HSQC-based methyl group selection via gradients in mutidimensional NMR spectroscopy of proteins

A quadruple-quantum filtered HSQC (QQF-HSQC) for the selection of methyl groups which minimizes the line-broadening effects of proton homonuclear couplings is presented. The scheme uses gradients for n-quantum coherence selection and solvent suppression. In contrast to the heteronuclear quadruple-quantum coherence (HQQC) approach, the QQF-HSQC allows for long constant-time (CT) evolution, making use of the generally favorable relaxation properties of methyl groups. The increase in resolution and concomitant gain in sensitivity is discussed in theory and demonstrated in practice on the 14-kDa human nonpancreatic synovial phospholipase A2 (hnps-PLA2). The constant-time version is particularly useful for obtaining high-resolution spectra as demonstrated on hnps-PLA2. The applicability of the CT-QQF-HSQC module in multidimensional experiments is demonstrated using a 3D CT NOESY-QQF-HSQC spectrum of the 31-kDa homodimeric IIAMan domain of the mannose transporter of E. coli.