Quantitative Measurement of Relaxation Interference Effects between 1HN 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 1 H N CSA and 1 H N − 15 N dipolar interaction in uniformly 15 N-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 1 H− 15 N bond vector the values of (σ ∥ − σ ⊥ )(3 cos 2 (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.