Peptide hairpins with strand segments containing alpha- and beta-amino acid residues: cross-strand aromatic interactions of facing Phe residues.
The incporation of beta-amino acid residues into the strand segments of designed beta-hairpin leads to the formation of polar sheets, since in the case of beta-peptide strands, all adjacent carbonyl groups point in one direction and the amide groups orient in the opposite direction. The conformational analysis of two designed peptide hairpins composed of alpha/beta-hybrid segments are described: Boc-Leu-betaPhe-Val-(D)-Pro-Gly-Leu-betaPhe-Val-OMe (1) and Boc-betaLeu-Phe-betaVal-D-Pro-Gly-betaLeu-Phe-betaVal-OMe (2). A 500-MHz 1H-NMR (nuclear magnetic resonance) analysis in methanol supports a significant population of hairpin conformations in both peptides. Diagnostic nuclear Overhauser effects (NOEs) are observed in both cases. X-ray diffraction studies on single crystals of peptide 1 reveal a beta-hairpin conformation in both the molecules, which constitute the crystallographic asymmetric unit. Three cross-strand hydrogen bonds and a nucleating type II' beta-turn at the D-Pro-Gly segment are observed in the two independent molecules. In peptide 1, the betaPhe residues at positions 2 and 7 occur at the nonhydrogen-bonding position, with the benzyl side chains pointing on opposite faces of the beta-sheet. The observed aromatic centroid-to-centroid distances are 8.92 A (molecule A) and 8.94 A (molecule B). In peptide 2, the aromatic rings must occupy facing positions in antiparallel strands, in the NMR-derived structure. Peptide 1 yields a normal "hairpin-like" CD spectrum in methanol with a minimum at 224 nm. The CD spectrum of peptide 2 reveals a negative band at 234 nm and a positive band at 221 nm, suggestive of an exciton split doublet. Modeling of the facing Phe side chains at the hydrogen-bonding position of a canonical beta-hairpin suggests that interring separation is approximately 4.78 A for the gauche+ gauche- (g+ g-) rotamer. A previously reported peptide beta-hairpin composed of only alpha-amino acids, Boc-Leu-Phe-Val-D-Pro-Gly-Leu-Phe-Val-OMe also exhibited an anomalous far-UV (ultraviolet) CD (circular dichroism) spectrum, which was interpreted in terms of interactions between facing aromatic chromophores, Phe 2 and Phe 7 (C. Zhao, P. L. Polavarapu, C. Das, and P. Balaram, Journal of the American Chemical Society, 2000, Vol 122, pp. 8228-8231).
Roy
Rituparna S
RS
Molecular Biophysics Unit, Indian Institute of Science, Bangalore-560012, India.
Gopi
Hosahudya N
HN
Raghothama
S
S
Gilardi
Richard D
RD
Karle
Isabella L
IL
Balaram
Padmanabhan
P
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D008970:Molecular Weight; D019906:Nuclear Magnetic Resonance, Biomolecular; D009842:Oligopeptides / Q000032:analysis* / Q000138:chemical synthesis / Q000302:isolation & purification; D010649:Phenylalanine / Q000737:chemistry*; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D021241:Spectrometry, Mass, Electrospray Ionization; D013237:Stereoisomerism; D014961:X-Ray Diffraction
D000596:Amino Acids; D009842:Oligopeptides; D010649:Phenylalanine