Garima Thakur

Surface chemistry of Alzheimer's disease: A Langmuir monolayer approach

Amyloid beta (1-40) and (1-42) peptides are the major constituents of hallmark senile plaques found in Alzheimers disease (AD) patients. Study of aggregation of Abeta (1-40) and (1-42) peptides and the truncated Abeta fragments could lead towards the mechanism of AD. Langmuir monolayer approach is one of the excellent methods to investigate the mechanism and origin of AD. Particularly, to study the steps involved in the formation and assembly of beta-sheet structures leading to formation of amyloid fibrils. Surface pressure- and surface potential-area isotherms provide information regarding the nature of short-range and long-range interactions between the molecules especially the lipids and the Abeta peptides. Spectroscopic methods like IRRAS, PM-IRRAS, FTIR-ATR, and GIXD at the air-water interface provide insight into the structural characterization, and orientation of the molecules in the Langmuir monolayer.

Cotinine reduces amyloid-β aggregation and improves memory in Alzheimer's disease mice

Alzheimers disease (AD) affects millions of people world-wide and new effective and safe therapies are needed. Cotinine, the main metabolite of nicotine, has a long half-life and does not have cardiovascular or addictive side effects in humans. We studied the effect of cotinine on amyloid-β (Aβ) aggregation as well as addressed its impact on working and reference memories. Cotinine reduced Aβ deposition, improved working and reference memories, and inhibited Aβ oligomerization in the brains of transgenic (Tg) 6799 AD mice. In vitro studies confirmed the inhibitory effect of cotinine on Aβ1-42 aggregation. Cotinine stimulated Akt signaling, including the inhibition of glycogen synthase kinase 3β (GSK3β), which promotes neuronal survival and the synaptic plasticity processes underlying learning and memory in the hippocampus and cortex of wild type and Tg6799 AD mice. Simulation of the cotinine-Aβ1-42 complex using molecular dynamics showed that cotinine may interact with key histidine residues of Aβ1-42, altering its structure and inhibiting its aggregation. The good safety profile in humans and its beneficial effects suggest that cotinine may be an excellent therapeutic candidate for the treatment of AD.

Surface chemistry of lipid raft and amyloid Aβ (1-40) Langmuir monolayer

Lipid rafts being rich in cholesterol and sphingolipids are considered to provide ordered lipid environment in the neuronal membranes, where it is hypothesized that the cleavage of amyloid precursor protein (APP) to Aβ (1-40) and Aβ (1-42) takes place. It is highly likely that the interaction of lipid raft components like cholesterol, sphingomylein or GM1 leads to nucleation of Aβ and results in aggregation or accumulation of amyloid plaques. One has investigated surface pressure-area isotherms of the lipid raft and Aβ (1-40) Langmuir monolayer. The compression-decompression cycles and the stability of the lipid raft Langmuir monolayer are crucial parameters for the investigation of interaction of Aβ (1-40) with the lipid raft Langmuir monolayer. It was revealed that GM1 provides instability to the lipid raft Langmuir monolayer. Adsorption of Aβ (1-40) onto the lipid raft Langmuir monolayer containing neutral (POPC) or negatively charged phospholipid (DPPG) was examined. The adsorption isotherms revealed that the concentration of cholesterol was important for adsorption of Aβ (1-40) onto the lipid raft Langmuir monolayer containing POPC whereas for the lipid raft Langmuir monolayer containing DPPG:cholesterol or GM1 did not play any role. In situ UV-vis absorption spectroscopy supported the interpretation of results for the adsorption isotherms.

Conformation of lysozyme Langmuir monolayer studied by infrared reflection absorption spectroscopy.

The surface chemistry and spectroscopy of the reduced lysozyme Langmuir monolayer were investigated at different pH values to compare with the native one. It was found that the limiting molecular area of the reduced lysozyme was not subphase pH dependent as the native lysozyme. 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. The interpretation of the results make plausible change of the conformation and orientation of the native lysozyme Langmuir monolayer with the subphase pH 3, 6, and 11.

Surface Chemistry and in Situ Spectroscopy of a Lysozyme Langmuir Monolayer

Surface pressure and surface potential-area isotherms were used to characterize a 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 the 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.

