Unraveling long range residual dipolar coupling networks in strongly aligned proteins

Long-range residual dipolar couplings (lrRDCs) have the potential to serve as powerful structural restraints in protein NMR spectroscopy as they can provide both distance and orientation information about nuclei separate in sequence but close in space. Current nonselective methods for their measurement are limited to moderate alignment strengths due to the sheer abundance of active couplings at stronger alignment. This limits the overall magnitude and therefore distance across which couplings can be measured. We have developed a double resonance technique for the inversion of individual coupled spin pairs, called Selective Inversion by Single Transition Cross Polarization (SIST-CP). This technique enables the selective recoupling of lrRDCs, thus allowing the complex multiplets occurring in strongly aligned systems to be disentangled. This technique is demonstrated in the context of an application to the measurement of (13)C'-(1)H(N) lrRDCs in strongly aligned proteins.