Orientation of TOAC amino-acid spin labels in α-helices and β-strands

The orientation of α-helices or β-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 α-helix or β-strand axis. Crystal structures of TOAC-containing peptides are used to derive the spin-label orientation relative to refined α-poly-l-alanine and β-poly-l-alanine structures. The spin-label z-axes of the two mirror-image TOAC twist-boat conformers are inclined at 13 ± 2° and 65 ± 3°, respectively, to the α-helix axis, or at 25 ± 3° and 32 ± 3° to the β-strand axis.