Gian Paolo Lorenzi

Regularly alternatingL,D-peptides. II. The double-stranded right-handed antiparallel ?-helix in the structure oft-Boc-(L-Phe-D-Phe)4-OMe

The crystal structure of Boc‐(L‐Phe‐D‐Phe)4‐OMe has been determined by x‐ray diffraction analysis. The peptide crystallizes in the triclinic system, space group P1 with a = 15.290 Å, b = 15.163 Å, c = 19.789 Å, α = 102.49°, β = 96.59°, γ = 74.22°, and Z = 2. The structure has been solved by coupling of the molecular replacement technique and expansion by tangent formula refinement of the set of known phases. Several cycles of Fourier calculations and least‐squares refinement led to the location of 194 atoms of the two independent octapeptide chains and few molecules of cocrystallized solvent (chloroform, water, and methanol). The isotropic refinement converged to R = 0.13 for the 3077 “observed” reflections.

A structural two-ring version of a tubular stack ofβ-rings in crystals of a cyclic D,L-hexapeptide

An X-ray analysis of single crystals (from MeOH) of cyclo(-D-Leu-L-MeLeu-D-Leu-L-MeLeu-D-Leu-L-MeLeu-) has been carried out. The analysis reveals that the molecules of the cyclopeptide occur in the crystals with two slightly different, almost hexagonal backbone conformations of the β-type, and that pairs of molecules with the same conformation interact through their nonmethylated face, forming dimeric units (units A and B) with six interannular H-bonds. This kind of pairing reproduces well that expected for a two-ring element in a stack of antiparalleβ-rings. The X-ray analysis has also revealed the presence in the A units of two water molecules, each at one of two equivalent sites located on the 3-fold axis of the units and equidistant from the center of gravity, and the presence in the B units of one water molecule at the center of the units. This provides experimental support for the idea that stacks ofβ-rings can serve as molecular channels.

A hairpin-shaped peptide conformation stabilized by multiple intramolecular H-bonds for a linear alternating D,L hexapeptide

Abstract Sequential L,D polypeptides have been the object of investigations by many authors (1–4), since it was realized that they are strictly related to a natural occurring peptide, gramicidin A, which forms monovalent cation selective transmembrane channels in biological membranes and lipid bilayers (5,6). Here we present the results obtained by x-ray analysis of a linear hexapeptide, Boc-(D-aIle-L-Ile)3-OMe, which, in the solid state, gives rise to a chain reversal stabilized by multiple intramolecular hydrogen bonds. It is the first description of a β-turn (C10) and an α-turn (C13) fused together and included in a larger 17-membered ring (C17) occurring in a linear peptide.

Novel tetrapodal molecules and reticular polyamides based on tetraphenylmethane

A series of tetrapodal derivatives of tetraphenylmethane were synthesized and characterized. Crystals obtained from tetrakis(4-acetamidophenyl)methane (1c) and from tetrakis[4-(4-aminobenzamido)phenyl]methane (2b) were analyzed by X-ray diffraction. The analyses pointed to the crystal packing problems faced by molecules of this kind by showing that the crystals, with composition1c·2DMF·2H2O and2b·2DMSO, respectively, contained cocrystallized solvent molecules. The solvent molecules were found in both cases to be held in place by H bonds; in the case of2b·2DMSO they occupied channels running along theb axis. Tetrakis(4-aminophenyl)methane (1b) was used in polycondensation reactions with terephthalic acid, under modified Yamazaki conditions, to produce rigid aromatic polyamide networks. The networks were obtained as gels encompassing the whole volume of the reaction mixture. The volume of the gels did not vary noticeably upon changing the solvent (1-methyl-2-pyrrolidone) with less polar solvents, but the gels collapsed upon drying. No crystallinity was observed.

Nanothin films of tubular oligopeptides : monolayers and Langmuir-Blodgett films of β4.4-helices with surface-pressure-dependent orientation

Abstract The orientation of β 4.4 helices of a D, L -alternating pentadecavaline, Boc- D -Val-( L -Val- D -Val) 7 -OMe, in monolayers at the air-water interface and in Langmuir-Blodgett films has been investigated. Compression of these helices in a Langmuir trough results in characteristic surface pressure-area curves exhibiting an initial rise, in which the surface pressure increases to 20 mN m -1 and the area decreases to 2.5 nm 2 helix -1 , followed by a plateau up to 30 mN m, -1 and 1.35 nm 2 helix -1 and by a second rise. At the end of this second rise, before the fims collapses, surface pressure and area are about 60 mN m -1 and 1.1 nm 2 helix -1 respectively. These results are interpreted as indicating that, at low surface pressures, the β 4.4 helices lie flat on the water, and that they can reorient themselves to assume an upright position when the pressure becomes high enough. Ellipsometric and grazing incidence reflection absorption measurements show that similar orientations relative to the substrate (silanized or gilded silicon) are obtained in LB films when these are assembled at surface pressures of 14 mN m -1 and 45 mN m -1 respectively.