Coexistence of Hydrogen-Bonded Loop and Extended Tetrapeptide Conformations

The conformations of a protected tetrathiopeptide have been analysed by isotope labelling and two-dimensional infrared spectroscopy (2D-IR). It has been found that Boc-Ala-Gly(=S)-Ala-Aib-OMe in acetonitrile, as well as its oxopeptide analogue, can adopt a hydrogen-bonded loop conformation in coexistence with less restricted structures. The two types of conformations interconvert too quickly to be resolved on the nuclear magnetic resonance (NMR) timescale, but give rise to different cross peaks in two-dimensional infrared spectra. The hydrogen bond between the Boc terminal group and the amide proton of Aib can be broken by photoisomerisation of the thioamide bond.

Intramolecular Disulfide Bridges as a Phototrigger To Monitor the Dynamics of Small Cyclic Peptides

Two cyclic disulfide-bridged tetrapeptides [cyclo(Boc-Cys-Pro-Aib-Cys-OMe) (1) and cyclo(Boc-Cys-Pro-Phe-Cys-OMe) (2)] have been monitored by time-resolved mid-IR spectroscopy in the C=O vibrational range. A conformational change is induced by cleavage of the intramolecular disulfide bridge upon UV excitation (lambda(exc) = 260 nm), giving rise to a pair of cysteinyl radicals (thiyl radicals), which diffuse apart allowing the peptide to change conformation before they undergo quenching. The amide I band reports on the dynamics of the peptide backbone, which evolves on a 100 ps time scale and then stays constant up to 10 micros at low enough concentrations ( approximately 100 mM). To probe specifically the lifetime of the free cysteinyl radicals, time-resolved UV laser flash photolysis has been applied. The concentration of the cysteinyl radical decays nonexponentially, but about 50% are still present after 1 ms. The photocleavable disulfide bridge hence may serve as an intrinsic, naturally occurring phototrigger to study peptide dynamics that opens a wide time-window from a few picoseconds to many hundreds of microseconds.

Biotinylated Stealth magnetoliposomes

Dimyristoylphosphatidylethanolamine (DC(14:0)PE) and the dioleoyl analogue (DC(18:1cis)PE) were mixed with alpha-biotinylamido-omega-N-succinimidoxycarbonyl-poly(ethylene glycol) (NHS-PEG-biotin) and quantitatively converted to alpha-biotinylamido-omega-(dimyristoylphosphatidylethanolamino-carbonyl)polyethylene glycol (DC(14:0)PE-PEG-biotin) and the dioleoyl analogue DC(18:1cis)PE-PEG-biotin, respectively. As shown by thin-layer chromatography and 1H NMR spectroscopy, PEGylation of both phosphatidylethanolamine types went to completion if the reaction was performed in organic solvent in the presence of triethylamine. The resulting derivatives were successfully incorporated into both classical phospholipid vesicles and a phospholipid bilayer surrounding nanometer-sized magnetite cores. In the latter case, the so-called activated Stealth(1) magnetoliposomes were produced which very efficiently immobilized streptavidinylated alkaline phosphatase.

Theoretical and experimental study of the vibrational spectrum of N-acetyl-L-alanine

The energies, vibrational frequencies and IR intensities of cis- and trans-N-acetyl-L-alanine (NAAL) are computed using the density functional theory (B3LYP) combined with the 6-311G(d, p) basis set. The trans conformer is characterized by an intramolecular NH ... O hydrogen bond leading to the formation of a five-membered ring and is by 23 kJ mol(-1) more stable than the cis conformer. The difference between the vibrational frequencies and IR intensities computed for the two conformers is discussed. The IR spectra at different temperatures and the Raman spectra of solid NAAL and its deuterated counterpart are investigated and discussed. The frequencies of the v(OH) vibration and the isotopic ratio suggest the formation of short OH ... O hydrogen bonds in the solid state. The NH group seems also to be involved in a weak hydrogen bond.

Synthesis, conformational studies, and molecular dynamics calculations of two cyclic tetrapeptides with 17- and 18-membered rings

Two cyclic tetrapeptides [Boc-cyclo(-Xxx-Pro-Asn-Lys-)OMe (Xxx = Asp or Glu)] were synthesized and investigated by NMR spectroscopy. They were designed in order to mimic the salt bridge found in physalaemin. Isomers of the urethane bond were observed in DMSO solution. The ROESY spectrum allowed the assignment of many signals of the minor isomer of both compounds. Conformational studies based on the temperature gradients of the NH chemical shifts, coupling constants, and ROEs revealed a similar conformation for the Asp analogue as proposed for physalaemin. A beta I turn with Pro and Asn in the corner positions was found for the major isomer. No hydrogen bonds were detected for the major isomer of the cyclic Glu analogue. Molecular dynamics calculations, using the NMR based initial structures, yielded sets of conformations in agreement with the experimental data. It is concluded that the salt bridge in physalaemin is best approximated by a lactam formed from the original amino acids.

Solid state and solution conformation of Boc-L-Met-Aib-L-Phe-OMe. Beta-turn conformation of a sequence related to an active chemotactic peptide analog

The conformation of the peptide Boc-L-Met-Aib-L-Phe-OMe has been studied in the solid state and solution by X-ray diffraction and 1H n.m.r., respectively. The peptide differs only in the N-terminal protecting group from the biologically active chemotactic peptide analog formyl-L-Met-Aib-L-Phe-OMe. The molecules adopt a type-II beta-turn in the solid state with Met and Aib as the corner residues (phi Met = -51.8 degrees, psi Met = 139.5 degrees, phi Aib = 58.1 degrees, psi Aib = 37.0 degrees). A single, weak 4----1 intramolecular hydrogen bond is observed between the Boc CO and Phe NH groups (N---O 3.25 A, N-H---O 128.4 degrees). 1H n.m.r. studies, using solvent and temperature dependencies of NH chemical shifts and paramagnetic radical induced line broadening of NH resonances, suggest that the Phe NH is solvent shielded in CDCl3 and (CD3)2SO. Nuclear Overhauser effects observed between Met C alpha H and Aib NH protons provide evidence of the occurrence of Met-Aib type-II beta-turns in these solvents.