Simulation of NMR data from oriented membrane proteins: practical information for experimental design

Several hundred solid state NMR dipolar couplings and chemical shift anisotropies were simulated for the polytopic membrane protein, bacteriorhodopsin, and for an idealized transmembrane peptide conforming to several different secondary structures (alpha- and 3(10)-helices and parallel and antiparallel beta-sheets), each at several tilt angles with respect to the bilayer normal. The use of macroscopically oriented samples was assumed. The results of these simulations suggest: (i) Because of the r-3 dependence of dipolar coupling, it is likely to prove difficult to successfully execute uniform isotopic enrichment strategies to generate large numbers of quantitatively interpretable structural measurements in oriented sample NMR studies of membrane proteins. (ii) There are a number of readily implementable specific isotopic labeling schemes which can yield data patterns sufficient to identify local secondary structure for transmembrane segments of idealized proteins which are tilted by < 10 degrees with respect to the bilayer normal. (iii) The measurement of dipolar coupling constants between 13C-, 19F-, and/or 3H-labeled side chains of proximal residues may prove effective as routes to long range tertiary structural data constraints.