M. Cotrait

Synthesis, characterization and crystal structures of two polyphilic mesogenic compounds

Abstract Three homologous series of semi-perfluorinated liquid crystals: 4-(2,2,3,3,4,4,4-heptafluoro-butyloxycarbonyl)phenyl, 4-(2,2,3,3,4,4,5,5-octafluoropentyloxycarbonyl)phenyl and 4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxycarbonyl)phenyl 4-[(n-perfluoroalkyl)alkyloxy]benzoates have been synthesized. Their mesomorphic properties were studied by DSC, microscopic observation and X-ray diffraction. Their layer spacings are smaller than the molecular length (L). The ratio d SA/L is about 0.7. The crystal structure of two derivatives of the first family have also been investigated. In both structures the molecules pack in smectic C-like sheets where neighbouring molecules are antiparallel, interacting through dipolar and van der Waals forces. The interactions between contiguous sheets, through the ends of perfluorinated chains are very weak. The X-ray diffraction results on the mesophases and on the crystalline structures of two compounds of the first family are compared. From this, we propose a mo...

Change of the lateral chain conformation in the solid and the nematic phase in laterally substituted nematogens

Abstract A new homologous series of 4-(4′-ethoxybenzoyloxy)-2-alkoxy-4′-(4-butoxysalicylaldimine)-azobenzenes has been synthesized. These compounds contain four rings in the main core and a lateral alkoxy branch on one of the inner rings and present enantiotropic nematic properties. The X-ray crystal structure (MoKα radiation, λ = 0.7071 A, graphite monochromator) of 4-(4′-ethoxybenzoyloxy)-2-butoxy-4′-(4-butoxysalicylaldimine)azobenzene was investigated (T = 293K, R = 0.059 for the 2855 observed reflections). This compound (C36H39N3O6) crystallizes in the monoclinic system with C2/c space group; (Z = 8); a = 28.052(7) A, b = 7.545(4) A, c = 33.823(7) A and β = 112.70(1)°. There is a strong chelated hydrogen bond between the OH group and the neighbouring nitrogen atom engaged in the imine bridge. In the solid state, the molecule is roughly linear with a 31 A length. The lateral butoxy chain is quasi-perpendicular to the long axis of the main core. The crystal cohesion is due to numerous Van der Waals inte...

Analysis of tachykinin-binding site interactions using NMR and energy calculation data of potent cyclic analogues of substance P

The three-dimensional structures of [Cys3,6,Tyr8]-, [Gly2,Cys3,6,Tyr8]- and [DCys3,Cys6]substance P, designed as conformational analogues of substance P, have been studied by 1H-NMR (500 MHz) in different solvents and by energy calculations. As previously observed for substance P and physalaemin, two tachykinins acting via the NK-1 receptor, [Cys3,6,Tyr8]substance P presents an alpha-helical structure of the 4----8 sequence in methanol. This structure is stabilized by a beta-turn III via the formation of three hydrogen bonds involving the Cys-6, Phe-7 and Tyr-8 NH groups. In contrast to substance P, two of these hydrogen bonds are still present in dimethyl sulfoxide and in water the Cys-6 NH hydrogen bond is the only one remaining, such that a beta-turn structure inside the ring can be envisaged. In close agreement with the NMR data, the energy calculations lead to three types of folding for the core of [Cys3,6,Tyr8]substance P: a beta-turn III, a less stable beta-turn I (delta E = 3 kcal), and a beta-turn II (delta E = 4.6 kcal). The structure of Gly-Leu-Met-NH2 is strongly affected by changing the hydrophobicity of the medium. The most stable calculated conformation is the helix; however, numerous unrelated structures are destabilized by about 2-3 kcal/mol. These data are analyzed and discussed in connection with the high potency of [Cys3,6,Tyr8]substance P for both the NK-1 and NK-3 binding sites; that is the internal region of tachykinins (non-homologous amino acids) might present a similar three-dimensional structure when bound to the receptors (which may be at the origin of some lack of selectivity), whereas paradoxically the selectivity may be due to the common C-terminal sequence.

Synthesis, properties and crystal structure of chiral semiperfluorinated liquid crystals with ferro and anticlinic smectic phases

A new chiral and semiperfluorinated series with ferro and anticlinic properties has been synthesized and characterized. The mesomorphic behaviour has been established on the grounds of both microscopic observations and DSC measurements. The non-chiral intermediate ethyl 4-semiperfluorinated alkyloxybenzoates exhibit SmA phases, unusual for compounds with a single phenyl ring. The final derivatives display SmA, SmC* and in several cases SmC*A phases. The longer fluorinated chains favour the SmA and SmC* phases at the expense of the SmC*A phase. Electro-optical measurements were carried out with the classical SSFLC geometry. The spontaneous polarization and tilt angle at saturation are higher than those of the hydrogenous homologues, around 140 nCcm-2 at 40 degrees C. One compound of the series, the 4,4,5,5,6,6,7,7,8,8,8-nonafluoroheptyloxy derivative, C36H35O7F9, Mx=750.6 g mol-1, crystallizes in the triclinic system, space group P1 with four independent molecules in the asymmetric unit (Z=4). The molecule...

