Majors in other disciplines often do honors research with Chemistry Faculty and those major departments are listed in parenthesis after their names.
Title: The Effect of the Cation-π Interaction on thr Sorption of Cationic Amines to Montmorillonite
Advisors: Dharni Vasudevan
Abstract: The cationic amine moiety is an important structural feature of many pharmaceuticals and pesticides that are being increasingly released into the environment. The sorption of these chemicals influences their environmental fate as well as the extent of human and ecosystem exposure. Currently, the primary mechanism of cationic amine sorption, cation exchange, is well understood, but there is a lack in understanding of secondary sorption mechanisms. As such, this study focused on the sorption of cationic benzylamines to montmorillonite, an important component of many soils systems. The goal was to investigate the effect of the cation-π interaction - a potential secondary sorption mechanism - on the sorption of cationic amines to montmorillonite. The cation-π interaction between two aromatic amines is defined as the electrostatic attraction between the π system of one amine and the cationic amine moiety of the other. To investigate the effect of the cation-π interaction, sorption isotherms were obtained for a benzylamines with varying electrostatic potentials (electron densities) at the center of the aromatic ring because the strength and stability of the cation-π interaction are dependent on electron density at the center of the ring. The importance of the cation-π interaction as a potential secondary sorption mechanism was explored by comparing the extents of sorption, the degree of non-linearity in the sorption isotherms of the test compounds, and the strengths of the cation-π interaction based on calculated electrostatic potentials. The examination of benzylamine sorption to Na- and Ca-montmorillonite provided indirect evidence for a secondary sorption mechanism. Observation of S-shaped isotherms that indicate increased affinity for the surface with increasing surface coverage suggest the likelihood of sorption by both cation exchange and an additional mechanism. At intermediate dissolved benzylamine concentrations, (Ci: 3 x 10-4 to 7 x 10-3 M) where the equilibrium sorption constant (Kd) showed a dramatic increase with an increased surface coverage and where the S-shape was prominent, the extent of benzylamine sorption was found to be greater than the extent of charge release (deduced from cation release from the solid surfaces). These observations further emphasized the presence of sorption mechanisms other than cation exchange. To determine if the cation-π interaction could be a potential secondary sorption mechanism, the sorption of two of the four compounds in the benzylamine series, benzylamine and 4-fluorobenzylamine, were compared. In the linear sorption range (Ci: 3 x 10-5 M to 1 x 10-4 M), 4-fluorobenzylamine, which is more hydrophobic than benzylamine, sorbed to a greater extent than benzylamine pointing to the contribution of hydrophobic exclusion (or desolvation penalty) to the extent of cation exchange. At intermediate concentrations (Ci: 3 x 10-4 M to 3 x 10-3 M), where sorption isotherms displayed an S-shape, benzylamine sorbed to a greater extent than 4-fluorobenzylamine. Furthermore, the change in the slope of the isotherm from low to intermediate concentrations was greater for benzylamine than 4-fluorobenzylamine indicating that the sorption isotherm for benzylamine displayed a larger degree of non-linearity. The greater extent of sorption and degree of non-linearity suggest that the secondary sorption mechanism is more favorable for benzylamine. The strength of the cation-π interaction is expected to be weaker for 4-fluorobenzylamine as compared to benzylamine because 4-fluorobenzylamine contains an electron withdrawing group, and therefore a lower electron density at the center of the aromatic ring. Considered together, the consistencies with respect to the strengths of the cation-π interactions and observations of the degree of non-linearity in the isotherms support our hypothesis that cation-π interactions are an important secondary sorption mechanism.
