Synthesis, Characterization, and Formation Constant Studies of Novel Bifunctional Ligands for Sensing Copper, Zinc, and Iron Alexis Kasparian, Lea Nyiranshuti, Feifei Wang, Dr. W. Rudolf Seitz Advisor: Dr. Roy Planalp University of New Hampshire, Parsons Hall, 23 Academic Way, Durham NH 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 bifunctional via an acrylamide 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. References [1] Yao, S. et al. Analyst , [2] Reddel, J. B.S. Thesis, University of New Hampshire, [3] Planalp, R.P. et al. Biochem. Soc. Trans , Preliminary 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 the more likely binding results. Two potential isomers of the ligand were hypothesized, assuming a tetradentate binding fashion with the ligand and completion of an octahedral geometry with water molecules (Figure 4). The isomers were then inserted into the equation below (Figure 5). Conclusions and Future Work The preliminary computed results show promise in regards to the specificity of the ligand. 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: Continuation of computational modeling to determine the most likely conformers of the ligand 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 Figure 1. Previously synthesized bifunctional metal-specific ligands containing acrylamido groups for polymerization. 1,2 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. Figure 3. Tachpyr molecule and subsequent theoretical modification for desired sensor ligand. Current Synthesis Routes and Results One synthesis route was explored (Figure 6), but after several attempts it was determined that the yield of conversion of 2 to 3 was impractical in our experience. The approach of Figure 7 appears more suitable. Figure 8. Model reaction for determination of reaction conditions using 2-aminomethyl pyridine with 1,3-propanediol di-p-tosylate. Figure 6. Original synthesis route. Conversion of 2 to 3 was very low yielding, prompting research into alternate methods for synthesis. 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. Figure 2. General setup of polymer for delivery and action of the ligand. Figure 7. Alternate synthesis route, in progress. Figure 5. Equation used to compare binding of the model ligand with different metal ions. The reactions assumed aqueous media and 6-fold binding in an octahedral fashion. Table 1. Calculated results of relative energies using the equation in Figure 5. Figure 4. Isomers A and B of the ligand, shown complexed to a metal ion center. Two water molecules complete the octahedral geometry. 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. 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 with the ligand, but since Cu II complexes tend to undergo Jahn-Teller distortion, further parameters need to be applied to determine copper’s true complex with the ligand and resultant energies. Total Energy Differences, right side - left side [Hartree][kcal/mol] Case Case Case Case Isomer AIsomer B 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. This project and presentation was funded by the UNH URA and Craig West Undergraduate Awards, and we thank all those who have helped to keep these programs going.