Measurement of residual dipolar couplings in methyl groups via carbon detection

Residual dipolar couplings (RDCs) provide both structural and dynamical information useful in the characterization of biological macromolecules. While most data come from the interaction of simple pairs of directly bonded spin-1/2 nuclei (¹H-¹⁵N, ¹H-¹³C, ¹H-¹H), it is possible to acquire data from interactions among the multiple spins of ¹³C-labeled methyl groups (¹H₃-¹³C). This is especially important because of the advantages that observation of ¹³C-labeled methyl groups offers in working with very large molecules. Here we consider some of the options for measurement of methyl RDCs in large and often fully protonated proteins and arrive at a pulse sequence that exploits both J-modulation and direct detection of ¹³C. Its utility is illustrated by application to a fully protonated two domain fragment from the mammalian glycoprotein, Robo1, ¹³C-methyl-labeled in all valines.