Metal/metal and metal/nitroxyl-labeled compounds for the development of EPR-based distance measurement techniques

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Electron paramagnetic resonance (EPR) techniques have become very useful tools to elucidate the structure of (bio)macromolecules and their assemblies in a disordered, native state. An in-cell application is on the horizon. For this purpose the molecules of interest are site-selectively spin labeled. For a long time this was done with nitroxyl radicals. Only recently Gd(III) came into focus as a spin label. One major objective of this project is to identify Gd(III) complexes having the spectroscopical properties required for a specific EPR technique (pulse and cw EPR) and developing them into ready-made spin labels for site-directed spin labeling of proteins. To sound the potential of Gd-based spin labels as well as those of the newly developing Gd-based EPR techniques compounds of the type Gd-spacer-Gd and Gd-spacer-nitroxyl will be prepared. The spacer keeps the two spin labels at a well-defined, tunable distance. The compounds will be water soluble to allow EPR experiments under conditions identical to those when biomolecules are studied. Model compounds with Cu(II) and Mn(II) as spin labels are included as synthetic targets in our project. This enables a broader fundamental exploration of EPR spectroscopical techniques that are applied to paramagnetic metal ions interacting with a second paramagnetic metal ion of the same or different kind or with a nitroxyl radical, a situation met with spin-labeled metallo-proteins.