Resonance assignments and structure determination of large and challenging proteins

NMR has become a method of choice for determining structures of proteins in near-physiological conditions, in particular for dynamic systems. However, in large proteins, line-broadening and spectral overlap often preclude resonance assignment of the many correlation maps needed for structural studies. Various well-described spectroscopic techniques and specific isotopic labeling strategies offer relief from these problems, but it is not always clear which samples should be employed to overcome a particular challenge, nor with which pulse sequences they should be used. Here, we review the challenges associated with assigning large proteins, discuss the many labeling schemes employed to overcome hurdles, and describe associated pulse sequences with focus on both practical concerns and the information content each sequence supplies.