerative diseases. The most common and highly growing form of dementia in the elderly population is Alzheimer's disease (AD) followed by Parkinson's disease (PD). The major proteins associated are amyloid beta (Abeta) and alpha-synuclein (alpha-syn) in AD and PD, respectively. These proteins are released or found near the neuronal membranes in the brain. Consequently to understand the behavior of the proteins using a model membrane system becomes an important facet of understanding these diseases. Langmuir monolayer approach was used to study the surface chemistry and spectroscopy of Abeta (1-40), Abeta(1-42) and alpha-synuclein. Moreover, surface chemistry of a model protein namely, lysozyme was investigated. In recent times, quantum dots (QDs) are considered as potential probes for bio-imaging. These particles can be beneficial when it comes to the investigation of neurodegenerative diseases. The effect of nanoparticles, i.e., CdSe/ZnS QDs on Abeta (1-42) morphology was investigated. Nevertheless, it was observed that the capping ligand plays a significant role in the surface chemistry of QDs when mixed with or conjugated to Abeta (1-42). Surface pressure- and surface potential-area isotherms were used to characterize the lysozyme Langmuir monolayer. The compression-decompression cycles and stability measurements showed a homogeneous and stable monolayer at the air-water interface. Salt concentration in the subphase and pH of the subphase were parameters controlling homogeneity and stability of the Langmuir monolayer. In situ UV-vis and fluorescence spectroscopies were used to verify the homogeneity of the lysozyme monolayer, and to identify the chromophore residues in the lysozyme. Optimal experimental conditions were determined to prepare a homogeneous and stable lysozyme Langmuir monolayer. The surface chemistry and spectroscopy of the reduced lysozyme Langmuir monolayer were investigated at different pH values and were compared to a native lysozyme. It was established that the limiting molecular area of the reduced lysozyme was not subphase pH dependent as was found for the native one. To explain this result in terms of the conformation and orientation of the lysozyme Langmuir monolayer at various subphase pH values, we have used Infrared Reflection Absorption Spectroscopy (IRRAS). The interpretation of the results suggests a change in the conformation and orientation of the native lysozyme Langmuir monolayer with the subphase pH 3, 6 and 11. The surface chemistry of Abeta (1-40) and its interaction with the lipid raft Langmuir monolayer were examined where the stability of the lipid raft Langmuir monolayer came out as an essential parameter. Lipid raft Langmuir monolayer in the presence or absence of ganglioside GM1 having POPC as one of the phospholipids was found to be very unstable and collapsed within 26 min. Whereas, the phospholipid DPPG improved the stability of the monolayer significantly when cholesterol was used in excess. We have examined the surface and spectroscopic properties of Abeta (1-42) mixed with or conjugated to dihydrolipoic acid (DHLA)- and polyethylene glycol (PEG)- capped CdSe/ZnS QDs. Surface pressure-area isotherms, in situ UV-vis absorption, and fluorescence spectroscopy were used to characterize the Abeta (1-42) mixed with or conjugated to QDs at the air-water interface. The capping of QDs played a role in surface chemistry as was determined by surface pressure-area isotherms and spectroscopic properties of the Langmuir monolayer. Furthermore Abeta(1-42) was bioconjugated to DHLA-capped CdSe/ZnS QDs. Upon conjugation of Abeta (1-42) to DHLA-capped QDs, the sample was incubated at 37oC, the process of fibrillation was inhibited as compared with a sample where Abeta (1-42) was simply mixed with the QDs. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were employed for the analysis of the samples. The morphology of fibrils and reduction in number of fibrils was substantial in the case of Abeta(1-42) conjugated to QDs. Reduction in fibrillation was also confirmed using a Thioflavin T assay. Moreover, quenching of tyrosine signal was observed in presence of the QDs, which indicates an interaction of QDs to the tyrosine residue in Abeta (1-42). The Surface chemistry and spectroscopy of alpha-syn, which is a natively unstructured protein important in the neuropathology of PD was investigated. IRRAS was utilized to investigate its conformation, alpha-syn was found to form a Langmuir monolayer in alpha-helical conformation with its helical axis parallel to the air-water interface.