 Therefore, multidimensional (2, 3 or 4D) experiments have been devised to deal with this problem.To facilitate these experiments, it is desirable to isotopically label the protein with 13C and 15N because the predominant naturally occurring isotope 12C is not NMR-active and the nuclear quadrupole moment of the predominant naturally occurring 14N isotope prevents high resolution information from being obtained from this nitrogen isotope.The most important method used for structure determination of proteins utilizes NOE experiments to measure distances between atoms within the molecule.