Structure cristalline et conformation en solution aqueuse de la carnosine (β-alanyl-L-histidine)

Abstract Carnosine (β-alanyl-L-histidine) is a biologically active molecule involved in muscular metabolism. It crystallises in the C; space group with a = 24.725 A b = 5,427 A c = 8,004 A β = 100,2° ( Z = 4) In the crystal, acid and basic groups are engaged in hydrogen bonds whose strength is evaluated through IR frequencies. Molecular conformation in the solid state is defined by τ 1 = /t-177° τ 2 = −38° φ = −96° ψ = +131° χ 1 = 181° χ 2 1 = 62° NMR study of carnosine in aqueous solution indicates that rotation about CH 2 -CH 2 is free and that the other angles take the following values: O⋍ −150° or −90° and X 1 = 165° or 315°. Infrared and Raman spectra suggest that τ 2 undergoes small changes when going from crystal to solution while ψ is close to +150°.

Photoinduced control of cation binding ability of non-conjugated bichromophoric receptors

Abstract Several receptors based on nonconjugated bichromophores displaying phototunable cation binding properties have been designed and studied. Some of these systems undergo a reversible photocyclisation to produce a crown ether or a cryptand; the photoswitching induces significant changes on a long time scale (hours to months after illumination) of the complexing ability of the material. Other bitopic receptors display a sudden and transitory change of cation concentration only during the singlet excited state lifetime.

Phasmid and biforked mesogens with thiobenzoate end groups

Abstract One phasmid and fourteen biforked mesogens with thiobenzoate end groups have been synthesized and studied. The phasmid displays a hexagonal columnar phase with a lattice parameter a = 45.5 A at 70°C. This phase is not observed for the biforked mesogens which exhibit only a nematic phase, a smectic C phase for short chains and an unknown phase for long chains. The tilt angle of the molecules in the smectic C phase is about 50° (the layer thickness is 38.2 A for a molecular length l = 61 A in the case of IA (n = 12). The crystal structure of compound IB (n = 12) is monoclinic and the molecules adopt a zigzag form.

ORIENTATIONAL ORDERING IN SOME NEMATOGENS DEVIATING FROM THE CLASSICAL ROD-SHAPE

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be rou...

Crystal structure of a new biforked mesogen

Abstract The crystal structure of a biforked mesogen derived from 3,4-dialkyloxyphenyl propanoic acid is described. The molecule crystallizes in the P21/c space group with two molecules per unit cell: a = 50·740 (3), b = 8·425 (2), c = 9·250 (1)A and β = 91·340 (7)˚; the final reliability factor is R = 0·086. Molecules adopt a zig-zag form: the non-aliphatic central moiety is 36·2 A long; the two alkyloxy chains are stretched (ttt) and are approximately parallel to the xy plane; both chains make an angle close to 120° with the central moiety. Molecules make sheets of about 51 A thickness; the polyaromatic central core makes an angle close to 60° (tilt angle) with the normal to the sheet and is quasi-parallel to the xy plane; interactions between sheets are very weak. Here, we try to given an explanation of the transition between the crystalline arrangement of molecules and the hexagonal symmetry of the mesophase for this compound.

Evidence for lateral chain conformation change in the solid phase of substituted nematogens

Abstract A new homologous series of 4-(4-chlorobenzoyloxy)-2-alkoxy-3-methyl-4′-(4-trans-pentyl-cyclohexanoyloxy) azobenzenes has been synthesized. These compounds contain four rings in the main core, a lateral alkoxy branch and an adjacent lateral methyl group on one inner ring. They present a large nematic range and a solid-solid phase transition for the first members of the series. The crystal structures of two similar compounds containing a lateral butoxy chain have been solved. The HM4 compound (C31H33N2O5Cl) is derived from the series, but does not have the terminal pentyl chain while the PM4 compound (C36H43N2O5Cl) possesses this terminal chain. HM4 and PM4 crystallize, respectively, in P21/c (Z = 4) and P1 (Z = 2) space groups. The final reliability factors are R = 0.138 for HM4 (the terminal cyclohexyl ring is largely disordered) and R = 0.041 for PM4, for 2204 and 1963 observed reflections, respectively. The four ring central core is linear and rigid for both compounds. The conformation of the b...