Fig. 1. Theory of decoupling using selective inversion to distinguish leucine from valine.

a Trajectory of the ¹³CH₃ leucine (red) and valine (blue, dashed) magnetization during a constant time encoding period of duration T = 1/¹J_CC = 28 ms. Selective decoupling of valine at time T/2 refocuses the coupling and produces positive peaks for valine at time T. Leucine is not decoupled, and the coupling interaction leads to negative peaks at time T. b The distributions of C^β and C^γ chemical shifts of valine (blue) and leucine (red) are distinct. Assuming an initial state of I_z, our decoupling pulse selectively inverts the C^β_Val, which selectively decouples the valine methyl resonance. The encoding of the methyl-carbon chemical shift continues unaffected during the selective homonuclear decoupling pulse.