Determination of the size of quantum dots by fluorescence spectroscopy

There has been a lack of quick, simple and reliable methods for determination of nanoparticle size. An investigation of the size of hydrophobic (CdSe) and hydrophilic (CdSe/ZnS) quantum dots was performed by using the maximum position of the corresponding fluorescence spectrum. It has been found that fluorescence spectroscopy is a simple and reliable methodology to estimate the size of both quantum dot types. For a given solution, the homogeneity of the size of quantum dots is correlated to the relationship between the fluorescence maximum position (FMP) and the quantum dot size. This methodology can be extended to the other fluorescent nanoparticles. The employment of evolving factor analysis and multivariate curve resolution-alternating least squares for decomposition of the series of quantum dots fluorescence spectra recorded by a specific measuring procedure reveals the number of quantum dot fractions having different diameters. The size of the quantum dots in a particular group is defined by the FMP of the corresponding component in the decomposed spectrum. These results show that a combination of the fluorescence and appropriate statistical method for decomposition of the emission spectra of nanoparticles may be a quick and trusted method for the screening of the inhomogeneity of their solution.

Surface chemistry and spectroscopy of human insulin Langmuir monolayer.

The human insulin (HI) protein was examined to elucidate its structure at the air-water interface. Optimal experimental conditions were determined to prepare a homogeneous and stable human insulin (HI) Langmuir monolayer. HI insulin Langmuir monolayer can be used to study interactions of HI with a membrane as Langmuir monolayers are used as an in vitro model of biological membranes. Surface pressure and surface potential-area isotherms were used to characterize the HI Langmuir monolayer. The compression-decompression cycles and stability measurements showed a homogeneous and stable monolayer at the air-water interface. However, higher surface pressures resulted in a higher decrease in area and less stability. In situ UV-vis and fluorescence spectroscopy were used to verify the homogeneity of the HI monolayer and to identify the chromophore residues in the HI. Domain formation was examined through epifluorescence and Brewster angle microscopies. The conformation of HI was examined by circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR) in the aqueous phase and at the air-water interface by infrared reflection absorption spectroscopy (IRRAS). HI was found to exist as a monomer in 2-D.

α-Synuclein in α-helical conformation at air–water interface: implication of conformation and orientation changes during its accumulation/aggregation

Alpha-synuclein, a natively unstructured protein important in the neuropathology of Parkinsons disease, was found to form a Langmuir monolayer in an alpha-helical conformation with its helical axis parallel to the air-water interface. This study sheds light on the role of vesicles in neuronal cells in the accumulation/aggregation of alpha-synuclein.

Directed self-assembly of proteins into discrete radial patterns

Unlike physical patterning of materials at nanometer scale, manipulating soft matter such as biomolecules into patterns is still in its infancy. Self-assembled monolayer (SAM) with surface density gradient has the capability to drive biomolecules in specific directions to create hierarchical and discrete structures. Here, we report on a two-step process of self-assembly of the human serum albumin (HSA) protein into discrete ring structures based on density gradient of SAM. The methodology involves first creating a 2-dimensional (2D) polyethylene glycol (PEG) islands with responsive carboxyl functionalities. Incubation of proteins on such pre-patterned surfaces results in direct self-assembly of protein molecules around PEG islands. Immobilization and adsorption of protein on such structures over time evolve into the self-assembled patterns.

Investigation of pH-induced protein conformation changes by nanomechanical deflection.

Broad-spectrum biosensing technologies examine sensor signals using biomarkers, such as proteins, DNA, antibodies, specific cells, and macromolecules, based on direct- or indirect-conformational changes. Here, we have investigated the pH-dependent conformational isomerization of human serum albumin (HSA) using microcantilevers as a sensing platform. Native and denatured proteins were immobilized on cantilever surfaces to understand the effect of pH on conformational changes of the protein with respect to the coupling ligand. Our results show that protonation and deprotonation of amino acid residues on proteins play a significant role in generating charge-induced cantilever deflection. Surface plasmon resonance (SPR) was employed as a complementary technique to validate the results.