Mechanism of selective polarization exchange amongst chemically similar and distinct protons during weak rf irradiation at fast magic angle spinning

Band Selective Spectral Spin-Diffusion (BASS-SD) is a method to obtain selective ¹H-¹H contacts between chemically similar protons within a distance range of 5-6 Å in fully protonated proteins. BASS-SD combines low-amplitude proton spinlock radio frequency (rf) pulses with fast MAS frequency to enable selective polarization exchange in fully protonated molecules. The selectivity of transfer is dictated by the bandwidth of the spinlock pulse and has been used to observe selective H^N-H^N, H^α-Η^α and H^methyl-H^methyl correlations. These proton-proton spatial contacts are similar to those observed in perdeuterated samples and serve as useful structural restraints towards de novo protein structure determination. This study employs bimodal Floquet theory to derive the first- and second-order effective Hamiltonians necessary to understand the spin dynamics during BASS-SD. Analytical calculations combined with numerical simulations delineate two different mechanisms for polarization transfer amongst the proton spins. The BASS-SD recoupling condition has been reoptimized to observe selective correlations between chemically different protons (e.g., H^N-H^α) while retaining the spatial contacts between chemically similar protons (e.g., H^N-H^N). The new BASS-SD condition is integrated with simultaneous and sequential acquisition approaches to generate four different types of structural restraints (H^N-H^N, H^α-Η^α, H^N-H^α, H^α-H^N) in one experiment. The approach has been demonstrated on microcrystalline U-[¹³C,¹⁵N] labeled GB1 protein at ∼ 95-100 kHz MAS.