A 1H paramagnetic relaxation study on the interaction of peptides with aminoxyl spin labels in apolar environments

The use of stable free radicals such as aminoxyl spin-labels has been recently exploited for obtaining information on solution structures of peptides (1,2) and proteins (3,4) from multidimensional NMR spectra. In those reports, it has been suggested that an efficient spin-labelling of water and dimethylsulphoxide (DMSO) solvents may be achieved and the paramagnetic solvent yields relaxation effects on backbone proton nuclei which are directly driven by the molecular surface accessibility. Upon the addition of the paramagnetic probe, the observation of absence in chemical shifts changes and of nuclear relaxation effects which are fully rationalised in terms of the solution structure, support a dynamic model for the interaction between the chemical probe and the biomolecule where weak collisional adducts favour the dipolar coupling of the unpaired electron with the outer nuclei of the investigated molecule.