Synthesis and Characterization of Mechanical Interconnected Molecules Containing Persistent Nitroxide Radicals

The development of tools for the observation of the molecular motions and determination of nanometer scale distances between components is a crucial task for molecular machines based on rotaxanes. In this contest, electron paramagnetic resonance (EPR) has several advantages compared to other analytical methods, and its use to measure distances between paramagnetic centers is growing exponentially. For distances <20 nm information can be obtained from the measurement of dipole-dipole interaction strength or electron spin exchange between the two paramagnetic centers in conventional CW-EPR experiments. Pulsed EPR techniques like PELDOR (pulse electron double resonance) are useful for measuring distances from 15 to 80 nm. In both cases crystalline samples are not required. While spin labeling of biological macromolecules with two or more synthetic nitroxide radicals is a very active field, the introduction of paramagnetic probes in mechanical interconnected molecules like rotaxanes for EPR studies remains substantially unexplored.

In this lecture the synthesis and the characterization of new examples of spin labeled rotaxanes will be described . The nitroxide labels are present as terminal stopper units or connected with the wheel. In all cases the preparation of the rotaxanes was achieved by azide-alchine click chemistry which was shown to be compatible with the presence of persistent radicals.^1,2

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