title: Orientation of TOAC amino-acid spin labels in alpha-helices and beta-strands.
abstract: The orientation of alpha-helices or beta-strands, e.g., in membranes, can be determined from EPR order parameters of (2,2,6,6-tetramethyl-piperidine-1-oxy-4-amino-4-carboxylic acid) TOAC amino-acid spin labels incorporated in the polypeptide backbone. This requires knowledge of the inclination of the nitroxide axes, relative to the alpha-helix or beta-strand axis. Crystal structures of TOAC-containing peptides are used to derive the spin-label orientation relative to refined alpha-poly-l-alanine and beta-poly-l-alanine structures. The spin-label z-axes of the two mirror-image TOAC twist-boat conformers are inclined at 13+/-2 degrees and 65+/-3 degrees , respectively, to the alpha-helix axis, or at 25+/-3 degrees and 32+/-3 degrees to the beta-strand axis.
authors.affiliation: Max-Planck-Institut für biophysikalische Chemie, Abt. Spektroskopie, 37070 Göttingen, Germany. dmarsh@gwd.de
authors.forename: Derek
authors.initials: D
authors.lastname: Marsh
mesh_terms: D000409:Alanine / Q000737:chemistry*; D003497:Cyclic N-Oxides / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D008051:Lipid Bilayers / Q000737:chemistry*; D008958:Models, Molecular; D010063:Ovum; D011487:Protein Conformation; D013113:Spin Labels
chemical_list: D003497:Cyclic N-Oxides; D008051:Lipid Bilayers; D013113:Spin Labels; D000409:Alanine