Exploration into the Synthesis and Analysis of a Novel Sensor for Biological Metal Ions

1. Exploration into the Synthesis and Analysis of a Novel Sensor for Biological Metal Ions Alexis Kasparian Advisor: Dr. Roy Planalp ahf28@wildcats.unh.edu; University of New Hampshire, Parsons Hall, 23 Academic Way, Durham NH 03824 Introduction The synthesis of ligands with particular affinities for metal ions allows their use in sensing. In particular, this research group has worked towards the synthesis of ligands with regard to the trace biological ions zinc(II), copper(II), and iron(II). Upon successful synthesis of ligands and determination of their formation constants with the ions, the ligands may be made functional via an acrylamido group for copolymerization with n-isopropylacrylamide (NIPA). The polyNIPAm system also contains sites that allow for addition of fluorophores.1 The fluorophores enable FRET under certain conditions, which is used for quantitative detection of metal ions (Figure 2). Current Synthesis Routes and Results The synthesis of the desired ligand is in progress (Figure 7). We are working to improve the outcome of the amine formation (3) by rigorous exclusion of water and oxygen, which may quench the formation of the necessary organocerium complex. The displayed plans do not include the acrylamido group for polymerization. Its synthesis could be accomplished with the introduction of a nitro group on the center carbon of 1,3-propanediol di-p-tosylate, which can then be converted to an amine for reaction to the resulting acrylamido moiety. Computational Results The model ligand was analyzed using Spartan software modeling programs. Energy comparisons of the ligand bound to the various ions were performed to determine likely binding results. Lowest energy conformers of the ligand were developed, assuming a tetradentate binding fashion with the ligand and completion of an octahedral geometry with water molecules (Figure 4). The conformers’ energies were then compared (Figure 5, Table 1). Cu(II) complex Zn (II) complex Fe(II) low spin complex Fe(II) high spin complex Figure 4. Four low energy conformers of the ligand attached to a metal ion center. Two water molecules complete the octahedral geometry. Figure 1. Previously synthesized metal-specific ligands with acrylamido groups for polymerization, left, and general polymer setup, right.1,2 Figure 7. Alternate synthesis route, in progress. To determine the most practical way for the addition of two amines onto a single di-tosylate, a model reaction is under exploration. This reaction (Figure 8) involves the use of 2-aminomethylpyridine, a readily available reagent. It is being reacted under different conditions to determine which method is most efficient for completion of reaction. Table 1. Calculated results of relative energies using the equation in Figure 5. Figure 5. Equation used to compare binding of the model ligand with different metal ions. Figure 2. Mechanism of action of the ligand-NIPA copolymer. The ratio of donor to acceptor emission, when FRET occurs or does not occur, is used to determine the amount of metal ion bound to the polymer. The results of the computations show that for Cases 1-3 the equilibrium will prefer the left side of the equation, with iron(II) bound to the ligand. Case 4 shows preference for copper binding, so it is possible that the ligand may have some affinity for copper(II). Figure 8. Model reaction for determination of reaction conditions using 2-aminomethyl pyridine with 1,3-propanediol di-p-tosylate. Link to Current Research To design a sensor selective for iron(II), the previously synthesized tachpyr molecule (Figure 3, left) was examined. Tachpyr is known to favor iron over zinc and is cytotoxic to cells, allowing its use as a cancer cell treatment.3 To produce a molecule with a reversible binding ability for sensing, tachpyr was modified (Figure 3, right). This novel molecule is currently under study. Synthetic routes are being explored and computational modeling is employed to determine how it will interact with the aforementioned metal ions. • Conclusions and Future Work • The computed results show promise in regards to the specificity of the ligand, and progress has been made towards its synthesis. The model reaction has been run under several different conditions and the products are under analysis. • Future work for the research group involves: • Successful ligand synthesis • Comparison of its crystal structure, if isolable, to the computed models • Titrations of the ligand to determine its pKa • Titrations of the ligand with iron(II), copper(II), and zinc(II) to determine formation constants • Addition of the ligand onto the polymer delivery system and resultant fluorescent studies with iron(II) concentrations Original Synthesis Route One synthesis route was explored (Figure 6), but after several attempts it was determined that the yield of conversion of 2 to 3 was too low for our purposes. The approach of Figure 7 appears more suitable. Figure 3. Tachpyr molecule and subsequent theoretical modification for desired sensor ligand. Acknowledgements Thank you to the UNH Department of Chemistry for their continuing support. My sincerest thanks to Dr. Planalp and Lea Nyiranshuti of UNH for their guidance, and Dr. Richard Johnson for his introduction to computational models. We thank the UNH URA and Craig West Undergraduate Award programs for funding. References [1] Yao, S. et al. Analyst. 2012. 137, 4734-4741. [2] Reddel, J. B.S. Thesis, University of New Hampshire, 2011. [3] Planalp, R.P. et al. Biochem. Soc. Trans. 2002. 30, 758-762 Figure 6. Original synthesis route. Conversion of 2 to 3 was very low yielding, prompting research into alternate methods for synthesis.