Conformations of heterochiral and homochiral proline-pseudoproline segments in peptides: context dependent cis-trans peptide bond isomerization.
The pseudoproline residue (Psi Pro, L-2,2-dimethyl-1,3-thiazolidine-4-carboxylic acid) has been introduced into heterochiral diproline segments that have been previously shown to facilitate the formation of beta-hairpins, containing central two and three residue turns. NMR studies of the octapeptide Boc-Leu-Phe-Val-(D)Pro-Psi Pro-Leu-Phe-Val-OMe (1), Boc-Leu-Val-Val-(D)Pro-Psi Pro-Leu-Val-Val-OMe (2), and the nonapeptide sequence Boc-Leu-Phe-Val-(D)Pro-Psi Pro-(D)Ala-Leu-Phe-Val-OMe (3) established well-registered beta-hairpin structures in chloroform solution, with the almost exclusive population of the trans conformation for the peptide bond preceding the Psi Pro residue. The beta-hairpin conformation of 1 is confirmed by single crystal X-ray diffraction. Truncation of the strand length in Boc-Val-(D)Pro-Psi Pro-Leu-OMe (4) results in an increase in the population of the cis conformer, with a cis/trans ratio of 3.65. Replacement of Psi Pro in 4 by (L)Pro in 5, results in almost exclusive population of the trans form, resulting in an incipient beta-hairpin conformation, stabilized by two intramolecular hydrogen bonds. Further truncation of the sequence gives an appreciable rise in the population of cis conformers in the tripeptide Piv-(D)Pro-Psi Pro-Leu-OMe (6). In the homochiral segment Piv-Pro-Psi Pro-Leu-OMe (7) only the cis form is observed with the NMR evidence strongly supporting a type VIa beta-turn conformation, stabilized by a 4-->1 hydrogen bond between the Piv (CO) and Leu (3) NH groups. The crystal structure of the analog peptide 7a (Piv-Pro-Psi(H,CH3)Pro-Leu-NHMe) confirms the cis peptide bond geometry for the Pro-Psi(H,CH3)Pro peptide bond, resulting in a type VIa beta-turn conformation.
Indian Institute of Science, Bangalore, Karnataka, India.
Srinivasarao
S
Raghothama
Upadhyayula Surya
US
Raghavender
Subrayashastry
S
Aravinda
Narayanaswamy
N
Shamala
Padmanabhan
P
Balaram
D006860:Hydrogen Bonding; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D010455:Peptides / Q000737:chemistry* / Q000235:genetics; D011392:Proline / Q000031:analogs & derivatives* / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013237:Stereoisomerism; D013844:Thiazoles / Q000737:chemistry*; D014961:X-Ray Diffraction
D010455:Peptides; D013844:Thiazoles; C505976:pseudoproline; D011392:Proline