Donald B. DuPré

Conformational studies of sphingolipids by NMR spectroscopy. I. Dihydrosphingomyelin

The conformational features of dihydrosphingomyelin (DHSM), the major phospholipid of human lens membranes, were investigated by 1H and 31P nuclear magnetic resonance spectroscopy. Several postulates emerge from the observed trends: (a) in partially hydrated samples of DHSM in CDCl3 above 13 mM, at which lipid-lipid interactions prevail, the amide proton is mostly involved in intermolecular H-bonds that link neighboring phospholipids through bridging water molecules. In the absence of water, the NH group is involved in an intramolecular H-bond that restricts the mobility of the phosphate group. (b) In the monomeric form of the lipid molecule, the amide proton of the major conformer is bound intramolecularly with one of the anionic and/or ester oxygens of the phosphate group. A minor conformer may also be present in which the NH proton participates in an intramolecular H-bond linking to the OH group of the sphingoid base. (c) Complete hydration leads to an extension of the head group as water molecules bind to the phosphate and NH groups via H-bonds, thus disrupting the intramolecular H-bonds prevalent at low concentrations.

Temperature, concentration, and molecular weight dependence of the twist elastic constant of cholesteric poly‐γ‐benzyl‐L‐glutamate

Aspects of the behavior of the twist elastic modulus K22 of a lyotropic liquid crystal formed by the α‐helical polypeptide poly‐γ‐benzyl‐L‐glutamate of three molecular weights (MW= 300 000, 310 000, and 550 000) in three solvents (dioxane, dichloromethane, and chloroform) were investigated as a function of concentration and temperature. We find that K22 in this system may be made to vary over a wide range by adjustment of solvent and polymer molecular weight. Only a slight concentration dependence of K22 throughout the liquid crystal range was, however, noted. The variations can be at least partially understood in terms of changing macromolecular interactions and recent statistical hard‐rod theoretical treatments of liquid crystal elasticity. The functional form for the temperature dependence of the order parameter was also determined for the system through the clearing point.

Liquid crystalline properties of solutions of persistent polymer chains

A synthesis of theories for the orientational entropy and excluded volume interaction for persistent chain polymers at intermediate degrees of chain flexibility, when extended to liquidlike densities, is shown to satisfactorily account for experimental data on the volume fractions of polymer at coexistence of isotropic and anisotropic phases, the concentration dependence of the order parameter within the lyotropic liquid crystal and the osmotic pressure equation of state for several extended chain polymers. Results of this continuum model are also compared with the virial approximation and treatments of the macromolecule as a rigid rod.

Conformational characterization of ceramides by nuclear magnetic resonance spectroscopy.

Ceramide (Cer) has been identified as an active lipid second messenger in the regulation of cell growth, differentiation, and apoptosis. Its analog, dihydroceramide, without the 4 to 5 trans double bond in the sphingoid backbone lacks these biological effects. To establish the conformational features that distinguish ceramide from its analogs, nuclear magnetic resonance spectral data were acquired for diluted samples of ceramides (C2- and C18-Cer), dihydroceramide (C16-DHCer), and deoxydihydroceramide (C18-DODHCer). Our results suggest that in both C2- and C18-Cer, an H-bond network is formed in which the amide proton NH is donated to the OH groups on carbons C1 and C3 of the sphingosine backbone. Two tightly bound water molecules appear to stabilize this network by participating in flip-flop interactions with the hydroxyl groups. In DHCer, the lack of the trans double bond leads to a conformational distortion of this H-bonding motif. Without the critical double bond, the degree with which water molecules stabilize the H bonds between the two OH groups of the sphingolipid is reduced. This structural alteration might preclude the participation of DHCer in signaling-related interactions with cellular targets.

Twist elastic constant of a cholesteric polypeptide liquid crystal

We have determined the critical magnetic field for the cholesteric to nematic transition of a pure lyotropic liquid crystal solution of an α‐helical polypeptide, poly‐γ‐benzyl‐L‐glutamate, by direct microscopic observation of the changing pitch of the structure as a function of magnetic field strength. The course of the transition is well accounted for by the theory of deGennes. An independent measurement of the anisotropy of diamagnetic susceptibility allows the determination of κ22, the twist elastic constant of the medium.

