Magic-Angle Spinning Frequencies beyond 300 kHz Are Necessary to Yield Maximum Sensitivity in Selectively Methyl Protonated Protein Samples in Solid-State NMR

In the last decade, proton detection in magic-angle spinning (MAS) solid-state NMR became a popular strategy for biomolecular structure determination. In particular, probe technology has experienced tremendous progress with smaller and smaller diameter rotors achieving ever higher MAS frequencies. MAS rotation frequencies beyond 100 kHz allow to observe and assign protons in fully protonated samples. In these experiments, resolution is however compromised as homogeneous proton-proton dipolar coupling interactions are not completely averaged out. Using a combination of experiments and simulations, we analyze the MAS frequency-dependent intensities of the ¹H,¹³C methyl correlation peaks of a selectively methyl protonated (CH₃) microcrystalline sample of the chicken α-spectrin SH3 domain (α-SH3). Extensive simulations involving nine spins employing the program SIMPSON allow to predict the MAS frequency dependence of the proton intensities. The experimental results are used to validate the simulations. As quantitative measure, we determine the characteristic MAS frequency, which is necessary to obtain >50% of the maximum achievable sensitivity. Our results show that this frequency is site-specific and strongly depends on the local methyl density. We find that the characteristic MAS frequency ranges from as low as 20 kHz up to 324 kHz with the average value of 135 ± 88 kHz for this particular sample at a magnetic field strength of 11.7 T. Inclusion of side chain dynamics in the analysis reduces the average characteristic MAS frequency to 104 ± 68 kHz within the range of 11-261 kHz. In case, >80% of the maximum sensitivity shall be achieved, MAS rotation frequencies of 498 ± 370 and 310 ± 227 kHz are required with and without including side chains dynamics in the analysis, respectively.