A novel triple-resonance NMR method is presented for the measurement of the protein backbone dihedral angle phi based on differential multiple-quantum relaxation induced by relaxation interference between 1Halpha(i)-13Calpha(i) dipolar and 13C'(i - 1) (carbonyl) chemical shift anisotropy mechanisms. The method employs a simultaneous transfer of 15N magnetization to the inter- and intra-residue 13Calpha carbons as well as the directly attached carbonyl carbon 13C'. Results obtained on 13C,15N-labeled ubiquitin demonstrate the potential of the method.