The determination of the elastic constants of a series of n‐alkylcyanobiphenyls by anisotropy of turbidity

Measurement of the anisotropy of turbidity of aligned nematic liquid crystals in three selected geometries can yield the splay, twist, and bend elastic constants (K11, K22, and K33, respectively) which, along with the viscosity coefficients of the medium, govern director fluctuations that lead to a strong light scattering. We have utilized this procedure to determine the temperature dependence of the elastic constants of a homologous series of n‐alkycyanobiphenyls (n‐CB, where n=5, 6, 7, 8) in the nematic range. K33≳K11≳K22 at all temperatures for each member of the series. The absolute values of the moduli are consistently higher than those obtained by magneto‐ and electro‐optic distortion methods (Frederik’s transitions). Odd–even effects are displayed as the number of methylene carbons in the alkyl chain increases. Distinctly larger K33 values for 8CB may be attributed to the short range smectic ordering in this liquid crystal, which has a lower temperature smectic‐A phase.

Light scattering of human lens vesicles in vitro

In passing through the lens, light crosses thousands of cell membranes. To explore the possible contribution of lipids to the scattering properties of the lens, we have carried out in vitro studies with lipids extracted from human lenses 1-90 years of age. Sphingomyelin and human lens lipids were extruded into large unilamellar vesicles (LUVs). The intensity of light scattered by human lens LUVs increased with age and lipid hydrocarbon chain order. Hydrocarbon chain order also correlated with light scattering intensity by sphingomyelin LUVs. Light scattered by LUVs composed of sphingomyelin (1-30 mg ml(-1)) was 20 to 100 times more intense than that scattered by the same concentration of alpha-crystallin in aqueous media. Increased lipid hydrocarbon chain order as well as variations in the headgroup and interfacial region of bilayers resulting from lipid compositional changes can influence membrane light scattering properties. In vitro measurements suggest that the contribution to light scattering by lipids may be significant and should not be disregarded in the investigation of factors and components that lead to the increase in light scattering by human lenses with age and cataract.

Temperature Dependence of the Anisotropy of Turbidity and Elastic Constants of Nematic Liquid Crystal Mixture E7

Abstract The three principal elastic constants, K 11, K 11, and K 33, of the eutectic nematic liquid crystal mixture E7 have been determined as a function of temperature by following changes in the anisotropy of turbidity of a magnetically aligned specimen. The agreement with values of K 22 obtained by a previous capacitance-voltage method is excellent, whereas the values of K 11 and K 33 derived from the absorption coefficients are systematically higher.

Optical rotatory behavior of polypeptide solutions in the liquid crystal and pretransitional regions

The optical rotatory behavior of solutions of the synthetic polypeptides poly‐γ‐benzyl‐l‐glutamate (PBLG) and poly‐γ‐benzyl‐d‐glutamate (PBDG) was studied in various solvents as a function of wavelength, concentration of polymer and temperature. The specific rotation in the liquid crystal was found to be inversely proportional to the pitch of the cholesteric structure and therefore does not follow the deVries equation previously thought to describe the optical rotatory dispersion of these liquid crystals. Solutions of PBLG and PBDG were also examined in the pretransitional region at concentrations just below the critical point for the formation of cholesteric spherulites. An enhancement of the optical rotatory power was observed which is attributable to short range chiral ordering of the long axes of the macromolecules. The sense of the emergent chiral ordering is strongly solvent dependent but the effects may be separated into three solvent classifications. Residual form optical activity was also found i...

The Effect of Trifluoroacetic Acid on the Viscoelastic Properties of a Polypeptide Liquid Crystal

Abstract The viscoelastic properties of a liquid crystal are characterized by a set of elastic moduli, Kij , and viscosity coefficients, yi . that are fundamental in the establishment of the mesophase. Together with the anisotropic diamagnetic (dielectric) susceptibility, these parameters determine the response of the material to external perturbations such as applied magnetic (electric) fields. The measurement of the twist elastic modulus, K 22, and the rotational viscosity coefficient, y 1, has been recently reported for lyotropic liquid crystalline solutions of the polypeptide poly-y-benzyl-L-glutamate. This system is of particular interest in that a number of molecular parameters (e.g. supporting solvent and molecular weight, i.e. length) can be easily varied for quantitative comparison with theory. We have examined K22. y1 , and the diamagnetic susceptibility in the presence of small amounts of a second solvent, trifluoroacetic acid (TFA), which is known to disrupt the α-helical conformation of this ...