Investigation of site-specific dynamic nuclear polarization on biomolecules

• Applicant: Professor Dr.-Ing. Björn Corzilius
• Subject Area: Analytical Chemistry
• Term: from 2012 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 232503709

During the initial Emmy Noether project it was possible to investigate the relevant mechanisms for dynamic nuclear polarization (DNP) under magic-angle spinning (i.e., solid effect and cross effect), particularly towards paramagnetic metal ion polarizing agents. In this regard, novel bis-Gd(III) metal complexes have been developed for cross effect DNP as well. Furthermore, ubiquitin has been introduced as a model protein by site-directed spin labeling using various Gd(III)-binding chelate labels. This has enabled the first DNP transfer between a bound metal ion and 13C within a protein. Experiments on spin-labeled mutants as well as deuterated proteins have identified the methyl-induced dipolar relaxation between 1H and 13C as impeding factor for the build-up of nuclear spin polarization. Preliminary experiments on direct 15N polarization are nevertheless highly promising and show enhancement factors >100 as well as indications of selective enhancement of signals arising from site chains within a certain distance from the electron spin.During the extension period three currently unsolved challenges of the project will be addressed in order to conclusively answer the respective questions (Which mechanisms have to be considered for high-field DNP with metal ions? What is the distance dependence of the direct DNP transfer process? Is it possible to obtain structural information directly from DNP parameters?). Towards one aspect, bis-Mn(II) complexes will be investigated and the interplay between nuclear Larmor frequency, zero-field splitting, and hyperfine interaction to the metal nucleus will be elucidated. This information is essential for the utilization of biologically relevant Mn(II) as DNP polarizing agent, for example, towards in-cell spectroscopy. The second aspect is the investigation of direct 15N polarization on Gd(III)-labeled ubiquitin. Due to the very small gyromagnetic ratio of 15N as well as the sparse network of nitrogen atoms in comparison to carbon in proteins, homonuclear spin-diffusion is greatly attenuated. This situation is expected to conserve the site-specificity of DNP. Lastly, DNA double helical model systems will be utilized for analysis of the distance dependence of DNP and paramagnetic relaxation enhancement (PRE) under DNP conditions. For this, one strand will be labeled with an electron spin while the complementary strand will be modified with a 13C,15N-isotope label. This information (i.e., distance dependence of DNP and PRE) is invaluable in the scientific community because no quantitative model exists at the current time due to the lack of convincing experimental indications and unfeasibility of theoretical modeling under DNP-relevant conditions.

• DFG Programme: Independent Junior Research Groups