Orientation of TOAC amino-acid spin labels in a-helices and b-strands

The orientation of a-helices or b-strands, e.g., in membranes, can be determined from EPR order parameters of (2,2,6,6-tetramethylpiperidine-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 a-helix or b-strand axis. Crystal structures of TOAC-containing peptides are used to derive the spin-label orientation relative to refined a-poly-L-alanine and b-poly-L-alanine structures. The spin-label zaxes of the two mirror-image TOAC twist-boat conformers are inclined at 13 ± 2 and 65 ± 3 , respectively, to the a-helix axis, or at 25 ± 3 and 32 ± 3 to the b-strand axis. 2006 Elsevier Inc. All rights reserved.