Student Honors Thesis

Sickle cell disease is a genetic disorder that affects the hemoglobin within red blood cells. A point mutation in the gene coding for the β-subunit of hemoglobin allows the mutant chain to interact with a hydrophobic pocket of another hemoglobin in a deoxygenated environment, causing polymerization of the proteins. This creates the characteristic sickle-shape of the diseased blood cells that can clog capillaries, leading to tissue damage and cell death. Currently, there are limited options for those affected with sickle cell disease. The research to be presented is focused on discovering peptides that can interact with the mutated hemoglobin and prevent aggregation. A novel proline-rich peptide ligand, ZSF39, was identified through a phage display against deoxygenated sickle cell hemoglobin. A combinatorial peptide library based on the structure of ZSF39 was synthesized and screened for binding affinity using an ELISA. A tightly binding peptide, LHSl, was discovered through the ELISA and found to have a significant inhibitory effect on the polymerization of sickle cell hemoglobin. This work represents a novel approach for the discovery of therapeutics for this debilitating disorder.

Research in our group focuses on environmentally friendly organic synthesis using bismuth compounds. With increasing environmental concerns, the need for environmentally friendly organic synthesis following Green Chemistry principles has assumed increased importance. The Pollution Prevention Act, passed in 1990, was especially important in increasing an interest in Green Chemistry, which is the design and redesign of chemical processes with a view of improving safety. Our group focuses on synthetic methodology i.e. the transformation of one functional group to another. Such transformations are of special relevance in the pharmaceutical industry for manufacture of life saving drugs. Many of the current organic synthesis methods utilize toxic and corrosive catalysts and reagents. In contrast, methods utilizing bismuth(III) salts are environmentally friendly because most bismuth compounds are non-toxic, non-corrosive, and inexpensive.

The goal of this project was to utilize bismuth(III) triflate and bismuth(III) bromide, as catalysts for the synthesis of cyclic acetals, the allylation of tetrahydropyranyl and tetrahydrofuranyl ethers, and the allylation of aldehydes. All these processes utilize many Green Chemistry principles such as the use of non-toxic reagents, room temperature reaction conditions and at times solvent-free conditions.

It has been long known that ammonia as well as many primary, secondary and tertiary amines, acts as a ligand forming complexes with the main transition elements.

Cyclic acetals (dioxolanes, dioxanes, and dithianes) are common protecting groups in organic synthesis but they can also be converted to other useful functional groups. A bismuth(III) triflate-catalyzed multi component reaction involving the allylation of cyclic acetals followed by in situ derivatization with acid anhydrides to generate highly functionalized esters and thioesters has been developed under solvent-free conditions. Most reagents used to date for the allylation of cyclic acetals are highly corrosive or toxic and are often required in stoichiometric amounts. In contrast, the use of a relatively non-toxic and non-corrosive bismuth(III) based catalyst makes this methodology benign and attractive.

The solvolysis of epoxycarbinyl substrates 1 has been the subject of several mechanistic studies. In spite of these investigations, it has not been established whether these solvolysis reactions proceed with anchimeric assistance from the epoxide oxygen and involve an oxabicyclobutonium ion intermediate or whether unassisted solvolysis occurs. Conflicting data in the literature suggest that the ability of the epoxide oxygen to provide anchimeric assistance is dependant upon structural and electronic features of the epoxycarbinyl substrate in question. The aim of this project was to study the nucleophilic substitution reactions of tetrahydrofurfuryl and tetrahydropyranyl sulfonates 2a and 2b as well as cyclopentylmethyl and cyclohexylmethyl sulfonates 3a and 3b, respectively, to probe the effect of a neighboring oxygen on solvolyses rates and product distribution. The rates of solvolyses of cyclopentylmethyl tosylate and tetrahydrofuranomethyl tosylate have been compared to determine if the oxygen lends anchimeric assistance to the leaving group in the solvolyses reactions of the latter. This research has led to an increased understanding of the role of the ether oxygen in reaction of tetrahydrofurfuryl tosylate.

