An NMR and quantum-mechanical investigation of tetrahydrofuran solvent effects on the conformational equilibria of 1,4-butanedioic acid and its salts

Vicinal proton-proton NMR couplings have been used to compare the influences of water and tetrahydrofuran (THF) as solvents on the conformational equilibria of 1,4-butanedioic (succinic) acid and its mono- and dianionic salts. An earlier NMR investigation (Lit, E. S.; Mallon, F. K.; Tsai, H. Y.; Roberts, J. D. J. Am Chem. Soc. 1993, 115, 9563-9567) showed that, in water, the conformational preferences for the gauche conformations for butanedioic acid and its monoanion and dianion were, respectively, approximately 84%, 66%, and 43%, essentially independent of the nature of the cation or concentration. We now report the corresponding gauche percentages calculated in the same way for 0.05 M solutions in THF to be 66%, 90-100%, and 46-64%. Substantial evidence was adduced for the rotational angle between the substituents in the monoanion being approximately 70 degrees. The positions of conformational equilibria of the salts in THF, particularly of the dianion, were found to be rather insensitive to concentration and temperature, but more sensitive to the amount of water present. Ab initio quantum-mechanical calculations for 1,4-butanedioate dianion indicate that, as expected for the gas phase, the trans conformation of the dianion should be heavily favored over the gauche, but, in both THF and water, the gauche conformation is calculated to predominate with rotational angles substantially less than 60 degrees. This conclusion is, in fact, generally consistent with the experimental vicinal proton couplings, which are wholly inconsistent with the trans conformation.     

Collisions of slow polyatomic ions with surfaces: dissociation and chemical reactions of C2H2+*, C2H3+, C2H4+*, C2H5+, and their deuterated variants C2D2+* and C2D4+* on room-temperature and heated carbon surfaces

Interaction of C2Hn+ (n = 2-5) hydrocarbon ions and some of their isotopic variants with room-temperature and heated (600 degrees C) highly oriented pyrolytic graphite (HOPG) surfaces was investigated over the range of incident energies 11-46 eV and an incident angle of 60 degrees with respect to the surface normal. The work is an extension of our earlier research on surface interactions of CHn+ (n = 3-5) ions. Mass spectra, translational energy distributions, and angular distributions of product ions were measured. Collisions with the HOPG surface heated to 600 degrees C showed only partial or substantial dissociation of the projectile ions; translational energy distributions of the product ions peaked at about 50% of the incident energy. Interactions with the HOPG surface at room temperature showed both surface-induced dissociation of the projectiles and, in the case of radical cation projectiles C2H2+* and C2H4+*, chemical reactions with the hydrocarbons on the surface. These reactions were (i) H-atom transfer to the projectile, formation of protonated projectiles, and their subsequent fragmentation and (ii) formation of a carbon chain build-up product in reactions of the projectile ion with a terminal CH3-group of the surface hydrocarbons and subsequent fragmentation of the product ion to C3H3+. The product ions were formed in inelastic collisions in which the translational energy of the surface-excited projectile peaked at about 32% of the incident energy. Angular distributions of reaction products showed peaking at subspecular angles close to 68 degrees (heated surfaces) and 72 degrees (room-temperature surfaces). The absolute survival probability at the incident angle of 60 degrees was about 0.1% for C2H2+*, close to 1% for C2H4+* and C2H5+, and about 3-6% for C2H3+.     

New ligand systems incorporating two and three 4,4'-bipyridine units. Characterization of bi- and trimetallic rhodium and iridium complexes

The synthesis of new ligand systems based on the bipyridine unit for bi- and trimetallic complexes, including a rare example of a chiral bimetallic complex, is presented. Ligands BBPX (bis-bipyridine-xylene, 3) and TBPBX (tris-bipyridine-bis-xylene, 4) were prepared in one step by reacting alpha,alpha'-dibromo-o-xylene (2) with 2 equiv of the monolithiated derivative of 4,4'-dimethyl-2,2'-bipyridine. Dilithium (S)-binaphtholate (5) reacted with 2 equiv of 4-bromomethyl-4'-methyl-2,2'-bipyridine (6), affording ligand (S)-BBPBINAP (bis-bipyridine-binaphtholate, 7). These ligands reacted cleanly with 1, 1.5, and 1 equiv of the rhodium dimer [Rh(2)Cl(2)(HD)(2)] (HD = 1,5-hexadiene), respectively. Chloride abstraction led to the isolation of the cationic complexes BBPX[Rh(HD)BF(4)](2) (8), TBPBX[Rh(HD)BF(4)](3) (10), and (S)-BBPBINAP[Rh(HD)BF(4)](2) (12). When BBPX (3), TBPBX (4), and (S)-BBPBINAP (7) were added to 2, 3, and 2 equiv of [Rh(NBD)(2)]BF(4) or [Rh(NBD)(CH(3)CN)(2)]BF(4) (NBD = norbornadiene), respectively, clean formation of BBPX[Rh(NBD)BF(4)](2) (9), TBPBX[Rh(NBD)BF(4)](3) (11), and (S)-BBPBINAP[Rh(NBD)BF(4)](2) (13) was observed. The neutral iridium complex (S)-BBPBINAP[IrCl(COD)](2) (14) was obtained by reaction of (S)-BBPBINAP (7) with 1 equiv of [Ir(2)Cl(2)(COD)(2)] (COD = cyclooctadiene). The complexes were fully characterized including X-ray structural studies of 8, 9, and 13, and preliminary studies on their catalytic activity were performed.     

