A topographically and conformationally constrained, spin-labeled, α-amino acid: Crystallographic characterization in peptides

Department of Chemistry, Institute of Biomolecular Chemistry, CNR, University of Padova, 35131 Padova, Italy.
European Journal of Allergy and Clinical Immunology (Impact Factor: 1.3). 07/2005; 65(6):564-79. DOI: 10.1111/j.1399-3011.2005.00258.x
Source: PubMed


2,2,6,6-Tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) is a topographically and conformationally restricted, nitroxide containing, C(alpha)-tetrasubstituted alpha-amino acid. Here, we describe the molecular and crystal structures, as determined by X-ray diffraction analyses, of a TOAC terminally protected derivative, the cyclic dipeptide c(TOAC)(2).1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) solvate, and five TOAC-containing, terminally protected, linear peptides ranging in length from tetra- to hepta-peptides. Incipient and fully developed, regular or distorted 3(10)-helical structures are formed by the linear peptides. A detailed discussion on the average geometry and preferred conformation for the TOAC piperidine ring is also reported. The X-ray diffraction structure of an intramolecularly cyclized side product resulting from a C-activated TOAC residue has also been determined.

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    • "position (D'Amore et al. 2003). In addition, the crystallographic characterization of some TOAC derivatives showed the piperidine ring in an approximate chair conformation, with the αamino group in the axial position and the α-carboxyl group in the equatorial position (Flippen-Anderson et al. 1996; Crisma et al. 2005). The nitroxyl group presents two absorption bands in the UV-visible region, ascribed to the n → π* (ʎ0420–450 nm, ɛ05–20 L mol −1 cm "
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