A new method for selective excitation in biomol. NMR uses two-fold single-transition cross-polarization between protons and nitrogen-15 or carbon-13 nuclei. Switching the frequencies between the forward and backward transfer steps allows one to select a multiplet pattern that is assocd. with a single pair of spins in a medium-size protein. The efficiency of the transfer benefits from so-called TROSY line-narrowing effects which arise from interference between relaxation mechanisms. [on SciFinder (R)]