Estimating cross-relaxation rates between methyl and neighboring labile proton spins in high molecular weight proteins

We show that water saturation leads to deleterious losses in sensitivity of methyl signals in selectively methyl-[¹³CH₃]-labeled protein samples of high molecular weight proteins dissolved in H₂O. These losses arise from efficient cross-relaxation between methyl protons and proximal labile protons in the protein structure. A phenomenological model for analysis of methyl intensity decay profiles that involves exchange saturation transfer of magnetization from localized proton spins of water to various labile groups in the protein structure that, in turn, efficiently cross-relax with protons of methyl groups, is described. Analysis of methyl intensity decay profiles with this model allows cross-relaxation rates (σ) between methyl and labile protons to be determined and permits identification of methyl sites in close proximity to labile groups in the protein structure.