1H‐Azepine‐4‐amino‐4‐carboxylic Acid: A New α,α‐Disubstituted Ornithine Analogue Capable of Inducing Helix Conformations in Short Ala‐Aib Pentapeptides

Abstract

A very efficient synthesis of orthogonally protected 1H-azepine-4-amino-4-carboxylic acid, abbreviated as Azn, a conformationally restricted analogue of ornithine, was realized. It was obtained on a gram scale in good overall yield in five steps, three of which did not require isolation of the intermediates, starting from the readily available 1-amino-4-oxo-cyclohexane-4-carboxylic acid. Both enantiomers were used for the preparation of pentapeptide models containing Ala, Aib, and Azn. Conformational studies using both spectroscopic techniques (NMR, CD) and molecular dynamics on model 5-mer peptides showed that the (R)-Azn isomer possesses a marked helicogenic effect.

Mechanism of Stabilization of Helix Secondary Structure by Constrained Cα-Tetrasubstituted α-Amino Acids

“…The average population of helical geometries (pop h% ) and the DSSP helical content (h % ), as well as φ and ψ dihedrals of the most representative structures obtained from cluster analysis (Table S1, S.I. ), show that the considered peptides fold into a right-handed 3 10 the presence of a i+4 → i H-bond, typical of α-helices, although with limited occupancies (7.11%, 11.2% and 6.82%, respectively). Surprisingly, the behavior of the norbornane 10 (1S,2R,4R)-IV is significantly different from that of the highly related norbornene (1R,2R,4R)-V and a possible reason will be given later.…”

“…10 However, to assess if the relative helical propensity of CTAAs is influenced by Aib, we also simulated the behavior of an L-Ala pentapeptide substituted at the central position 3 by (R)-I or (1R,2R,4R)-V, selected as representatives of the least and the most performing CTAAs, respectively. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 a Calculated as the sum of 3 10 -and α-helix content of CTAA, averaged with respect to the 25-50, 50-75, 75-100 ns time intervals.…”

“…Other peculiarities in the QTAIM analyses performed on the right-handed 3 10 showed only the typical BCP network of helical secondary structures, with a ∑ ߩ ሺ‫ݎ‬ ሻ = 0.1002 a.u.. Conversely, in peptide 6 we noticed the presence of an additional intra-residue C-H···O BCP, involving the methylene C7-H and the backbone carbonyl group of (1R,2R,4R)-V, with a ߩሺ‫ݎ‬ ሻ = 0.0135 a.u., which is in the range of strong H-bonds ( Figure 8 and Table S8). …”

“…The simulation protocol was previously optimized to reproduce the X-ray conformation of Boc-L-Ala-Aib-L-Ala-Aib-AibOMe 66 and further validated through the reproduction of both NMR and X-ray geometries of related CTAA containing pentapeptides. 10,59 It is known that several polypeptide force fields are affected by a certain degree of helical propensity. [67][68][69][70] Thus, we preliminarily evaluated the ability of different force field and solvent model combinations to reproduce the folding of a selection of helical, turn-like and random coil peptides, made by 5-15 residues, and ff99SB coupled with the implicit solvent model by Onufriev, Bashford, and Case (igb = 5).…”

“…Superimposed right-handed 310 -helices of peptides 3 ((1R,2R,4R,)-IIIa; pink) and Ala-(1R,2S,4R)-IIIbwr-Ala-Aib-Ala (ochre). B. Superimposed right-handed 3 10 -helices of peptides 11 ((1R,2S,4R)-IIIb; orange) and Ala-(1R,2S,4R)-Vbdm-Ala-Aib-Ala (purple).…”