Crystallographic characterization of conformation of 1-aminocyclopropane-1-carboxylic acid residue (Ac3c) in simple derivatives and peptides.
The molecular and crystal structures of the C alpha,alpha-dialkylated alpha-amino acid residue 1-aminocyclopropane-1-carboxylic acid hemihydrate (H2+-Ac3c-O-.1/2 H2O) and nine derivatives and dipeptides have been determined by X-ray diffraction. The derivatives are pBrBz-Ac3c-OH, Piv-Ac3c-OH, Z-Ac3c-OH, the alpha-and beta-forms of t-Boc-Ac3c-OH, Z-Ac3c-OMe, and the 5(4H)-oxazolone from pBrBz-Ac3c-OH; the dipeptides are H-(Ac3c)-OMe and c(Ac3c)2. The values determined for the torsion angles about the N-C alpha (phi) and C alpha-C' (psi) bonds for the single Ac3c residue of Piv-Ac3c-OH, the alpha- and beta-forms of t-Boc-Ac3-OH and Z-Ac3c-OMe, and the C-terminal Ac3c residue of H-(Ac3c)2-OMe correspond to folded conformations in the "bridge" region of the Ramachandran map. The structures of pBrBz-Ac3c-OH and Z-Ac3c-OH, however, are unusual in having a semi-extended conformation for the phi, psi angles. The N-terminal Ac3c residue of H-(Ac3c)2-OMe adopts a novel type of C5 conformation, characterized inter alia by an (amino) N. . .H-N (peptide) intramolecular hydrogen bond. While the acyl N alpha-blocking groups form trans amides (pBrBz-Ac3c-OH and Piv-Ac3c-OH), the urethane groups may adopt either the trans [Z-Ac3c-OH and t-Boc-Ac3c-OH (alpha-form)] or the cis amide conformations [t-Boc-Ac3c-OH(beta-form) and Z-Ac3c-OMe]. The five- and six-membered rings of the 5(4H)-oxazolone and the 2,5-dioxopiperazine, respectively, are planar. The four independent molecules in the asymmetric unit of the free alpha-amino acid are zwitterionic.
Valle
G
G
Department of Organic Chemistry, University of Padova, Italy.
Crisma
M
M
Toniolo
C
C
Holt
E M
EM
Tamura
M
M
Bland
J
J
Stammer
C H
CH
D000596:Amino Acids; D000598:Amino Acids, Cyclic; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003460:Crystallization; D006860:Hydrogen Bonding; D008968:Molecular Conformation; D015394:Molecular Structure; D010455:Peptides; D014961:X-Ray Diffraction
D000596:Amino Acids; D000598:Amino Acids, Cyclic; D010455:Peptides; C023863:1-aminocyclopropane-1-carboxylic acid