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

An experiment is presented that allows the quantitative measurement of the cross-correlation rate between H-1(N) CSA and H-1(N)-N-15 dipolar interaction in uniformly N-15-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, tau, with a reference experiment in which it is inactive. The CSA/DD cross-correlation rates of 75 residues of the HU protein from Bacilus stearothermophilus were obtained from the linear fits of CSA/DD to reference ratios recorded for five values of tau and at two different B-o fields. After correction for the mobility of the H-1-N-15 bond vector the values of (sigma(parallel to) -sigma(perpendicular to))(3 cos(2)(theta) -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 alpha-helical conformation show values of 9 +/- 4 ppm, whereas residues in beta-sheet conformation show substantially higher values of 16 +/- 6 ppm.