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-corr elation rate is obtained from the intensity ratio of an experiment in which the CSA/DD cross-correlation is active for a fixed time, tau, wi th a reference experiment in which it is inactive. The CSA/DD cross-co rrelation rates of 75 residues of the HU protein from Bacilus stearoth ermophilus were obtained from the linear fits of CSA/DD to reference r atios 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 val ues 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 me asurements. The data also show a dependence upon hydrogen bonding and secondary structure: residues in alpha-helical conformation show value s of 9 +/- 4 ppm, whereas residues in beta-sheet conformation show sub stantially higher values of 16 +/- 6 ppm.