Carbon‐13 Centerband‐Only Detection of EXchange with Dynamic Nuclear Polarization

The magic‐angle spinning NMR technique, Centerband‐Only Detection of EXchange (CODEX), can be used to determine the oligomerization state of molecules when combined with site‐specific labeling. Calibrated with amino acid crystals, the method is successfully applied to proteins, primarily combined with 19 F labeling. The use of 13 C spins for CODEX‐based oligomer determination in proteins is hampered by limited sensitivity of 13 C spins due to the low gyromagnetic ratio of 13 C and the presence of natural abundance background spins which contribute to the observed CODEX decay. The use of CODEX is proposed in conjunction with dynamic nuclear polarization (DNP) at low temperature to increase sensitivity. It is necessary to correct for effects of 13 C present at natural abundance. A (PDSD) proton driven spin diffusion‐based correction is demonstrated to be effective when the isotropic chemical shifts of the natural abundance background are distinct from the labeled site. Using a 13 C‐ ζ ‐phenylalanine‐labeled GB1 sample, it is demonstrated that the autocorrelation peak decay observed in a series of PDSD spectra can be utilized to correct for the additional dephasing and recover the expected CODEX decay curve. With 13 C‐ γ ‐phenylalanine labeling and 13 C‐depleted background, mixing times up to 1500 s are demonstrated.