Influence of N-terminal residue stereochemistry on the prolyl amide geometry and the conformation of 5-tert-butylproline type VI beta-turn mimics.
The effects of N-terminal amino acid stereochemistry on prolyl amide geometry and peptide turn conformation were investigated by coupling both L- and D-amino acids to (2S, 5R)-5-tert-butylproline and L-proline to generate, respectively, N-(acetyl)dipeptide N'-methylamides 1 and 2. Prolyl amide cis- and trans-isomers were, respectively, favored for peptides 1 and 2 as observed by proton NMR spectroscopy in water, DMSO and chloroform. The influence of solvent composition on amide proton chemical shift indicated an intramolecular hydrogen bond between the N'-methylamide proton and the acetamide carbonyl for the major conformer of dipeptides (S)-1, that became less favorable in (R)-1 and 2. The coupling constant (3J(NH,alpha)) values for the cis-isomer of (R)-1 indicated a phi2 dihedral angle value characteristic of a type VIb beta-turn conformation in solution. X-ray crystallographic analysis of N-acetyl-D-leucyl-5-tert-butylproline N'-methylamide (R)-lb showed the prolyl residue in a type VIb beta-turn geometry possessing an amide cis-isomer and psi3-dihedral angle having a value of 157 degrees, which precluded an intramolecular hydrogen bond. Intermolecular hydrogen bonding between the leucyl residues of two turn structures within the unit cell positioned the N-terminal residue in a geometry where their phi2 and psi2 dihedral angle values were not characteristic of an ideal type VIb turn. The circular dichroism spectra of tert-butylprolyl peptides (S)- and (R)-1b were found not to be influenced by changes in solvent composition from water to acetonitrile. The type B spectrum exhibited by (S)-1b has been previously assigned to a type VIa beta-turn conformation [Halab L, Lubell WD. J. Org. Chem. 1999; 64: 3312-3321]. The type C spectrum exhibited by the (R)-lb has previously been associated with type II' beta-turn and alpha-helical conformations in solution and appears now to be also characteristic for a type VIb geometry.
Département de chimie, Université de Montréal, Québec, Canada.
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Halab
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Lubell
D000577:Amides / Q000737:chemistry*; D002725:Chloroform / Q000737:chemistry; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008958:Models, Molecular; D010452:Peptide Biosynthesis; D011392:Proline / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000737:chemistry*; D011487:Protein Conformation; D013237:Stereoisomerism
C407508:5-tert-butylproline; D000577:Amides; D002725:Chloroform; D011392:Proline