Conformation and Orientation of Tetraalanine in a Lyotropic Liquid Crystal Studied by Nuclear Magnetic Resonance

Abstract

The (1)H NMR spectra of two isotopomers of tetraalanine deuterated on the two external methyl groups and on the two internal ones, respectively, were recorded in the lyotropic solvent cesium pentadecafluorooctanoate (CsPFO)/water. Eight dipolar couplings could be estimated from the spectra. The set of dipolar couplings was fitted assuming that one rigid conformer is present. Of the four major conformers considered, selected on the basis of theoretical calculations, the one characterized by the two couples of internal dihedral angles in the Ramachandran region of PPII resulted to be the only one to fit the set of couplings within experimental error. The data indicate that the molecule is oriented with the long molecular axis tilted with respect to the surface of the micelles formed by CsPFO.

Conformation and Orientation of an Oligopeptide in a Lyotropic Mesophase by1H NMR

“…In particular, the combination of Polarized Raman, Fourier transform-IR and vibrational circular dichroism indicated that the dihedral angles of the two internal residues of cationic tetraalanine are close to the Ramachandran coordinates of the polyproline II helix (PPII). Further evidence supporting these results was given by us in a previous paper [11], where we presented a liquid-crystal 1 H NMR study on the conformation and orientation of tetraalanine in the lyotropic solvent cesium pentadecafluorooctanoate=water (CsPFO=D 2 O). This system is known to form bilayer-like disk-shaped micelles with average alignment of the disks perpendicular to an external magnetic field [12], which induces alignment also on solute molecules.…”

“…The doublets due to methyl protons resonate at 1.66, 1.53, and 1.46 ppm, the intensity of the signal centered at 1.53 ppm being double than each of the others. On the basis of previous assignment [11], these signals were attributed to the methyl groups of the N-terminal alanyl residue, of the two inner residues, and of the C-terminal residue, respectively. Because the H N protons readily exchange with D 2 O, they are not observed in the 1 H spectrum.…”

“…The experimental evidence so far acquired in the literature [6,11] gives strong indication that in aqueous environment the inner residues adopt a PPII conformation. As far as the dihedral angles of the external residues are concerned, no clear indication of preferred values are reported.…”

“…CsPFO was prepared according to the procedure reported in the literature [8]. A 4 was dissolved in the CsPFO=D 2 O solution, prepared as reported in the literature [11], at a concentration of 0.3 wt%. The isotropic-nematic phase transition occurs at 308 K and the nematic range of stability is approximately 10 K.…”

“…The set of conformers analyzed was generated from conformer 1 of Ref. [11] varying the angles w 1 and u 4 in steps of 30 in the range À 180 -180 , and in steps of 5 in the regions between À110 and À130 for w 1 and between À70 and À85 for u 4 , where the experimental values were best reproduced on the basis of a preliminary screening.…”