Self-assembling molecular cages containing biradicals for increased efficiency of DNP/solid-state NMR for membrane proteins


In order to increase the inherently low signal intensity of solid-state NMR spectroscopy the technique has been combined with Dynamic Nuclear Polarization (DNP) where the samples are doped with free radicals, the unpaired electrons polarized by powerful microwave irradiation and the electron polarization transferred to the nuclei. With the development of specific bi-radicals for DNP/solid-state NMR applications impressive enhancements close to the theoretical limit have been obtained for model compounds. In contrast, the experimental DNP enhancements in real samples lack much behind due to the in-homogenous distribution of free radicals and interactions with other molecules.

In particular for membrane samples and/or in the absence of a glassy matrix aggregation and surface accumulation of the biradicals limit DNP efficiency, and in such systems a potential to increase the DNP/NMR signal intensity by 1-2 orders remains.

In order to bridge this gap, we propose to develop and test molecular cages for the biradicals in the form of either self-assembling transmembrane peptide oligomers or membrane-anchored cyclodextrins. Such systems will disperse better the bi-radicals in a favorable local membrane environment and at the same time separate the paramagnetic radicals from the nuclei of interest. Thereby such caged bi-radicals exhibit improved relaxation properties of electrons and nuclei and at the same time the nuclear spins are kept at a favorable distance from the paramagnetic centers. The gain in DNP enhancement promises to much boost the structural investigation of membrane proteins by solid-state NMR.