Mixed-time parallel evolution in multiple quantum NMR experiments: Sensitivity and resolution enhancement in heteronuclear NMR

A new strategy is demonstrated that simultaneously enhances sensitivity and resolution in three- or higher-dimensional heteronuclear multiple quantum NMR experiments. The approach, referred to as mixed-time parallel evolution (MT-PARE), utilizes evolution of chemical shifts of the spins participating in the multiple quantum coherence in parallel, thereby reducing signal losses relative to sequential evolution. The signal in a given PARE dimension, t₁, is of a non-decaying constant-time nature for a duration that depends on the length of t₂, and vice versa, prior to the onset of conventional exponential decay. Line shape simulations for the ¹H- ¹⁵N PARE indicate that this strategy significantly enhances both sensitivity and resolution in the indirect ¹H dimension, and that the unusual signal decay profile results in acceptable line shapes. Incorporation of the MT-PARE approach into a 3D HMQC-NOESY experiment for measurement of H^N-H^N NOEs in KcsA in SDS micelles at 50°C was found to increase the experimental sensitivity by a factor of 1.7±0.3 with a concomitant resolution increase in the indirectly detected ¹H dimension. The method is also demonstrated for a situation in which homonuclear ¹³C- ¹³C decoupling is required while measuring weak H3'-2'OH NOEs in an RNA oligomer.