Spectroscopic Signatures of Divalent Metal ion Binding to Anionic Surfactants and Local Mechanics of Embedded Groups in Two-Dimensional Water Networks Through Cryogenic Cluster

点击翻译按钮获取中文摘要

In this project funded by the Chemical Structure, Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Mark A. Johnson of Yale University is using advanced instrumentation to determine how large molecules flex in response to the presence of calcium ions that are critical to biological function. Although calcium is well known to be essential for the formation of permanent structures like bone, it is also present in the blood (a liquid that is basically salty water). Most of the salt is just dissolved NaCl, common table salt, where the positive charge carrier is the sodium ion, Na+. Calcium has much stronger interactions with molecules because it carries two positive charges (Ca2+). Direct contacts with doubly charged (divalent) ions like Ca2+ control how large molecules (like proteins) undergo chemical reactions, but it has been difficult to identify exactly where these ions are attached to the proteins using conventional methods of chemical analysis. Professor Johnson and his research group are able to determine these contact points by measuring changes in the oscillation frequencies of the molecules' chemical bonds. These oscillation frequencies typically correspond to infrared (IR) radiation, the same type of light used in night vision goggles. With IR lasers and other specialized instruments, the changes in the oscillation frequencies of biological molecules with and without Ca2+ ions binding to them can be detected. These changes provide clues to where on the molecule the Ca2+ ion is attached. The measurements are carried out with custom-built instruments, much of which are actually built by the students involved in the project. Thus, in addition to gaining knowledge about metal-ion interactions with large molecules in biological systems, the students also receive training in instrument design and construction, as well as in computer modeling of both the instruments and the chemical systems. This research may have implications in human health. This project uses recent advances in the combination of mass spectrometry with laser spectroscopy to establish how the infrared signatures of divalent metal ion binding to surfactants and peptides, observed in solution, are generated at the molecular level. In this approach, ionic assemblies are extracted from solution using electrospray ionization, captured in a temperature controlled radiofrequency (FRF) ion trap, and characterized with vibrational spectroscopy. The vibrational spectra are obtained in an action mode by the photoinduced mass loss caused by the evaporation of weakly bound adducts such as rare gas atoms or H2 molecules. Because these assemblies often occur in multiple isomeric forms upon cooling, the spectrometer is equipped with the ability to carry out isomer-specific spectroscopy using a two-color, IR-IR photobleaching approach. This method isolates the spectra of different structural isomers by systematically removing each of them from the ion ensemble with a tunable IR laser, while monitoring changes across the spectrum with a second IR laser. An important class of such isomers is that created when an isotopically labeled water molecule is introduced into a cluster containing many water molecules. Because chemical exchange of H and D atoms between water molecules does not occur in small clusters, it is possible to isolate a single, intact H2O molecule in a surrounding network consisting entirely of D2O molecules. This situation provides an opportunity to monitor the spectral response associated with the individual hydrogen bonding sites within an extended water network. Part of this program is, therefore, directed at elucidating fundamental aspects of water structure and dynamics, which are important for understanding and predicting the behavior of water when it is in contact with biological media such as enzymes and DNA. The broader impacts of this research include the development of new types of instrumentation for chemical analysis of complex media and the formulation of a "rule book" for decoding the structural information contained in the vibrational bands of anionic domains bound to divalent metal ions in aqueous solution. The students involved in this project integrate a wide spectrum of skills including laser development, electronics (digital, analogue, high voltage and RF), computer control over complex experiments, electronic structure calculations, construction and use of high vacuum apparatus, fundamental aspects of ion beam and mass spectrometer design, cryogenics, and synthesis.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.