Two major problems are present in NMR of larger proteins: Overlapping resonances and increased linewidths. For this reason, isotopic labeling and 3D NMR experiments have been succesfully introduced to overcome such difficulties. Basically, the incorporation of selective ¹³C-labeling in proteins affords the possibility to expand the suite of NMR experiment to be applied. In addition to the conventional experiments described for unlabeled proteins, some new NMR experiments can be applied taking advantage from the improved sensitivity for the ¹³C nucleus:

• Heteronuclear 2D ¹H-¹³C correlation experiments using proton detection as, for instance, HSQC and HMQC experiments. Usually, constant-time versions are preferable to avoid carbon-carbon coupling constants.
• Protein dynamics can be studied from ¹³C relaxation rates and from ¹H{¹³C} NOE data extracted from closely related HMQC and HSQC pulse trains.
• Other inverse 2D and 3D NMR experiments such as, for instance, ¹H-¹³C TOCSY-HSQC or ¹H-¹³C NOESY-HSQC experiments.

Use of several approaches have been described that use selective ¹³C-labeling at specific sites to improve structure determination and resonance assignments. selective ¹³C labeling on the e position of phenyalalanine residues ( 99JACS1611 and 02JB231-24 ). This approach eliminates the presence of 13C-13C coupling constants and reduce 1H line widths and spectral overlapping.