PAIN with and without PAR: variants for third-spin assisted heteronuclear polarization transfer

Abstract

In this article, we describe third-spin assisted heteronuclear recoupling experiments, which play an increasingly important role in measuring long-range heteronuclear couplings, in particular N-15-C-13, in proteins. In the proton-assisted insensitive nuclei cross polarization (PAIN-CP) experiment (de PaA << pe et al. in J Chem Phys 134:095101, 2011), heteronuclear polarization transfer is always accompanied by homonuclear transfer of the proton-assisted recoupling (PAR) type. We present a phase-alternating experiment that promotes heteronuclear (e.g. N-15 -> C-13) polarization transfer while simultaneously minimizing homonuclear (e.g.C-13 -> C-13) transfer (PAIN without PAR). This minimization of homonuclear polarization transfer is based on the principle of the resonant second-order transfer (RESORT) recoupling scheme where the passive proton spins are irradiated by a phase-alternating sequence and the modulation frequency is matched to an integer multiple of the spinning frequency. The similarities and differences between the PAIN-CP and this het-RESORT experiment are discussed here.