| Popis: |
Crystal and molecular structures for the heterochiral sequence N-formyl-l-methionine-d-phenylalanine, 1, and its tertiary butyl ester, 2, are reported. The solid-state peptide conformation is compared to that observed in solution by n.m.r. techniques. For N-f-Met-d-Phe, 1, the crystal was orthorhombic, space group P21 2121 with a cell of dimensions a = 5.061(3), b = 16.575(5) and c = 19.656(6) A at ambient temperature of 293K; V = 1649(2)A3, Z = 4, Dm= 1.31(2)gcm−3, Dx= 1.307gcm−3, μ(Mokx) = 2.047cm−1. For N-f-Met-d-Phe-OtBu. 2, the crystal was monoclinic, space group P21 with a cell of dimensions a = 9.963(2), b = 11.147(2), c = 19.166(3)A. β= 102.31(1) at 273K; V = 2080(2) A3 Z = 4, Dm= 1.22(2)gcm−3, Dx= 1.215gcm−3μ(MoKx) = 1.714cm−1. The structures were solved from diffractometer data and refined to conventional final R = 0.046, Rw= 0.053 for F-Met-d-Phe (1301 observations. I ≥ 3σ(I)) and to R = 0.056, Rw= 0.064 for the t-butylester (2411 observations. I ≥ 3σ(I)). The l-d acid, 1, crystallizes in an extended β-sheet conformation with trans-planar peptide bond; the principal torsion angle values are φ1=– 141.2(4).Ψ1= 149.6(4)3, φ2= 157.4(4)°. The methionine side chain adopts a common coiled conformation with x11= - 58.0(5).x21= 175.1(4), x31= 76.5(5). The Phe side chain adopts the statistically least favored g orientation in contrast to the most populated rotamer in solution. The crystal structure is composed of parallel β-sheets held together by four weak intermolecular contacts including two C-H O contacts from the alpha carbons to the formyl and peptide carbonyl oxygens. Sheet layers are joined in a head-to-tail fashion through a very short (2.569(4) A) contact between the carboxyl OH and the formyl oxygen and may be further stabilized by a C-H—O interaction between the formyl proton and the carboxyl OH. Two crystallographically independent molecules are observed in the crystal structure of N-f-l-Met-d-Phe-OtBu, 2. These are distinct conformational isomers differing principally at both the N and C termini. Particularly noteworthy is the synplanar orientation of the ester C = O with respect to the peptide nitrogen in molecule A. which contrasts to the antiplanar orientation in molecule B. Additionally, the formyl group is coiled more towards the C-terminus in molecule B. Principal torsion angles are φ1(A) = - 120.6(5), Ψ1(A) = 102.0(6)°, φ2(A) = 128.1(6), φ1(B) = - 94.6(5), Ψ1(B) = 91.9(6)°. 121.7(6)°. The peptide bond is trans-planar in both molecules. Side chain dispositions are essentially identical in both structures. The Met side chain adopts the zig-zag trans-planar conformation while the Phe residue adopts an orientation near + 60° in agreement with rotamer populations observed by solution n.m.r. Typical peptide intermolecular H-bonding is observed in the ester crystal structure; both the peptide and formyl groups participate in the proposed H-bonding scheme. |
