Quantitative measurement of relaxation interference effects between 1H(N) CSA and 1H-15N dipolar interaction: Correlation with secondary structure

An experiment is presented that allows the quantitative measurement of the cross-correlation rate between 1H(N) CSA and 1H(N)-15N dipolar interaction in uniformly 15N-enriched samples. The CSA/DD cross-correlation rate is obtained from the intensity ratio of an experiment in which the CSA/DD cross-correlation is active for a fixed time, τ, with a reference experiment in which it is inactive. The CSA/DD cross-correlation rates of 75 residues of the HU protein from Bacillus stearothermophilus were obtained from the linear fits of CSA/DD to reference ratios recorded for five values of τ and at two different B(o) fields. After correction for the mobility of the 1H-15N bond vector the values of (σ(||) - σ(perpendiular to))(3 cos2(θ) - 1)/2, containing information about the chemical shielding anisotropy, were derived for individual amide protons. The average value of 13 ± 5 ppm compares well with the results from previous solid state NMR measurements. The data also show a dependence upon hydrogen bonding and secondary structure: residues in α-helical conformation show values of 9 ± 4 ppm, whereas residues in β-sheet conformation show substantially higher values of 16 ± 6 ppm.