Assessing the effects of time and spatial averaging in 15N chemical shift/15N-1H dipolar correlation solid state NMR experiments.

The effect of time and spatial averaging on (15)N chemical shift/(1)H-(15)N dipolar correlation spectra, i.e., PISEMA spectra, of alpha-helical membrane peptides and proteins is investigated. Three types of motion are considered: (a) Librational motion of the peptide planes in the alpha-helix; (b) rotation of the helix about its long axis; and (c) wobble of the helix about a nominal tilt angle. A 2ns molecular dynamics simulation of helix D of bacteriorhodopsin is used to determine the effect of librational motion on the spectral parameters. For the time averaging, the rotation and wobble of this same helix are modelled by assuming either Gaussian motion about the respective angles or a uniform distribution of a given width. For the spatial averaging, regions of possible (15)N chemical shift/(1)H-(15)N dipolar splittings are computed for a distribution of rotations and/or tilt angles of the helix. The computed spectra show that under certain motional modes the (15)N chemical shift/(1)H-(15)N dipolar pairs for each of the residues do not form patterns which mimic helical wheel patterns. As a result, the unambiguous identification of helix tilt and helix rotation without any resonance assignments or on the basis of a single assignment may be difficult.