Estimates of methyl 13C and 1H CSA values (Δσ) in proteins from cross-correlated spin relaxation

Simple pulse schemes are presented for the measurement of methyl C-13 and H-1 CSA values from H-1 - C-13 dipole/C-13 CSA and H-1 - C-13 dipole/H-1 CSA cross-correlated relaxation. The methodology is applied to protein L and malate synthase G. Average C-13 CSA values are considerably smaller for Ile than Leu/Val ( 17 vs. 25 ppm) and are in good agreement with previous solid state NMR studies of powders of amino acids and dipeptides and in reasonable agreement with quantum-chemical DFT calculations of methyl carbon CSA values in peptide fragments. Small averaged H-1 CSA values on the order of 1 ppm are measured, consistent with a solid state NMR determination of the methyl group H-1 CSA in dimethylmalonic acid.