EPR Spectroscopy in Nanoscale Biophysics & Soft Matter Science

Human serum albumin (HSA) is a versatile transport protein for various endogenous compounds and drugs. In this project, we focus on its highly relevant transport function for fatty acids in the circulatory system. While extensive crystallographic data on the HSA–fatty acid binding exist, we use a new spectroscopic approach to gain information on the functional structure of HSA in solution. Using spin-labeled (i.e. paramagnetic) stearic acids and applying the EPR-technique double electron–electron resonance spectroscopy (DEER), the functional protein structure can be directly accessed from the ligands’ point of view.

We currently expand our approach to study the detailed impact of solvation on the solution structure as well as the transport capabilities.

We study the self-organization of molecules in solution - from the realm of small spherical ions via organic molecules that bear charges to polyelectrolytes. Solvation in aqueous solutions and ionic liquids is of major interest for questions such as protein folding or fine-tuning the reactivity of catalysts in solution. Our main tools in these studies are high-field EPR spectroscopy (W-band), high-field electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation spectroscopy (HYSCORE).

Using HYSCORE we could reveal weak but distinct interactions between several main group element radicals and physically dissolved dinitrogen in solution. These interactions are the basis for a well-defined coordination of N₂ to the paramagnetic centers. The complexes formed are primary of the van der Waals-type but also feature a non-negligible orbital overlap between dinitrogen and the radicals’ SOMO. Our spectroscopic findings are strongly supported by experiments with isotope-labeled ¹⁵N₂, spectral simulations and quantum chemical and density functional theory (DFT) calculations. Formation of such complexes is unexpected when considering the small polarizability of dinitrogen but steric effects like formation of a binding “pocket” may facilitate their formation. The sensitivity, selectivity, and resolution of HYSCORE with respect to the detection of weak hyperfine couplings is needed to identify such complexes that are probably the result of a subtle interplay of induced van der Waals forces between N₂, the solvent and the dissolved radicals.

The controlled and directed release of small drug molecules from macromolecular host systems is an interdisciplinary field of pharmaceutical research that borders synthetic chemistry, materials science and physical chemistry. Many biological (e.g. cellulose) and synthetic (e.g. pluronics triblock copolymers) polymeric materials are being used or are being tested for use to host small drug molecules and release them at the intended site of action. CW EPR spectroscopy on spin probes as tracers for drug molecules is well suited to deliver a large variety of information on guest molecule dynamics, release kinetics, and accessibility by solvent. We use pulse EPR techniques and high-field (W-band) EPR spectroscopy to obtain more detailed information on the local guest-host interactions and the distribution of drug molecules.

Upon addition of oligo- or polyamines such as spermine to DNA a drastic increase in flexibility of this otherwise rigid-rod like polyelectrolyte is observed that eventually leads to precipitation of large (e.g. toroidal, donut-shaped) aggregates of DNA.
This effect is not fully understood on a fundamental polymer-physical basis and EPR spectroscopy may help understanding the polyamine-DNA interaction especially before the appearance of the aggregates. Since this effect also plays a key role in the packing of DNA in bacteriophage capsids, it could even be envisioned for designing gene delivery vectors.

The application of ¹³C (or other low gamma nuclei) in nuclear magnetic resonance (NMR) spectroscopy and imaging for clinical diagnosis has been constrained by the extremely long acquisition times that are required to obtain sufficient signal-to-noise ratios under physiological conditions. Reasons for that are: low natural abundance of ¹³C, low concentration of ¹³C-compounds, physiological temperature etc. However, this obstacle can be overcome by in vitro hyperpolarization (signal enhancements up to more than 10000 have been achieved) of a ¹³C containing molecule with long spin lattice relaxation time via DNP (Dynamic Nuclear Polarization) and subsequent injection into patients. DNP requires electron spin saturation and transfer of the higher electron spin polarization on the nuclei of interest. We are currently developing a mobile DNP polarizer for clinical applications working at low magnetic fields (0.3T), which we also want to use to combine EPR spectroscopy and DNP-NMR spectroscopy for use in the field of soft matter research.

Stable or at least persistent molecular radicals of main group elements are rare and most of the known examples are either formed by certain combinations of electron-rich atoms of high electronegativity, such as O, N, F, or stabilized by bulky substituents or extensive delocalization of the unpaired electron. Radicals of the heavier elements of Groups 13 to 15 were not structurally characterized until 1993 All efforts to synthesize stable mononuclear radicals of Tl, Pb, and Bi have so far failed.
Together with the group of Prof. Klinkhammer at the University of Mainz, who are among the first groups to synthesize stable, well structured radical species of the heavy element atoms Pb and Sn in the oxidation state (III), we have started a systematic investigation comprising the synthesis of new well-defined Pb- and Sn-radical species and their extensive CW and pulse EPR-spectroscopic characterization.