Observing in-phase single-quantum ¹⁵N multiplets for NH₂ / NH₃⁺ groups with two-dimensional heteronuclear correlation spectroscopy

Two-dimensional (2D) F1-¹H-coupled HSQC experiments provide 3:1:1:3 and 1:0:1 multiplets for AX₃ and AX₂ spin systems, respectively. These multiplets occur because, in addition to the 2 S_y H_z ^a → 2 S_y H_z ^a process, the coherence transfers such as 2 S_y H_z ^a → 2 S_y H_z ^b occurring in t₁ period provide detectable magnetization during the t₂ period. Here, we present a 2D F1-¹H-coupled ¹H-¹⁵N heteronuclear correlation experiment that provides a 1:3:3:1 quartet for AX₃ spin system and a 1:2:1 triplet for AX₂. The experiment is a derivative of 2D HISQC experiment [J. Iwahara, Y.S. Jung, G.M. Clore, Heteronuclear NMR spectroscopy for lysine NH₃ groups in proteins: unique effect of water exchange on ¹⁵N transverse relaxation. J. Am. Chem. Soc. 129 (2007) 2971-2980] and contains a scheme that kills anti-phase single-quantum terms generated in the t₁ period. The purge scheme is essential to observe in-phase single-quantum multiplets. Applications to the NH₂ and NH₃ ⁺ groups in proteins are demonstrated.