Aromatic interactions in model peptide ß-hairpins: ring current effects on proton chemical shifts.

Crystal structures of eight peptide ß-hairpins in the sequence Boc-Leu-Phe-Val-Xxx-Yyy-Leu-Phe-Val-OMe revealed that the Phe(2) and Phe(7) aromatic rings are in close spacial proximity, with the centroid-centroid distance (R(cen)) of 4.4-5.4 Å between the two phenyl rings. Proton NMR spectra in chloroform and methanol solution reveal a significant upfield shift of the Phe(7) C(δ,δ') H(2) protons (6.65-7.04 ppm). Specific assignments of the aromatic protons have been carried out in the peptide Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe (6). The anticipated ring current shifts have been estimated from the aromatic ring geometrics observed in crystals for all eight peptides. Only one of the C(δ,δ') H proton lies in the shielding zone with rapid ring flipping, resulting in averaging between the two extreme chemical shifts. An approximate estimate of the population of conformations, which resemble crystal state orientation, may be obtained. Key nuclear Overhauser effects (NOEs) between facing Phe side chains provide support for close similarity between the solid state and solution conformation. Temperature dependence of aromatic ring proton chemical shift and line widths for peptide 6 (Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe) and the control peptide Boc-Leu-Val-Val-(D)Pro-Gly-Leu-Phe-Val-OMe establish an enhanced barrier to ring flipping when the two Phe rings are in proximity. Modeling studies suggest that small, conformational adjustment about C(α)-C(ß) (χ(1) ) and C(ß)-C(γ) (χ(2) ) bonds of both the Phe residues may be required in order to permit unhindered, uncorrelated flipping of both the Phe rings. The maintenance of the specific aromatic ring orientation in organic solvents provides evidence for significant stabilizing interaction.

ß-Turn analogues in model αß-hybrid peptides: structural characterization of peptides containing ß(2,2)Ac6c and ß(3,3)Ac6c residues.

The effect of gem-dialkyl substituents on the backbone conformations of ß-amino acid residues in peptides has been investigated by using four model peptides: Boc-Xxx-ß(2,2)Ac(6)c(1-aminomethylcyclohexanecarboxylic acid)-NHMe (Xxx = Leu (1), Phe (2); Boc = tert-butyloxycarbonyl) and Boc-Xxx-ß(3,3)Ac(6)c(1-aminocyclohexaneacetic acid)-NHMe (Xxx = Leu (3), Phe (4)). Tetrasubstituted carbon atoms restrict the ranges of stereochemically allowed conformations about flanking single bonds. The crystal structure of Boc-Leu-ß(2,2)Ac(6)c-NHMe (1) established a C(11) hydrogen-bonded turn in the αß-hybrid sequence. The observed torsion angles (α(ϕ≈-60°, ψ≈-30°), ß(ϕ≈-90°, θ≈60°, ψ≈-90°)) corresponded to a C(11) helical turn, which was a backbone-expanded analogue of the type III ß turn in αα sequences. The crystal structure of the peptide Boc-Phe-ß(3,3)Ac(6)c-NHMe (4) established a C(11) hydrogen-bonded turn with distinctly different backbone torsion angles (α(ϕ≈-60°, ψ≈120°), ß(ϕ≈60°, θ≈60°, ψ≈-60°)), which corresponded to a backbone-expanded analogue of the type II ß turn observed in αα sequences. In peptide 4, the two molecules in the asymmetric unit adopted backbone torsion angles of opposite signs. In one of the molecules, the Phe residue adopted an unfavorable backbone conformation, with the energetic penalty being offset by a favorable aromatic interaction between proximal molecules in the crystal. NMR spectroscopy studies provided evidence for the maintenance of folded structures in solution in these αß-hybrid sequences.

Entrapment of a water wire in a hydrophobic peptide channel with an aromatic lining.

A one-dimensional water wire has been characterized by X-ray diffraction in single crystals of the tripeptide Ac-Phe-Pro-Trp-OMe. Crystals in the hexagonal space group P6(5) reveal a central hydrophobic channel lined by aromatic residues which entraps an approximately linear array of hydrogen bonded water molecules. The absence of any significant van der Waals contact with the channel walls suggests that the dominant interaction between the "water wire" and "peptide nanotube" is electrostatic in origin. An energy difference of 16 kJ mol(-1) is estimated for the distinct orientations of the water wire dipole with respect to the macrodipole of the peptide nanotube. The structural model suggests that Grotthuss type proton conduction may, through constricted hydrophobic channels, be facilitated by concerted, rotational reorientation of water molecules.

Chain length effects on helix-hairpin distribution in short peptides with Aib-DAla and Aib-Aib segments.

The Aib-D Ala dipeptide segment has a tendency to form both type-I'/III' and type-I/III ß-turns. The occurrence of prime turns facilitates the formation of ß-hairpin conformations, while type-I/III turns can nucleate helix formation. The octapeptide Boc-Leu-Phe-Val-Aib-DAla-Leu-Phe-Val-OMe (1) has been previously shown to form a ß-hairpin in the crystalline state and in solution. The effects of sequence truncation have been examined using the model peptides Boc-Phe-Val-Aib-Xxx-Leu-Phe-NHMe (2, 6), Boc-Val-Aib-Xxx-Leu-NHMe (3, 7), and Boc-Aib-Xxx-NHMe (4, 8), where Xxx=DAla, Aib. For peptides with central Aib-Aib segments, Boc-Phe-Val-Aib-Aib-Leu-Phe-NHMe (6), Boc-Val-Aib-Aib-Leu-NHMe (7), and Boc-Aib-Aib-NHMe (8) helical conformations have been established by NMR studies in both hydrogen bonding (CD3OH) and non-hydrogen bonding (CDCl3) solvents. In contrast, the corresponding hexapeptide Boc-Phe-Val-Aib-DAla-Leu-Phe-Val-NHMe (2) favors helical conformations in CDCl3 and ß-hairpin conformations in CD3 OH. The ß-turn conformations (type-I'/III) stabilized by intramolecular 4→1 hydrogen bonds are observed for the peptide Boc-Aib-D Ala-NHMe (4) and Boc-Aib-Aib-NHMe (8) in crystals. The tetrapeptide Boc-Val-Aib-Aib-Leu-NHMe (7) adopts an incipient 3(10)-helical conformation stabilized by three 4→1 hydrogen bonds. The peptide Boc-Val-Aib-DAla-Leu-NHMe (3) adopts a novel α-turn conformation, stabilized by three intramolecular hydrogen bonds (two 4→1 and one 5→1). The Aib-DAla segment adopts a type-I' ß-turn conformation. The observation of an NOE between Val (1) NH↔HNCH3 (5) in CD3OH suggests, that the solid state conformation is maintained in methanol solutions.