Growth and Dissolution of Organic Crystals with “Tailor-Made” Inhibitors—Implications in Stereochemistry and Materials Science

When crystals of organic compounds are grown in the presence of growth inhibitors, there is a change in crystal morphology. A stereochemical correlation exists between the crystal structure, its modified morphology, and the molecular structure of the inhibitor. This correlation has been successfully exploited for the efficient resolution of conglomerates, the engineering of organic crystals with desired morphologies, the direct and relative assignment of the absolute configurations of chiral molecules and crystals, and for the design of a new model for the spontaneous generation of optical activity. In an analogous way dissolution of organic crystals in the presence of these growth inhibitors induces etch pits at preselected faces. The effect of solvent on crystal growth has been analyzed in some model systems. The experimental results are complemented by atom-atom potential energy calculations.     

Detection of paralytic shellfish poison by rapid cell bioassay: antagonism of voltage-gated sodium channel active toxins in vitro

Although cytotoxicity assays provide several advantages over mouse bioassays, sodium channel-blocking marine toxins, such as those associated with paralytic shellfish poison (PSP), require prolonged incubation periods of 24-48 h. This is in marked contrast to in vitro detection of sodium channel-enhancing marine toxins such as ciguatoxins or brevetoxins which can be accomplished in as few as 4-6 h. We developed a modified PSP cell bioassay that is as rapid as in vitro methods for sodium channel-enhancing toxins. The cell bioassay is based on a saxitoxin-dependent antagonism of the rapid in vitro effects of brevetoxin or ciguatoxin. Comparative analysis of naturally incurred PSP residues by both antagonism cell bioassay and the mouse bioassay demonstrated significant correlation. The simplicity, sensitivity, and enhanced kinetics of the new antagonism cell bioassay format provide the basis for development of a practical alternative to conventional mouse testing for PSP.     

Red and yellow solvates of chloro­(2,2′:6′,2′′‐terpyridine)platinum(II) chloride and Pt⋯Pt inter­actions

Crystallization of chloro­(2,2′:6′,2′′‐terpyridine)platinum(II) chloride from dimethyl sulfoxide yields a red polymorph, [PtCl(C₁₅H₁₁N₃)]Cl·C₂H₆OS, (I), which exhibits stacking along the a axis through pairs of Pt⋯Pt(−x, −y, −z) inter­actions of 3.3155 (8) Å. The cations are further associated through close Pt⋯Pt(1 − x, −y, −z) distances of 3.4360 (8) Å. Recrystallization from water gives a mero­hedrally twinned yellow–orange dihydrate form, [PtCl(C₁₅H₁₁N₃)]Cl·2H₂O, (II), with pairwise short Pt⋯Pt(1 − x, 2 − y, −z) contacts of 3.3903 (5) Å but no long‐range stacking through the crystals. Inter­pair Pt⋯Pt(−x, 2 − y, −z) distances between cation pairs in the hydrate are 4.3269 (5) Å.     

Artificial photosynthetic reaction centers: mimicking sequential electron and triplet-energy transfer.

