CODEX and its slow-down under fast MAS

Dipolar spin exchange in CODEX and its slow-down under fast MAS 

In 13C-enriched proteins, deAzevedo et al.25 have circumvented the l3C spin-diffusion problem by performing a 15N CODEX experiment followed by l5N-l3C transfer and 13C detection. The dipolar couplings between the 15N nuclei, which are separated by three bonds and have smaller gyromagnetic ratios, are more than an order of magnitude reduced relative to those of 13C in a uniformly labeled protein. 

A second dipolar effect in CODEX is the relaxation-induced dipolar exchange due to dipolar couplings to heteronuclei,82 83 in particular 14N and 15N, which undergo significant longitudinal relaxation during tm. The relaxation removes a heteronuclear coherence term from the density operator and thus reduces the stimulated echo. This effect is described in detail in ref. 83. 

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