Rotational spectrum of cis-cis HOONO

The pure rotational spectrum of cis-cis peroxynitrous acid, HOONO, has been observed. Over 220 transitions, sampling states up to J'=67 and Ka'=31, have been fitted with an rms uncertainty of 48.4 kHz. The experimentally determined rotational constants agree well with ab initio values for the cis-cis conformer, a five-membered ring formed by intramolecular hydrogen bonding. The small, positive inertial defect Delta=0.075667(60) amu A2 and lack of any observable torsional splittings in the spectrum indicate that cis-cis HOONO exists in a well-defined planar structure at room temperature.     

Kinetic evidence for the existence and mechanism of formation of a barrel stave structure from pore-forming dendrimers

A dendritic approach to the construction of a homologous series of pore-forming amphiphiles has been developed, based on the use of spermidine, spermine, lysine, and cholic acid. A kinetic analysis of Na+ transport across bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine by three dendritic amphiphiles has provided the strongest evidence to date for a barrel stave structure.     

Ligand manipulation and design for ruthenium metathesis and tandem metathesis-hydrogenation catalysis

Results of investigations into tandem ring-opening metathesis polymerization (ROMP)-hydrogenation are reviewed, in which hydrogen and 3-chloro-3-methyl-1-butyne provide simple chemical toggles to switch between metathesis and hydrogenation chemistry, enabling multiple tandem catalysis in chlorocarbon solvent. In the presence of methanol, hydrogenation of metathesis polymers can be carried out under 1 atm H₂. Issues of ligand design are examined in developing new Ru-diphosphine catalysts with improved selectivity, and an important decomposition pathway is identified for RuCl₂(PP)(CHR) systems (PP=chelating diphosphine).     

A quantitative assessment of the influence of permanent kinks on the mixing behavior of phospholipids in cholesterol-rich bilayers

The mixing behavior of a phospholipid containing a cis-cyclopropyl moiety (1) with one that contains two myristoyl groups (3a) has been investigated in fluid bilayers via the nearest-neighbor recognition (NNR) method. In the absence of cholesterol, these lipids mix ideally. In the presence of cholesterol, they show a modest preference for homo-phospholipid association. A trans-form of 1 (i.e., 2) was found to have similar behavior, except that the influence of cholesterol in promoting homo-phospholipid association was greater. Similar results have been found in membranes in which 3a is replaced with a phospholipid bearing two palmitoyol chains (3b). In this case, the effect of the kink is approximately twice as great. The implications of these findings, with respect to the "trans-fatty acid debate", are briefly discussed     

Probing the hydration of lipid bilayers using a solvent isotope effect on phospholipid mixing

Nearest-neighbor recognition experiments, which have been carried out using exchangeable dimers derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine,and 1-palmitoyl-2-oleoyl-sn-glycerophosphoethanolamine, indicate that replacement of H2O by D2O can significantly influence phospholipid mixing, but only in bilayers that are saturated and devoid of cholesterol. These findings, together with those of previous electron spin resonance spin-labeling studies,indicate that mammalian membranes, which are rich in cholesterol and unsaturated phospholipids, are ideal hydrophobic barriers.     

Glued Langmuir-Blodgett bilayers having unusually high He/CO2 permeation selectivities

Single Langmuir-Blodgett bilayers derived from 5,11,17,23,29,35-hexakis[(N,N,N-trimethylammonium)-N-methyl-37,38,39,40,41,42-hexakis-n-hexamedecyloxy-calix[6]arene hexachloride (1), which have been ionically cross-linked (i.e., "glued together") with poly(acrylic acid), have been found to exhibit He/CO2 permeation selectivities as high as 150. This degree of selectivity, for a membrane that is less than 6 nm in thickness, is without precedent. In principle, materials of this type could lead the way to improved membranes for hydrogen purification.     

Total synthesis of 7-deoxypancratistatin-1-carboxaldehyde and carboxylic acid via solvent-free intramolecular aziridine opening: phenanthrene to phenanthridone cyclization strategy

Solid-state silica-gel-catalyzed opening of aziridine 6 provided phenanthrene 7, whose oxidative cleavage, recyclization, and further elaboration furnished the C-1 aldehyde and carboxylic acid derivatives of 7-deoxypancratistatin for potential analogue synthesis.     

Thermally gated liposomes

The combined use of a pore-forming amphiphile, 1 (derived from lysine, cholic acid, and spermine), and thermally sensitive liposomes (made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) to create "thermally gated liposomes" (TGLs) has been demonstrated. Thus, at temperatures that lie above the gel to liquid-crystalline phase transition temperature of DPPC (i.e., 41 degrees C), 1 creates pores within DPPC membranes through which entrapped aqueous solutes, such as carboxyfluorescein, can readily pass. Below this temperature, efflux rates are greatly reduced. The potential of TGLs as devices for the targeted delivery of therapeutic agents is briefly discussed.     

Self-cleaning resins

This paper introduces a fundamentally new concept in adsorbents, whereby the sorption of an aqueous solute by a cross-linked polymer is controlled by a gel to liquid-crystalline phase transition. To demonstrate proof of principle, a bilayer forming surfactant, N,N-dioctadecyl,N,N-dimethylammonium bromide (DODAB) has been immobilized onto a cation exchange resin, Dowex 50WX2, and its thermotropic phase behavior and solute-adsorption properties have been investigated. Examination by a combination of differential scanning calorimetry and X-ray scattering has confirmed the retention of a gel to liquid-crystalline phase transition of the surfactant, occurring between 296 and 318 K. Adsorption measurements that were made for 4-chlorotetrahydropyran, 1,2-dichloroethane, and benzyl alcohol have also confirmed uptake by the resin in the liquid-crystalline phase and release in the gel phase.