Site-directed parallel spin-labeling and paramagnetic relaxation enhancement in structure determination of membrane proteins by solution NMR spectroscopy

A method of parallel spin-labeling with paramagnetic and diamagnetic labels is introduced and it shows that distances in the range 15-24 A can be readily determined. The protein was labeled at 11 water-exposed and lipid-covered sites, and 320 paramagnetic relaxation enhancement (PRE) distance restraints were measured and the addition of these restraints resulted in significant improvement of the calculated backbone structure of outer membrane protein A (OmpA).