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A New Dispersive Vibrational Circular Dichroism Instrument: Development, Testing, and Application

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Title: A New Dispersive Vibrational Circular Dichroism Instrument: Development, Testing, and Application
Author(s): Lakhani, Ahmed
Advisor(s): Keiderling, Timothy A.
Contributor(s): Luke Hanley; Michael Trenary
Department / Program: Chemistry
Graduate Major: Chemistry
Degree Granting Institution: University of Illinois at Chicago
Degree: PhD, Doctor of Philosophy
Genre: Doctoral
Subject(s): vibrational circular dichrosim chiral S/N polarization modulation peptides and proteins IR
Abstract: The primary goal of this thesis is to focus on the design and construction of a new dispersive vibrational circular dichrosim (VCD) instrument optimized for the measurement of mid-IR bands such as amide I and amide II vibrational modes of peptides and proteins (C=O stretching, and CN stretching-NH bending, respectively). VCD is a differential absorption (ΔA = AL – AR) of left and right circularly polarized light by the excitation of molecular vibrational transitions in a chiral environment. The new VCD design was implemented to make a more compact VCD instrument for biological molecules, to increase signal to noise (S/N) ratio, to simultaneously collect the signals resulting from sample transmission and polarization modulation, and to digitally normalize these signals following a design of Diem. In addition to describing the building of the new VCD instrument itself, we also collected spectra for peptides and proteins with different dominant secondary structures (alpha-helix, beta-sheet, and random coil), in order to compare the new dispersive VCD spectrometer to our previously constructed analogue-based dispersive VCD, and to some commercial Fourier Transform IR based VCD designs. In addition, we compared identical samples from old and new VCD instruments with comparable resolution and total measurement time. Our new compact dispersive VCD instrument exhibited improved S/N for the amide I′ band region for biological molecules as compared to a previously built instrument in our laboratory. Our first application studied 310-helical peptides with the new VCD spectrometer. The 310-helical synthesized peptides were studied in non-aqueous solvents, and we compare data gathered from the new VCD, IR, and Raman spectra of 310-helical synthesized peptides with two different amino-acid chain lengths. Simulations of the 310-helical peptide IR, Raman, and VCD provided a means of interpretation, since predictions of the conformational dependence of the relative separations of 13C=O and 12C=O features and the exciton splitting of the 13C=O band in the doubly labeled species were in agreement with those seen experimentally. A second application studied Amyloid fibrils, which are often associated with certain degenerative disorders. This study revealed a number of intriguing spectral properties using polyglutamic acid at low pH as a model system. Our results demonstrate how both IR absorption and enhanced VCD are measurable in such complex systems and provide spectra sensitive to subtle packing defects and the super helical structure of amyloid fibrils, thus shedding more light on the relationship between the fibrils’ structure and their optical traits. We further detail ongoing or potential projects that would, moving forward, improve the performance of the VCD.
Issue Date: 2012-12-07
Genre: thesis
Rights Information: Copyright 2011 Ahmed Lakhani
Date Available in INDIGO: 2012-12-07
Date Deposited: 2011-12

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