Title: Phosphate Source-Sink Dynamics in Androscoggin River Sedminents
Advisor: Dharni Vasudevan
Abstract: The Androscoggin River in central Maine has historically had varying levels of inorganic phosphate input from many sources, including pulp and paper mills, agricultural runoff, and wastewater treatment plants. Since the passage of the Clean Water Act, water quality has greatly improved. As the river becomes cleaner, however, questions remain over whether the phosphate currently bound to the sediment will reenter the water, potentially resulting in eutrophication, and adversely affecting the ecosystem. As such, the objectives of this study were to determine the orthophosphate concentration range in the river water at which the sediments will act as a source (release) or sink (uptake) of phosphate and to elucidate the influence of sediment characteristics on the ability of the sediments to retain phosphate. To examine these source-sink dynamics, sediments from two sampling locations along the Androscoggin, Gulf Island Pond (GIP), and Merrymeeting Bay (MMB), were used in sorption experiments from which the equilibrium phosphorus concentration (EPC) was extrapolated. The EPC value has been extensively used to establish aqueous phosphate concentrations at which there is no net phosphate release or uptake by the sediment. At all locations, the EPC values (0.5-1.0 µmol P/L) were in the same range as sediment pore water (0.7-1.4 µmol P/L) and surface water phosphate concentrations (1.1-1.8 µmol P/L). The similarity in these ranges indicates that phosphate in the sediment is in equilibrium with phosphate in the river water. This suggests that as phosphate concentrations in the river water fall below 0.5 µmol P/L, the sediments will become a new source (release) of phosphate to the river water, potentially hindering further ecosystem recovery along the Androscoggin. The influence of sediment characteristics on the ability of river sediments to retain phosphate was also compared at GIP and MMB. Sediments from GIP were found to retain higher concentrations of phosphate and also had higher surface areas, extractable iron (Fe), and extractable aluminum (Al) contents compared to MMB sediments. This indicates that the ability of sediments to sorb phosphate was greatly influenced by the number of reactive sites on the sediment, as indirectly measured by surface area, and directly measured by the number of surface bound Fe and Al sites available for phosphate surface complexation. Similar to trends observed in extractable Fe and Al content, GIP sediments were also found to have a higher extractable phosphorus content compared to MMB. Despite the high sediment bound phosphorus concentration, GIP sediments also had higher phosphate retention capacities. These observations demonstrate that the retention of additional phosphate by the sediments in GIP via sorption processes is not inhibited by the native phosphorus concentrations in the sediments. Furthermore, GIP is known to experience anoxic conditions during the summer months. Anoxic conditions promote the reduction of FeIII in the sediments and the release of FeII and iron bound phosphate into the water column. The higher iron bound phosphate content in the sediments at GIP indicates that anoxic conditions in the summer could contribute to additional phosphate release.
Title: Discovering Helicobacter pylori's glycoproteins using metabolic oligosaccharide engineering
Advisor: Danielle Dube
Title: Controlling cis/trans Isomerism in Thiopeptids using the Thione-Aromatic n→π* Interaction
Advisor: Ben Gorske
Abstract: Numerous biological functions involve the interaction between two or more proteins. The formation of these complexes is dictated by the structures of the binding partners; therefore, they must be structurally well-defined by the non-covalent and weakly covalent interactions that govern protein folding and thereby function. Small peptide molecules can disrupt protein-protein interactions, thereby serving as therapeutic agents. However, peptides have disadvantages due to proteolytic instability. Peptoids are oligomers of N-substituted glycines that represent a class of potential therapeutic peptidomimetics because of their biostability. A significant challenge for peptoids has been the control of cis- and trans-isomerism in the peptoid backbone. The present work has therefore focused on the stabilization of cis-peptoid rotamers that would be useful for the design of peptidomimetic polyproline type I (PPI)-like helices. In this context, the formation of cis-peptoid rotamers is promoted by delocalization of the electron lone pair n of the carbonyl oxygen into the antibonding π* orbital of the aromatic side chain (designated n→π*). It was hypothesized that increasing the nucleophilicity of the lone pair by substituting the carbonyl oxygen with a more polarizable sulfur atom would strengthen the n→π* carbonyl-aromatic interaction. A library of peptoids and their novel “thiopeptoid” counterparts were thus synthesized and their relative rotamer populations were determined by 1H NMR spectroscopy. The aromatic side chains were electronically tuned by electron-withdrawing and electron-donating groups to gain an understanding of the n→π* interaction, with additional consideration given to the position of the substitution. The thiopeptoids were also analyzed in different solvent systems to assess the presence of non-covalent interactions. The results clearly demonstrate that, with sufficient aryl electrophilicity, peptoid sulfur substitutions strengthen the n→π* interaction. This research provides the experimental basis for the synthesis of novel thiopeptoids and affords a new approach to stabilize cis-rotamers in peptoid systems. The findings are applicable to the generation of stabilized PPI-like helices with increased biological activity for potential therapeutic applications.
