Biocompatible lipidic formulations: phase behavior and microstructure

Biocompatible systems formulated for use in the food, cosmetic, and pharmaceutical fields are characterized. Ternary phase diagrams of mixtures of natural lipids (glycerol trioleate, glycerol monooleate, diglycerol monooleate, and lecithin) and water were investigated by means of optical microscopy in polarized light and by multinuclear NMR spectroscopy. All systems showed a microemulsion region at high oil content and a large area of coexistence of two liquid crystalline (hexagonal and lamellar) phases. 1H and 13C NMR self-diffusion measurements were used to characterize microstructural features of the microemulsions. On water dilution, the two-phase liquid crystalline region transforms into a creamy emulsion area where the droplets of water are stabilized by both the lamellar and the hexagonal phases, as indicated by 2H NMR measurements. Due to the very effective dispersing action of the two liquid crystalline phases, these emulsions show a high stability toward phase separation.     

Understanding the role of stereoelectronic effects in determining collagen stability. 2. A quantum mechanical/molecular mechanical study of (Proline-Proline-Glycine)(n) polypeptides

The importance of vicinal and long-range interresidue effects in determining the stability of the collagen triple helix has been investigated by quantum mechanical (QM) and molecular mechanical (MM) computations on suitable model polypeptides, taking into account solvent effects by the polarizable continuum model (PCM). At the QM level, the PII conformation corresponds to an energy minimum for pentapeptide analogues incorporating the sequence Gly-Pro-Pro-Gly, irrespective of the down or up puckering of the pyrrolidine ring. However, our computations indicate that the alternation of down and up prolines characterizing collagen and collagen-like peptides is not due to an intrinsic preference of the Pro-Pro-Gly sequence. This result is confirmed by MM computations of longer polypeptides. Next, MM computations on model triple helices show that a better packing is obtained for specific values of backbone dihedrals, which, in turn, favor the alternation of down and up prolines along each chain.     

Thermal and Microstructural Analysis of the Y–Ba–Cu–O/Ag Equilibrium Around the Critical Composition YBa2Cu3Ox/Ag 35 mass%

We studied the Y–Ba–Cu–O/Ag equilibrium diagram in oxygen atmosphere around the composition YBa₂Cu₃O_x/Ag35 mass%. We found a high thermal effect: the peritectic decomposition temperature of Y-123 phase is lowered from 1040 to 940°C. We demonstrate here that the nature of the phenomenon is not chemical. We explained it as the result of a mechanical segregation of Y-123 decomposition products from Y-123 phase, performed by silver. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quantum spin Hall effect in graphene

We study the effects of spin orbit interactions on the low energy electronic structure of a single plane of graphene. We find that in an experimentally accessible low temperature regime the symmetry allowed spin orbit potential converts graphene from an ideal two-dimensional semimetallic state to a quantum spin Hall insulator. This novel electronic state of matter is gapped in the bulk and supports the transport of spin and charge in gapless edge states that propagate at the sample boundaries. The edge states are nonchiral, but they are insensitive to disorder because their directionality is correlated with spin. The spin and charge conductances in these edge states are calculated and the effects of temperature, chemical potential, Rashba coupling, disorder, and symmetry breaking fields are discussed.     

Continuum theory for nanotube piezoelectricity

We develop and solve a continuum theory for the piezoelectric response of one-dimensional nanotubes and nanowires, and apply the theory to study electromechanical effects in boron-nitride nanotubes. We find that the polarization of a nanotube depends on its aspect ratio, and a dimensionless constant specifying the ratio of the strengths of the elastic and electrostatic interactions. The solutions of the model as these two parameters are varied are discussed. The theory is applied to estimate the electric potential induced along the length of a boron-nitride nanotube in response to a uniaxial stress.     

Kinetics and mechanism of the thermal reactions of endo- and exo-5-cyanobicyclo[2.2.2]oct-2-ene and methyl-substituted derivatives in the gas phase

The thermal reactions of endo- and exo-5-cyanobicyclo-[2.2.2]oct-2-ene and their trans- and cis-6-methyl-substituted derivatives have been investigated in the gas phase between 518 and 630 K. Each product decomposes by two parallel first-order retro-Diels-Alder reactions, a main one with formation of cyclohexa-1,3-diene and a minor one with elimination of ethene. Slight isomerizations are also observed. The kinetic results can be explained in terms of a biradical mechanism. The rate-determining step is shown to depend on the amount of resonance energy in the biradical. Heats of formation and entropies of the bicyclo[2.2.2]oct-2-enes studied are estimated.     

An SFG/DFG investigation of CN− adsorption at an Au electrode in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl) amide ionic liquid

In this paper we report an SFG/DFG investigation of the adsorption of CN^? – used as a probe molecule to study the electrochemical double-layer structure – at a polycrystalline Au electrode in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl) amide ([BMP][TFSA]) room-temperature ionic liquid (RTIL). The adsorption of CN^? yielded single SFG and DFG bands in the range from ca. 2125 to 2135 cm^?1, exhibiting a Stark tuning of ca. 3 cm^?1 V^?1. Vibrational resonances – corresponding to modes of the RTIL coadsorbed with CN^?, were found in the range from ca. 1200 to 1500 cm^?1. The study of the double-layer structure was complemented by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, from which the differential double-layer capacity (C_DL) was estimated.