ORIENTATIONS OF HELICAL PEPTIDES IN MEMBRANE BILAYERS BY SOLID-STATE NMR-SPECTROSCOPY

The orientations of helical peptides in membrane bilayers provide impo rtant structural information that is directly relevant to their functi onal roles, both alone and within the context of larger membrane prote ins. The orientations can be readily determined with solid state NMR e xperiments on samples of N-15-labeled peptides in lipid bilayers align ed between glass plates. The observed N-15 chemical shift frequencies can be directly interpreted to indicate whether the peptide's helix ax is has a trans-membrane or an in-plane orientation. In order to distin guish between these possibilities on the basis of a single spectral pa rameter, e.g. the easily measured N-15 chemical shift frequency, it is necessary to demonstrate that the secondary structure of the peptide is helical, generally by solution NMR spectroscopy of the same peptide in micelle samples, and that it is immobile in bilayers, generally fr om solid state NMR spectra of unoriented samples. Six different 20-30 residue peptides are shown to have orientations that fall into the cat egories of trans-membrane or in-plane helices. A model hydrophobic pep tide was found to be trans-membrane, several different amphipathic hel ical peptides were found to have either trans-membrane or in-plane ori entations, and a leader or signal peptide, generally regarded as hydro phobic, was found to have a significant population with an in-plane or ientation.