The aim of this project was to investigate blue and UV light signal transduction in the induction of the genes encoding the flavonoid biosynthetic enzymes phenylalanine ammonia lyase (PAL) and chalcone synthase (CHS) in Arabidopsis cell culture. First, the induction of the flavonoid biosynthetic pathway was characterised in the cell culture. PAL and CHS transcripts accumulated transiently over a few hours in response to blue/UVA, UVA and UVB light. The transient light induction of PAL and CHS gene expression resembles that found in whole Arabidopsis plants. In addition, flavonol accumulation was demonstrated in response to blue and UVB light, showing that light treatment of the cell culture leads not only to PAL and CHS gene expression but also to activation of the biosynthetic pathway ultimately resulting in the production of flavonoids. Pharmacological inhibitors were used to investigate the role of anion channels in the signal transduction pathways leading to PAL and CHS gene expression. The anion channel inhibitor anthracene-9-carboxylate was found to inhibit the blue/UVA, UVA and UVB light induction of PAL and CHS and had no effect on control gene inductions, suggesting a possible role for anion channel activity in the signal transduction pathways. Attempts were made to measure any light induced changes in extracellular chloride concentration using a chloride selective electrode inserted into the cell culture. No change in chloride concentration was detected. Previous pharmacological studies had suggested a requirement for calcium channels and pumps in the induction of PAL and CHS (e.g. Christie and Jenkins, 1996). To enable direct measurement of light induced changes in cytosolic calcium concentration, aequorin expressing cell cultures were produced. High fluence rate UVA light treatment induced an apparent transient increase in cytosolic calcium concentration in the cell cultures, but this did not appear to relate to PAL and CHS gene expression.

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The objective of this research was to investigate the microscopic processes occurring at the interfaces of mixed anion III-V heterostructures grown by molecular beam epitaxy (MBE). In particular, efforts were made to characterize the dominant chemical and physical mechanisms that lead to anion exchange and to develop semi-empirical models capable of predicting the atomic scale structure and composition at the interfaces of mixed anion heterostructures for the As/P and Sb/As material systems. This research considers the MBE growth of 20-period superlattices (SLs) formed by allowing a dissimilar anion flux to impinge on a static group-V stabilized surface. Statistical experimental design was used to determine the effects of substrate temperature, V/III growth flux ratio, and anion exposure time on the anion exchange process. The superlattice structures were analyzed via high resolution x-ray diffraction (HRXRD) and simulation, RHEED analysis, x-ray photoelectron spectroscopy (XPS), and other techniques, which allowed for the determination of both chemical composition and atomic structure at the interfaces. Finally, a semi-empirical hybrid neural network was developed that quantifies the effects of MBE growth processes at the interfaces of mixed anion III-V heterostructures by incorporating a first principles kinetic model with back-propagation neural networks.

The primary goal of this research concerns the synthesis and characterization of hybrid main group Lewis acids. Initially, the focus of this work was on the synthesis of derivatives possessing unusual bonding interactions enforced by a rigid 1,8- naphthalenediyl scaffold. After discovering a route to a new dilithio reagent, silicon based derivatives featuring R3Si-F->CR3 + and R3C-H->SiFR3 interactions of 2.703(2) and 2.32(2) Angstrom respectively were successfully synthesized and fully characterized. Another hybrid Lewis acid based on the 1,8-naphthalenediyl scaffold that was studied was a trinuclear B2/Hg Lewis acid. This molecule has been shown to bind two fluoride anions sequentially, and the binding events can be followed by differential pulsed votammetry. The final part of this work concerns the reactivity and redox behavior of main group systems. It has been shown that the p-phenylene linker in 4-dimesitylboryl-1- diarylmethylium benzenes effectively reduces electrochemical communication between the carbocation and borane moieties when compared to systems without the linker. Reduction of these species produces a derivative whose EPR signal is only slightly influenced by the ^11 B center. These findings have been further substantiated by theoretical calculations. Finally, the redox properties of alpha-phosphonio- and alpha- phosphonyl-carbocations have been studied. Chemical reduction of both species leads to a predominately carbon centered radical with coupling to the ^31P center of 18 and 19.7 G respectively. The alpha-phosphonio carbocations, however, also undergo ligand exchange reactions with pyridine derivatives suggesting that these species can also be referred to as ligand stabilized carbodications.

