Side chain assignments of Ile δ1 methyl groups in high molecular weight proteins:: An application to a 46 ns tumbling molecule

A sensitive 3D NMR pulse scheme, (H)C(CA)NH-COSY, is presented for the assignment of C-13(delta1) IIe chemical shifts in large perdeuterated, methyl-protonated proteins. The nonlinearity of branched amino acids, such as IIe, significantly degrades the quality of TOCSY schemes which transfer magnetization from methyl carbons to the backbone C-13(alpha) positions, and in applications to high molecular weight proteins (correlation times on the order of 40-50 ns), this compromises the sensitivity of spectra used for methyl assignment. The experiment presented utilizes COSY-based transfer steps and refocuses undesirable C-13-C-13 scalar couplings that degrade the efficiency of TOCSY transfers. The (H)C(CA)NH-COSY scheme is tested on an N-15,C-13,H-2-[Leu, Val, IIe (delta1 only)]-methyl-protonated maltose binding protein (MBP)/beta-cyclodextrin complex at 5degreesC (molecular tumbling time 46 +/- 2 ns), facilitating the assignment of C-13(delta1) chemical shifts for 18 of the 19 IIe residues for which backbone assignments were previously obtained. Both sensitivity and resolution of the resulting spectra are shown to be significantly better than those for a similar TOCSY-based approach.