Orientation of TOAC amino-acid spin labels in alpha-helices and beta-strands.

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.