Finite-pulse radio frequency driven recoupling with phase cycling for 2D (1)H/(1)H correlation at ultrafast MAS frequencies.

The first-order recoupling sequence radio frequency driven dipolar recoupling (RFDR) is commonly used in single-quantum/single-quantum homonuclear correlation 2D experiments under magic angle spinning (MAS) to determine homonuclear proximities. From previously reported analysis of the use of XY-based super-cycling schemes to enhance the efficiency of the finite-pulse-RFDR (fp-RFDR) pulse sequence, XY8(1)4 phase cycling was found to provide the optimum performance for 2D correlation experiments on low-γ nuclei. In this study, we analyze the efficiency of different phase cycling schemes for proton-based fp-RFDR experiments. We demonstrate the advantages of using a short phase cycle, XY4, and its super-cycle XY4(1)4 that only recouples the zero-quantum homonuclear dipolar coupling, for the fp-RFDR sequence in 2D (1)H/(1)H correlation experiments at ultrafast MAS frequencies. The dipolar recoupling efficiencies of XY4, XY4(1)4 and XY8(1)4 phase cycling schemes are compared based on results obtained from 2D (1)H/(1)H correlation experiments, utilizing the fp-RFDR pulse sequence, on powder samples of U-(13)C,(15)N-l-alanine, N-acetyl-(15)N-l-valyl-(15)N-l-leucine, and glycine. Experimental results and spin dynamics simulations show that XY4(1)4 performs the best when a high RF power is used for the 180° pulse, whereas XY4 renders the best performance when a low RF power is used. The effects of RF field inhomogeneity and chemical shift offsets are also examined. Overall, our results suggest that a combination of fp-RFDR-XY4(1)4 employed in the recycle delay with a large RF-field to decrease the recycle delay, and fp-RFDR-XY4 in the mixing period with a moderate RF-field, is a robust and efficient method for 2D single-quantum/single-quantum (1)H/(1)H correlation experiments at ultrafast MAS frequencies.