The FNR protein is a transcription factor that allows Escherichia coli to undergo anaerobic cellular respiration. It is known to positively regulate the expression ofseveral genes required for anaerobic respiration as well as negatively regulate genes responsible for aerobic respiration. Consequently, FNR is active under anaerobic conditions and inactive under aerobic conditions. Although the tertiary structure ofFNR is unknown, previous studies have indicated that FNR is inactive in the monomeric state and active in the dimeric state. Thus, it is believed that in anaerobic conditions, FNR undergoes a confonnational change from the monomeric to dimeric state. The mechanism involved in going from the monomeric to dimeric state is not completely understood, but it is thought to be triggered by the acquisition ofa [4Fe-4S]2+ cluster in the N-tenninal region ofFNR. The acquisition ofthe cluster causes a confonnational change to be transmitted through the allosteric domain to the dimerization helix resulting in the active dimeric species. Infonnation regarding the environment of amino acid residues in the dimerization helix in both the active and inactive fonns of FNR could be helpful in eliciting a better understanding ofthe dimerization mechanism. Such environmental conditions can be detennined by the fluorescent properties ofthe amino acid, tryptophan. Surface exposed tryptophan residues are expected to have a longer Amax than those buried in the hydrophobic core. In order to gain insight into the environment ofthe amino acids on the dimerization helix we have created tryptophan mutants (LWI46, MW147, MW157, KW163, and KW164) that either lay on or near the helix. The mutants KW163 and KW164 all lie on the periphery ofthe helix while LW146, MW147 and MW157 lie on the helix. Ofthe five mutants, KW163 retained anaerobic activity most similar to that of the wild type indicating that its structure is similar to the wild type protein with the exception ofthe single amino acid substitution. By comparing the fluorescence ofthe active and inactive forms of KW163, we hope to gain a better understanding ofthe dimerization mechanism ofFNR.

The synthesis of bifunctional thioureas and the corresponding thiourea S,S,S-trioxides has been examined. Two methods for the synthesis of the bisthioureas were employed. One involved the treatment of a diamine with silicon tetraisothiocyanate in benzene. The second involved treatment of the amine with ammonium thiocyanate in dilute acid. This latter synthesis was superior because of the ease of its use, the high yields obtained, and the purity of the products. Though this synthesis worked well for the preparation of phenylene-l,4-bis(thiourea), it yielded only bisthiocyanate salts in the synthesis of aliphatic thioureas. The oxidation of the bisthioureas was carried out using peracetic acid or hydrogen peroxide to give the corresponding thiourea S,S,S-trioxides.

Aqueous solutions of HONO (ranging from 0.010M to 0.057M) and NO2 (ranging from 0.025M to 0.035M) were each photolyzed with nm ultraviolet (UV) light. In the presence of benzene scavenger, DH radical intermediate was indicated by formation of p-nitrosophenol (PNP). Ultraviolet/visible (UV/vis) absorption spectra of photolyzed aqueous HONO/benzene solutions showed the presence of PNP by its characteristic absorption at 298 nm. UV/vis absorption spectra of photolyzed aqueous NO -benzene solutions showed no evidence of PNP formation. Other compounds such as scavengers were toluene, benzoic acid, and terephthalic acid. UV/vis spectra of photolyzed aqueous HOND/scavenger solutions showed an Int n e road peak in the 295-310 nm range, Indicating that the scavenger was hydroxylated by OH, formed from HONO photolytic dissociation, and subsequently nitrostated by reaction with excess HONO. Hydrugen peroxide, a known OH producer, was photolyzed in the presence of benzene to verify the proposed OH-scavenging sequence under varying pH condItions. UV/vis spectra showed evidence of hydrocybenzene formation upon photolysis. The thermal decomposition of HONO was studied and a kinetic order with respect to HONO, of 0.5+-0.5 was determined. Quantitative data concerning the photochem cal de omposition of HONO was too inconsistent to make reasonable comparisons to thermal decomposition data.

A synthetic model system for the special pair in the photosynthetic reaction center of bacteria has been prepared from electrostatically paired porphyrin and phthalocyanine molecules. Use of rigorously dry propylene carbonate as the solvent allowed complex fonnation to occur between tetra-(N-methyl-4-pyridyl)porphyrin hexafluorophosphate (H2TMPyP(PF6)4) and tetraphenylphosphonium tetrasulfatophthalocyanine (Ph4P)4H2TSPc molecules. The stoichiometry of the complex was determined to be 1:1 by Job's method and the pair formation constant K1 was estimated to be 1x108.T he dependence of K1 on ionic strength was studied at various concentrations of tetra-n-butylammonium tetrafluoroborate BU4NBF4. With increasing ionic strength K1 decreases as expected according to the Debye-Huckellimiting law. Using an ionic strength of 2 mM (BU4NBF4) in PC, the extent of pairing was found to be almost 100%. Under these conditions tetrasulfatophthalocyanine and the electrostatically paired complex were oxidized at E1/2's of 0.453 and 0.350 V vs SCE (aq), respectively. From evaluation by the Nemst equation of the measured potential difference between paired and unpaired tetrasulfatophthalocyanine, the ratio of [C]K2 / [C+]K1 is estimated to be 55. K2 is the equilibrium constant for the formation of the singly-oxidized pair. The significance of the value of the ratio leads to an estimate of K2 = 6 x 109 at an ionic strength of 2.0 mM.