Measurement of long-range cross-correlation rates using a combination of single- and multiple-quantum NMR spectroscopy in one experiment

A method is described to determine long-range cross-correlations between the modulations of an anisotropic chemical shift (e.g., of a C′ carbonyl carbon in a protein) and the fluctuations of a weak long-range dipolar interaction (e.g., in cross-correlation between the same C′ carbonyl and the H^N proton of the neighboring amide group). Such long-range correlations are difficult to measure because the corresponding long-range scalar couplings are so small that Redfield's secular approximation is often violated. The method, which combines features of single- and double-quantum NMR spectroscopy, allows one to cancel the effects of dominant short-range dipolar interactions (e.g., between the CSA of the amide nitrogen N and the dipolar coupling to its attached proton H^N) and is designed so that the secular approximation is rescued even if the scalar coupling between the long-range dipolar coupling partners is very small. The cross-correlation rates thus determined in ubiquitin cover a wide range because of local motions and variations of the CSA tensors.