Fermentation of a skim milk concentrate with Streptococcus thermophilus and chymosin: structure, viscoelasticity and syneresis of gels

Fermentation of a reconstituted skim milk concentrate (8% protein) was investigated to elucidate the effects of various fermentation parameters on the structural, rheological and visual (wheying-off) properties of the resulting gels (pH 4.60). Fermentation trials were performed with non-exocellular polysaccharide-producing strains of Streptococcus thermophilus at various fermentation temperatures and at various chymosin levels. Oscillatory vane rheometry carried out on the intact gels (at 4 °C) showed that the level of chymosin had a significant impact on the gel strength (storage modulus G′). This can be explained by the arrangement of casein micelles into more compact aggregates and the enhanced fusion of aggregated casein micelles as revealed by transmission electron microscopy for the gels fermented with chymosin. Wheying-off of the stirred gels as measured by a centrifugation test (at 4 °C) and pore size of the intact gel structures investigated by scanning electron microscopy both increased with increasing level of chymosin and increasing fermentation temperature (resulting in an increase in acidification rate). A higher level of syneresis (wheying-off) is explained by the larger pore size, since larger pores present a lower resistance to the outflow of whey from the gel.     

Syntheses of seven-membered rings: ruthenium-catalyzed intramolecular [5+2] cycloadditions

The Ru-catalyzed intramolecular [5+2] cycloaddition of cyclopropylenynes is investigated with respect to the regio- and diastereoselectivity as well as the functional group compatibility of the reaction. Evidence for the mechanism as occurring through a ruthenacyclopentene intermediate is elucidated from 1) the study of the diastereoselectivity of the cycloaddition; 2) the effect of variation of substituents on the regioselectivity of cyclopropyl bond cleavage in 1,2-trans- and 1,2-cis-disubstituted cyclopropanes and 3) examples that clearly do not involve ruthenacyclohexene as intermediates as products still incorporate the cyclopropyl moiety. The scope and limitations of the Ru-catalyzed cycloaddition are discussed and compared with the Rh-catalyzed reaction. The potential power of this methodology towards natural product total synthesis is demonstrated by the formation of several polycyclic systems with the chosen reaction conditions and readily available cyclopropylenyne substrates.     

A heterocyclic peptide nanotube

An open-ended hollow tubular structure is designed based on hydrogen-bond-directed self-assembly of a chimeric cyclic peptide subunit comprised of alternating alpha- and epsilon-amino acids. The design features a novel 1,4-disubstituted-1,2,3-triazole epsilon-amino acid and its utility as a peptide backbone substitute. The N-Fmoc-protected epsilon-amino acid was synthesized in high yield and optical purity in three steps from readily available starting materials and was employed in solid-phase peptide synthesis to afford the desired cyclic peptide structure. The cyclic peptide self-assembly has been studied in solution by (1)H NMR and mass spectrometry and the resulting tubular ensemble characterized in the solid state by X-ray crystallography.     

Penta­chloro‐1,3,6‐tris­(diethyl­phenyl­phosphino)­dirhenium(II,III)

The title complex [systematic name: penta­chloro‐1κ³Cl,2κ²Cl‐tris(diethylphenylphosphino)‐1κP,2κ²P‐dirhenium(II,III)(Re—Re)], 1,3,6‐Re₂Cl₅(PEt₂Ph)₃ or [Re₂Cl₅(C₁₀H₁₅P)₃], consists of dirhenium mol­ecules with eclipsed structures similar to those of previously characterized 1,3,6‐Re₂Cl₅(PR₃)₃ compounds. The Re—Re bond distance is 2.2262 (3) Å and the metal–metal bond order is 3.5.     

Carbon-13 NMR microstructural characterization of dichlorocarbene adducts of polybutadiene

¹³C-NMR has been used to examine a number of dichlorocarbene adducts of cis- and trans-polybutadiene prepared in a two-phase system. Dichlorocarbene was generated by reaction of aqueous or solid NaOH or KOH with CHCl₃ in the presence of a phase transfer catalyst. Monomer compositions, comonomer sequence lengths, and stereochemical information were obtained for the resulting polymers. The polymers examined here were stereochemically pure and were treated as simple copolymers. Samples prepared using aqueous NaOH can be described as essentially random copolymers over the entire range of monomer composition. Samples prepared using solid alkali-metal hydroxides contain a higher fraction of blocked units than a polymer of comparable composition prepared using aqueous NaOH. This blockiness can coincide with the presence of two glass transition temperatures and a two-phase morphology as seen by transmission electron microscopy. Fractionation of a substantially blocked sample yielded a chlorine-poor fraction which was a random copolymer and a chlorine-rich fraction which was more blocked than the original unfractionated material.