Structural fitting of PISEMA spectra of aligned proteins.

An algorithm for fitting protein structures to PISEMA spectra is described, and its application to helical proteins in aligned samples is demonstrated using both simulated and experimental results. The formulation of the algorithm in terms of rotation operators yields compact recursion relations that provide a fast and effective way of obtaining peptide plane orientations from chemical and torsion angle constraints. The algorithm in combination with experimental solid-state NMR data results in a method for determining the backbone structures of proteins, since it yields the orientation of a helix as a whole, including its tilt and twist angles, and describes kinks and curves with atomic resolution. Although the algorithm can be applied in an "assignment-free" manner to spectra of uniformly labeled proteins, the precision of the structural fitting is improved by the addition of assignment information, for example the identification of resonances by residue type from spectra of selectively labeled proteins.