Despite an ever growing number of reports concerning the anion-π interaction, controversy surrounding the nature of these weak supramolecular interactions continues. In an effort to further explore the nature and properties of anion-π interactions, experimental and computational methods were employed to study their occurrence in inorganic, organic and biological systems. As part of ongoing research in the Dunbar group on the topic of anion-π based supramolecular interactions, the ligand 3,6-bis(2′-pyrimidyl)-1,2,4,5-tetrazine (bmtz) was synthesized and reacted with [Cu(NCMe)4][BF4] to form the octanuclear complex [Cu8(bmtz)6][BF6]6·6MeCN. Crystallographic evidence indicates an in situ reduction of two of the complexed bmtz ligands to radical anions. A second blue compound has also been observed in this reaction, and recent work has resulted in a direct synthesis of this compound. Preliminary results indicate that this second compound contains Cu(II) centers, as expected. Further work is necessary to identify the second blue compound. In an effort to explore the fundamental question of whether or not anion-π interactions can occur between complex anions and olefins a series of Density Functional Theory and ab initio computations have been preformed for the tetracyanoethylene, 7,7,8,8-tetracyanoquinodimethane, 7,7,8,8-tetracyano-1,2,4,5-tetrafluoroquinodimethane and octacyanoquinodimethane molecules with the anions tetrafluoroborate and hexaflurophosphate. These optimizations indicate favorable interactions for all cases and are further supported by critical point analysis, performed using the Atoms in Molecules theory, and Natural Bond Orbitals analysis. DFT and NBO computations were also employed to explore the simultaneous anion-π and π-type charge transfer interactions observed between 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile and the chloride, bromide and iodide anions, as observed previously by the Dunbar group. Computations involving chloride and bromide anions are in full agreement with the previously reported spectroscopic and crystallographic evidence. Finally, the question of whether or not anion-π interactions occur in proteins was investigated by searching the Protein Data Bank for interactions between chloride or iodide anion and the aromatic moieties of the phenylalanine, tyrosine and tryptophan residues. Computer scripts were specifically written for this search and revealed promising interactions between chloride anions and all three amino acids. Procedural and statistical considerations preclude these examples from being definitive.

In efforts to expand the limited amount of functional groups available for anion recognition, a series of highly acidic, strongly hydrogen bond-donating groups were envisaged as suitable candidates. These included the thoroughly studied N-aryl sulfonamides along with the less utilized N-acyl sulfonamides and tetrazoles. These groups were affixed to a well-understood supramolecular platform in calix[4]arene and their binding affinities for various halides and oxyanions probed. It was found that although in its least energetically favourable conformation that is orthogonal to the aryl group to which it was bound, the tetrazole proved a superior anion-binding element. Noting that tetrazoles prefer co-planarity with aryl neighbours, a series of pyrrolyl-tetrazole anion binding compounds were prepared, first a simple bidentate pyrrolyl-tetrazole which when tested for anion binding affinity demonstrated some of the strongest binding with anions for a bidentate compound ever observed, especially chloride. It was then conceived to hybridize this new binding motif with the well-known amidopyrrole moiety and two new tetrazolyl-amidopyrroles were constructed. When compared to an ester-functionalized pyrrolyl-tetrazole, binding strength with halides was not much different, leading to the postulation that the amide N-H may just be a spectator in the binding event, and the electron-withdrawing nature of the adjacent carbonyl was what led to the binding potency. Nonetheless, a new class of diversifiable anion binders with superior strength to analogous amidopyrroles has been constructed and could perhaps be used in a variety of functional applications. / Graduate

Zugl.: Duisburg, Essen, Univ., Diss., 2007