Title: Synthesis of Cobalt Oxide Nanoparticles in Methane-Oxygen Co-flow Flames
Advisors: Jeff Nagle and Norm Laurendeau
Abstract: The photocatalytic breakdown of water molecules by sunlight into hydrogen and oxygen offers a potentially affordable and renewable alternative energy source. Before hydrolysis can be implemented on a large scale, however, improvements must be made to lower the photocatalytic cost and to increase the efficiency of energy capture and storage. The addition of a catalyst to drive the oxidation reaction would greatly speed this process, but few materials are able to both absorb visible light and induce splitting of water molecules. Much recent work has been focused on identifying an ideal oxidation catalyst; it must be naturally abundant, inexpensive, capable of self-repair, and stable in the harsh oxidizing environment of a fuel cell. Cobalt nanoparticles, in particular, have shown great promise. Currently, such particles are being synthesized through costly and time-consuming multi-step wet phase syntheses. Flame synthesis offers an attractive single-step alternative method, with the advantages of creating a thermally-stable product and being easily scaled up for use in industrial settings. Using a coflow burner, we were able to synthesize cobalt nanoparticles in a single-step process, using cobalt nitrate hexahydrate as a chemical precursor. Analysis of these particles by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that the product particles were nanoparticle-sized, while inductively-coupled plasma-optical emission spectroscopy (ICP-OES) and electron dispersive spectrometry (EDS) revealed that the cobalt-derived nanoparticles were composed of cobalt, likely CoII.
Title: Electronic Spectroscopy of Allowed and Forbidden Transitions in Long Polyenes
Advisor: Ron Christensen
Abstract: We analyzed the symmetry-allowed and symmetry-forbidden electronic transitions of unsubstituted and methyl-substituted polyenes with 5-23 conjugated double bonds (N). A HPLC purification process using a C30 column and mobile phases with various ratios of methyl-tert-butyl ether/ methanol was developed to balance the quality of separation and the retention times of the polyenes to enhance the purification of the samples for further spectroscopic analysis. The strongly allowed electronic transitions, S0→S2 (0-0) band energies were identified using absorption spectroscopy. The energies were analyzed as a function of N with two simple models: particle-in-a-box theory and Simple Hückel Molecular Orbital theory. Incorporation of bond alternation into the models accounts for the non-zero energies of the S0→S2 transitions in the long polyene limit. The symmetry-forbidden S0→S2 (0-0) transitions were investigated using fluorescence spectroscopy for the N=5 and N=7 polyenes. The absorption and fluorescence spectra of both the substituted and unsubstituted N=5 and N=7 polyenes were obtained and the effects of substitution were analyzed. The unsubstituted N=5 and N=7 polyene emission spectra were smoothed and fit to Gaussian curves to determine the energies of the (0-0) bands. The S0→S2 (0-0) band for the unsubstituted N=7 polyene was then compared to the S1 energy estimated from transient absorption experiments. The measurement of the S0→S2 and S0→S2 (0-0) bands for unsubstituted N=5 and N=7 polyenes shows that the S2-S1 energy difference increases from 3,190 ± 24 cm-1 to
3,930 ± 81 cm-1 with increasing conjugation length. This compares with S2-S1 energy differences of 3,775 cm-1 (N=5) and 4,825 cm-1 (N=7) for simple, dimethyl-substituted polyenes.
Title: Synthesis of an azidosugar substrate to selectively label Helicobacter pylori's pseudaminic acid
Advisor: Danielle Dube
Title: Synthesis and Characterization of Closthioamide Derivatives
Advisor: Ben Gorske
Abstract: Closthioamide, a recently discovered natural secondary metabolite isolated from the bacterium Clostridium cellulolyticum, has been previously shown to induce potent antibacterial effects due to its rich abundance of thioamide moieties. To further investigate which specific structural components within the compound might confer enhanced bioactivity, closthioamide derivatives were synthesized for characterization via antimicrobial assays. β-Thiopeptoids, which exhibit high structural similarity to closthioamide, were used as a scaffold to construct the derivatives and to probe the role of hydrogen bonding in closthioamide bioactivity. Using an α-peptoid that was easily synthesized, an efficient purification technique entailing preparative TLC was developed. This optimized method was applied towards the purification of β-peptoids, which served as precursors to β-thiopeptoids. Adequate yields of β-peptoids for thionation were afforded by introducing a stronger Fmoc deprotection step at the start of β-peptoid solid-phase synthesis. A protocol for the subsequent thionation of a β-peptoid was developed, which resulted in a distinct, nonpolar product that might be representative of a β-thiopeptoid. With an effective protocol for β-thiopeptoids in hand, different classes of closthioamide derivatives may be designed to identify essential structural elements that result in improved bioactivity. Knowledge acquired from this study may ultimately be valuable for the future design of de novo thioamide antibiotics.
Title: Effect of Soil Properties on Cationic Amine Sorption
Advisor: Dharni Vasudevan