The proposal is based on computational and experimental studies of binding of metal ions to synthetic organic compounds that serve as matrices in MALDI experiments. With permission from the Program Officer, the mass spectral studies were extended to include electrospray ionization mass spectrometry (ESI-MS).
During the preceding year, the formation of adducts of a variety of compounds including MALDI matrices and alkali and alkaline earth metal ions were studied. Both computational and experimental approaches were used, namely DFT calculations using the software Gaussian 09 and ESI-MS techniques. The results from these studies are at present preliminary, and will be reported in future.
During ESI-MS studies, a serendipitous discovery was made that the ionophore valinomycin, well known for its cation binding properties, also binds to anions. This avenue was explored both experimentally and computationally, resulting in a publication in the journal Supramolecular Chemistry. This work was carried out in collaboration with Professor Megumi Fujita (Department of Chemistry), a former recipient of an ACS-PRF grant, and Professor Swamy Mruthinti (Department of Biology) and five undergraduate students. The details are summarized below.
The binding selectivity for halide ions for valinomycin is found to be in the order Cl->Br-~F->>I- based on ESI-MS experiments in methanol. 1H NMR studies in acetone-d6 and CD3CN reveal the binding selectivity of Cl->Br->>F-~I-. NMR studies and DFT calculations support a bracelet-like structure for the binding of a chloride ion to valinomycin. Association constants of 531±45 and 57±2 M-1 were obtained via NMR titrations in acetone-d6 for chloride and bromide ions, respectively.