Identification of spin diffusion pathways in isotopically labeled biomolecules

One-dimensional NOE experiments applicable to labeled macromolecules are presented which allow the manipulation of specific spin diffusion pathways and thus unambiguously identify clandestine spins through which the direct NOE is mediated. A treatment of spin diffusion using average Liouvillian theory is shown to describe adequately these phenomena. Experiments are carried out on an 15N-labeled sample of human ubiquitin.