A simple method to measure ¹³CH₂ heteronuclear dipolar cross-correlation spectral densities

Here, we report a method to simultaneously determine CH₂ cross-correlation spectral densities and T₁ relaxation times in the laboratory and rotating frames. To accomplish this, we have employed an indirect approach that is based on measurement of differences in relaxation rates acquired with and without cross-correlation terms. The new method, which can be employed using multidimensional NMR and standard relaxation pulse sequences, is validated experimentally by investigation of a selectively ¹³C-enriched hexadecapeptide and the uniformly ¹³C-enriched immunoglobulin-binding domain of streptococcal protein G (GB1). Use of this approach makes determination of CH₂ cross-correlation spectral densities in uniformly ¹³C-enriched proteins now routine and provides novel information concerning their internal motions.