Synthesis of Enantiomerically Pure cis‐ and trans‐4‐Amino‐1‐oxyl‐2,2,6,6‐tetramethylpiperidine‐3‐carboxylic Acid: A Spin‐Labelled, Cyclic, Chiral β‐Amino Acid, and 3D‐Structural Analysis of a Doubly Spin‐Labelled β‐Hexapeptide

Abstract

Amination of 3‐carboxymethyl‐1‐oxyl‐2,2,6,6‐tetramethyl‐4‐piperidone (1) with either (R)‐ or (S)‐α‐methylbenzylamine gave corresponding enamines 2. Whereas the reduction with NaBH3CN/CH3COOH afforded predominantly a mixture of two possible cis diastereomers of 3, (1′R,3S,4S)/(1′R,3R,4R) or (1′S,3R,4R)/(1′S,3S,4S), which could be separated by crystallisation of their HCl salts, the use of NaBH4/(CH3)2CHCOOH as the reducing agent resulted in a mixture of one trans‐ and one cis diastereomer of 3 (1′R,3S,4R)/(1′R,3R,4R) or (1′S,3R,4S)/(1′S,3S,4S) in varying proportions depending upon the conditions used. The stereochemistry of the four diastereomers of 3 was clearly established by X‐ray diffraction analysis of one of them, combined with 1H NMR spectroscopic studies after nitroxide reduction. Removal of the chiral auxiliary from the separated diastereomers of 3 by hydrogenation and regeneration of the nitroxide radical gave expected amino esters 4. A model β‐hexapeptide containing (3R,4S)‐β‐TOAC combined with (1S,2S)‐2‐aminocyclohexane carboxylic acid was synthesised by solution methods and its preferred conformation (314‐helix) was assessed by FTIR absorption, CD, and EPR spectroscopy.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis and Conformational Characterisation of Hexameric β‐Peptide Foldamers by Using Double POAC Spin Labelling and cw‐EPR

“…Recently, DeGrado and co‐workers29 showed that the amide I′ spectrum of a helical β‐peptide in D 2 O is quite complex because it is characterised by three bands centred at $\tilde \nu $ =1612, 1624, and 1650 cm −1 . Finally, our ACHC/β‐TOAC model hexapeptide mentioned above12 shows a main band at $\tilde \nu $ =1654 cm −1 accompanied by weak shoulders at $\tilde \nu $ =1624 and 1682 cm −1 in CDCl 3 . All of these β‐peptides are folded in the 14‐helix structure.…”

“…[4][5][6][7] We have explored the extension of this approach to bpeptide foldamers. [11] A model b-hexapeptide, containing two (3R,4S)-b-TOAC residues [12] combined with four (1S,2S)-2-aminocyclohexane carboxylic acid (ACHC) residues (Scheme 1), was synthesised and the preferred conformation (i.e., 3 14 -helix) [1,2] was assessed in particular by continuous-wave (cw) EPR spectroscopic analysis. [12] Herein, we have expanded this area by utilising the enantiopure five-membered ring (3R,4R)-4-amino-1-oxyl-2,2,5,5-tetramethylpyrrolidine-3-carboxylic acid (POAC).…”

“…[11] A model b-hexapeptide, containing two (3R,4S)-b-TOAC residues [12] combined with four (1S,2S)-2-aminocyclohexane carboxylic acid (ACHC) residues (Scheme 1), was synthesised and the preferred conformation (i.e., 3 14 -helix) [1,2] was assessed in particular by continuous-wave (cw) EPR spectroscopic analysis. [12] Herein, we have expanded this area by utilising the enantiopure five-membered ring (3R,4R)-4-amino-1-oxyl-2,2,5,5-tetramethylpyrrolidine-3-carboxylic acid (POAC). [13][14][15][16] This b-amino acid has a backbone chemical structure identical to that of (1S,2S)-2-aminocyclopentane carboxylic acid (ACPC), the homo-oligo(b-peptide)s of which [17] are folded in the 2.6 12 -(also termed 12-) helical conformation in the crystal state and in secondary-structure supporting solvents.…”

Semi‐Rigid Nitroxide Spin Label for Long‐Range EPR Distance Measurements of Lipid Bilayer Embedded β‐Peptides

“…β‐TOAC was designed as a spin label with reduced mobility, as required for the investigation of β‐peptide structures in solution . Nevertheless, similar to the α‐TOAC the β‐analogue, is likely to influence the natural secondary structure formation . Therefore, we report the synthesis of the conformationally semi‐rigid 4‐(3,3,5,5‐tetramethyl‐2,6‐dioxo‐4‐oxylpiperazin‐1‐yl)‐ d ‐β 3 ‐homophenylglycine (β 3 ‐hTOPP) spin label for β‐peptides.…”

Peptide–Membrane Interactions by Spin-Labeling EPR

“…Balog et al, 2003; M. R. Balog et al, 2004; Kalai, Schindler, Balog, Fogassy, & Hideg, 2008; Tominaga et al, 2001; Wright et al, 2007). The most commonly used one is 2,2,6,6-tetramethyl-N-oxyl-4-amino-4-carboxylic acid (TOAC, Figure 1, 8 ) (Rassat & Rey, 1967; Schreier, Bozelli, Marín, Vieira, & Nakaie, 2012; A.…”