The presence of a paramagnetic center in a protein alter the NMR parameters of nuclei to different extents depending on their nature, proximity and bond connectivities with the paramagnetic center ( 99COP145 , 01METH314 , 02PROG249 , 02CONC259 ). Thus, Insertion of paramagnetic spin labels at specific sites allows the exploration of molecular structure at long distance by monitoring: Chemical shift changes ( 98JACS10992 ) Measuring residual dipolar couplings ( 95PNAS9279 and 98JMR256-135 ) owing to the inherent molecular alignment NMR Experiments NMR Constraints Applications