An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylporphyrin (P), and a bis(heptafluoropropyl)porphyrin (P(F)), C-P-P(F) , and the related triad in which the central porphyrin has been metalated to give C-P(Zn)-P(F) have been synthesized and characterized by transient spectroscopy. These triads are models for amphipathic triads having a carboxylate group attached to the P(F) moiety; they are designed to carry out redox processes across lipid bilayers. Triad C-P-P(F) undergoes rapid singlet-singlet energy transfer between the porphyrin moieties, so that their excited states are in equilibrium. In benzonitrile, photoinduced electron transfer from the first excited singlet state of P and hole transfer from the first excited singlet state of P(F) yield the initial charge-separated state C-P(.) (+)-P(F) (.) (-). Subsequent hole transfer to the carotenoid moiety generates the final charge-separated state C(.) (+)-P-P(F) (.) (-), which has a lifetime of 1.1 mus and is formed with a quantum yield of 0.24. In triad C-P(Zn)-P(F) energy transfer from the P(Zn) excited singlet to the P(F) moiety yields C-P(Zn)-(1)P(F) . A series of electron-transfer reactions analogous to those observed in C-P-P(F) generates C(.) (+)-P(Zn)-P(F) (.) (-), which has a lifetime of 750 ns and is formed with a quantum yield of 0.25. Flash photolysis experiments in liposomes containing an amphipathic version of C-P(Zn)-P(F) demonstrate that the added driving force for photoinduced electron transfer in the metalated triad is useful for promoting electron transfer in the low-dielectric environment of artificial biological membranes. In argon-saturated toluene solutions of C-P-P(F) and C-P(Zn)-P(F) , charge separation is not observed and a considerable yield of triplet species is generated upon excitation of the porphyrin moieties. In both triads triplet energy localized in the P(F) moiety is channeled to the carotenoid chromophore by a triplet energy-transfer relay mechanism. Certain photophysical characteristics of these triads, including the sequential electron transfer and the triplet energy-transfer relay mechanism, are reminiscent of those observed in natural reaction centers of photosynthetic bacteria.     

Metallophilic interactions in iodido(2,2′:6′,2′′‐terpyridine)platinum(II) diiodidoaurate(I)

In the title compound, [PtI(C₁₅H₁₁N₃)][AuI₂], the [PtI(terpy)]⁺ cations (terpy is 2,2′:6′,2′′‐terpyridine) stack in pairs about inversion centers through Pt...Pt interactions of 3.5279 (5) Å. The [AuI₂]⁻ anions also exhibit pairwise stacking, with Au...I distances of 3.7713 (5) Å. The [PtI(terpy)]⁺ cations and [AuI₂]⁻ anions aggregate forming infinite arrays of stacked ...({[PtI(terpy)]⁺...[PtI(terpy)]⁺}...{[AuI₂]⁻...[AuI₂]⁻})... units.     

Detection of Salmonella in fresh cheese,poultry products,and dried egg products by the ISO 6579 Salmonella culture procedure and the AOAC official method: collaborative study

Three food types were analyzed for the presence of Salmonella by the AOAC culture method and by the International Organization for Standardization (ISO 6579:2002) culture method. Paired test portions of each food type were simultaneously analyzed by both methods. A total of 21 laboratories representing federal government agencies and private industry, in the United States and Europe, participated in this interlaboratory study. Foods were artificially contaminated with Salmonella and competing microflora if naturally contaminated sources were not available. No statistical differences (p < 0.05) were observed between the AOAC and ISO culture methods for fresh cheese and dried egg products. A statistically significant difference was observed for one of the 2 lots of poultry from the first trial. The poultry meat used in this run was radiation sterilized, artificially contaminated with Salmonella and competitive flora, and then lyophilized. A second trial was conducted with 2 separate lots of raw ground chicken that were naturally contaminated. The results from the second trial showed no statistical difference between the 2 culture methods. A third trial involving 4 laboratories was conducted on 2 separate lots of naturally contaminated raw poultry. Again, no statistically significant differences occurred. It is recommended that ISO 6579:2002 culture method for Salmonella be adopted Official First Action for the analysis of fresh cheese, fresh chilled and frozen poultry, and dried egg products.     

Method extension study to validate applicability of AOAC Official Method 996.14 Assurance polyclonal enzyme immunoassay for detection of Listeria monocytogenes and related Listeria spp. from environmental surfaces: collaborative study

Test portions from 3 environmental surface types, representative of typical surfaces found in a food production facility, were analyzed by the Assurance Listeria Polyclonal Enzyme Immunoassay (EIA) and the U.S. Department of Agriculture/Food Safety and Inspection Service (USDA/FSIS) culture method for Listeria monocytogenes and related Listeria species. In all cases, naturally contaminated environmental test samples were collected from an actual food production facility by sponge or swab. Test samples from concrete surfaces were collected by both swab and sponge; sponge test samples were collected from rubber surfaces, and swabs were used to sample steel surfaces. Test portions from each surface type were simultaneously analyzed by both methods. A total of 23 collaborators, representing government agencies, as well as private industry in both the United States and Canada, participated in the study. During this study, a total of 550 test portions and controls was analyzed and confirmed, of which 207 were positive and 336 were negative by both methods. Six test portions were positive by culture, but negative by the EIA. Three test portions were negative by culture, but positive by the EIA. Two test portions were negative by EIA and by culture, but confirmed positive when EIA enrichment broths were subcultured to selective agars. The data reported here indicate that the Assurance Listeria EIA method and the USDA/FSIS culture method are statistically equivalent for detection of L. monocytogenes and related Listeria species from environmental surfaces taken by sponges or swabs.