Understanding proton transfer is intimately linked with rationalizing the gas-phase fragmentation chemistry of biomolecules in mass spectrometry. This talk aims to explore mechanisms of proton transfers during ionization and controlled gas-phase ion activation. In the ionization studies, the kinetic trapping of solution-phase structures into the gas phase is shown to be dependent on the solvents and energetic considerations in the electrospray source. The systematic studies support a protic bridge-type mechanism which may (or may not) enable proton transfer. In the gas-phase activation studies, UV activation causes distonic cleavage at C-I bonds, triggering subsequent rearrangement chemistry. In all of these studies, the structures of the species of interest are characterized via infrared ion spectroscopy supported by quantum-chemical calculations.

Recent innovations in speed, accuracy and sensitivity have established mass spectrometry (MS) based methods as a key technology for the analysis of kinetic intermediates and folding mechanism of peptides, protein, DNA and DNA-protein complexes. In particular, Ion Mobility Spectrometry – Mass Spectrometry provides a powerful tool for the identification of structural motifs, and when complemented with theoretical calculations, it permits a better understanding of the main motifs that drive the dynamics across the free energy landscape. We have recently introduced a Trapped Ion Mobility Spectrometry coupled to Mass Spectrometry (TIMS-MS) as a high-throughput technique for the study of conformational states of biomolecules, as well as the kinetic intermediates involved during their folding as a function of the molecular environment (e.g., pH, organic and salt content). While this description holds true for most contemporary IMS analyzers, the higher resolving power (e.g., R= 150-250, 3x larger than traditional IMS systems) and the unique ability to hold and interrogate molecular ions for kinetic studies (e.g., millisecond-second time scale) provides TIMS-MS with unique capabilities for the study and interrogation as a function of the time after desolvation. Recently combined with hydrogen-deuterium exchange, HDX-TIMS-MS, a more detailed description of the accessible surface area and the folding can be achieved over time.  That is, HDX-TIMS-MS has a significant advantage in the flexibility to interrogate, at the single molecule level, the molecular interactions that define the conformational space. In addition to IMS-MS separation, significant information on the type of interactions that stabilize tridimensional structures can be obtain using Action spectroscopy (IRMPD). In the present talk, recent results that reveal the kinetic intermediates and the main folding pathways for small molecules, peptides, proteins, DNA and DNA-protein complexes will be discussed.

Recently we reported the effects of elastic interactions and resulting bowing moments on strain in bulk GaAs/Si composites¹ using optically pumped nuclear magnetic resonance (OPNMR).² We have also reported the effects of strain in GaAs/AlGaAs quantum wells, where the heavy-hole/light-hole energy splitting was affected by both strain and quantum confinement.³ In that 30 nm wide GaAs well, our calculations show that only modest quantum confinement effects were present, and strain effects dominated the modifications of the electronic band structure, optical absorption, and OPNMR action spectrum. Here we present our progress-to-date on an OPNMR study of a series of GaAs/AlGaAs quantum wells where the well-width is varied. In collaboration with Sandia National Laboratories, a series of quantum well arrays (QWA) with widths of 28, 14, 7, and 4 nm were grown via molecular beam epitaxy.  By comparing the photon energy dependences of the OPNMR, magneto-optical absorption and photoluminescence spectra to electronic band structure calculations, we expect a more complete understanding of the OPNMR photo-physics to emerge. The QWA films were epoxy-bonded to a transparent, single crystal sapphire wafer, and the sacrificial GaAs substrate was etched down to the stop-etch layer using selective wet chemical etching. Magneto-photoluminescence experiments ensured the complete removal of the bulk GaAs growth substrate. The exploitation of strain, quantum confinement, and magnetic field to enhance and control the optically pumped nuclear spin hyperpolarization could have applications to nuclear spin electronics and quantum computing.

 

 

[1] Wood, R.M.; Tokarski III, J.T.; McCarthy, L.A.; Stanton, C.J; Bowers, C.R., Characterization of elastic interactions in GaAs/Si composites by optically pumped nuclear magnetic resonance, Journal of Applied Physics, 2016, 120, 085104.

[2] Kuhns, P. L.; Kleinhammes, A.; Schmiedel, T.; Moulton, W. G.; Chabrier, P.; Sloan, S.; Hughes, E.; Bowers, C. R., Magnetic-field dependence of the optical Overhauser effect in GaAs, Physical Review B, 1997, 55, 7824-7830.

[3] Wood, R. M.; Saha, D.; McCarthy, L. A.; Tokarski, J. T.; Sanders, G. D.; Kuhns, P. L.; McGill, S. A.; Reyes, A. P.; Reno, J. L.; Stanton, C. J.; Bowers, C. R., Effects of strain and quantum confinement in optically pumped nuclear magnetic resonance in GaAs: Interpretation guided by spin-dependent band structure calculations. Physical Review B, 2014, 90 155317.

Tuning magnetic interactions between multiple spin carrying metals, to illicit desired magnetic/electronic properties, has long been a goal in molecular magnetism research. A great deal of this work has focused on exchange coupled clusters where the magnetic ions are weakly coupled through a bridging group. Here, we report on our efforts to understand the relationship between structure and physical properties of multi-metallic compounds with a direct metal-metal bond. To this end, we have performed a combined electron paramagnetic resonance and ⁵⁷Fe Mössbauer spectroscopic investigation of several such compounds with varying spin states and coordination environments. We have rationalized the observed spectroscopic parameters in terms of ligand field theory and quantum chemical calculations.

Monolayer-protected clusters (MPCs) are an emerging class of photonic materials that can be synthesized and isolated with atomic precision. Control over MPC composition results, in part, from electron filling of Superatom orbitals,  yielding colloidal metal nanoparticles of specific magic sizes. These synthetic advances overcome many limitations of inherently heterogeneous colloidal metal nanoparticle syntheses. Recently, post-synthetic electrochemical methods for manipulating the oxidation state of stable MPCs have been demonstrated. Here, femtosecond time-resolved and magneto-optical spectroscopy studies of a family of MPCs in the 1-2 nm size range will be presented. These results show that the optical, electronic and magnetic properties of MPCs are extremely sensitive to the electronic configuration of Superatom orbitals. For example, the magnetic properties of Au₂₅(SR)₁₈, where SR represents an alkanethiol, can be switched reversibly by oxidative opening of the eight-electron Superatom P orbital. Collective interactions between assembled MPCs also exhibit spin-dependent magnetic phenomena not present in the isolated building blocks. Magnetic Circular Dichroism and time-dependent spectroscopy on dimerized 20-atom MPCs reveal inter-particle spin-dependent dynamics not observed for the monomer. Importantly, these results indicate that the magnetic properties of gold MPCs result from the electronic configuration of metal-based Superatom orbitals