Heteronuclear J Coupling Precession during Spin-Lock and Adiabatic Pulses. Use of Adiabatic Inversion Pulses in High-Resolution NMR

Abstract

The J-coupled precession of I spins in an IS J-coupled system is scaled during S irradiation by the factor sin α where α is the tilt of the effective RF field applied to the S spins. This property is used in a spin-locked version of INEPT, dubbed SLINEPT, to transfer polarization from I to S in a mixed IS and IS2 system, both simultaneously and efficiently, as exemplified by N to H transfer in a uniformly 15N-enriched protein. Of more general use is the utilization of this property during adiabatic inversion pulses. In this context, it is shown both theoretically and experimentally that sech/tanh adiabatic inversion pulses may be applied during the entire J modulation delay period of any IS pulse sequence. As a result, the length of such adiabatic pulses is not of critical importance, and they may be used widely in high-resolution NMR: as selective-inversion pulses over 1 kHz or more, as very broadband inversion pulses over more than 100 kHz, or as selective-inversion pulses at any intermediate spectral width. It is also shown that other adiabatic modulation schemes such as sin/cos or constant/tan are not competitive with the sech/tanh method for these purposes.