ENG 5000-001: Intro to Methods in English Studies

The rheological behavior of a non-Brownian Newtonian concentrated suspension is investigated under oscillatory shear at different strain amplitudes and at two frequencies. The data are in agreement with the literature showing that the complex viscosity varies both with the number of oscillations imposed to the material and with the strain amplitude. We here also show that the response to the oscillatory shear depends on the applied frequency. This result is unexpected for a Newtonian suspension and implies the existence of some mechanism able to introduce a characteristic time into the system. Can it be cage elasticity

HPMC Hydrogel Formation Mechanisms Unveiled by the Evaluation of the Activation Energy

Aqueous solutions of hydroxypropyl methylcellulose (HPMC) show inverse thermoreversible gelation, i.e., they respond to small temperature variations exhibiting sol–gel transition during heating, and reversibly gel–sol transition during cooling. According to the pertinent literature on HPMC aqueous systems, at room temperature, the loss modulus (G”) is higher than the storage modulus (G’). During the heating ramp, the viscoelastic response follows a peculiar path: initially, G” and G’ smoothly decrease, then drop to a minimum and finally increase. Eventually, G’ overcomes G”, indicating the gel formation. A recent explanation of this behaviour considers a two‐step mechanism: first, phase separation occurs, then fibrils form from a polymer-rich phase and entangle, leading to a three‐dimensional network. Based on this, our research focuses on the rheological analysis of the different steps of the sol–gel transition of an HPMC aqueous solution. We perform different viscoelastic tests: thermal ramps, time sweeps, and frequency sweeps at selected characteristic temperatures. We couple classical analysis of the SAOS experiments with an innovative approach based on the evaluation of the activation energy (Ea), made possible by the instrument intrinsic temperature oscillations around the target value. Results show that Ea can be a valid tool that contributes to further clarifying the peculiar microstructural evolution occurring in this kind of thermoreversible gel

Metabolites of the new Caledonian sponge Cladocroce incurvata

The deep-water New Caledonian sponge #Cladocroce incurvata$ contains two "polyketide" metabolites. Cladocrocin A (1) appears to be derived from fatty acid with ethyl side chains, thus incorporating butyrate units. Cladocroic acid (2) is a straight chain fatty acid which incorporates a terminal enyne functionality and a cycloproprane ring directly attached to the carboxylic acid function. The structures were elucidated by interpretation of spectral data, and the cis stereochemistry of the cyclopropane ring in cladocroic acid (2) was derived after the synthesis of cis - and trans - 2, 3 - methanohexanoic acid models and nmr spectral comparisons. (Résumé d'auteur

A novel group of polyhydroxycholanic acid derivatives from the deep water starfish Styracaster caroli

Three novel polyhydroxysteroid constituents have been isolated from the starfish #Stracaster caroli$ collected at a depht of 2000 m off New Caledonia. These, designated carolisterols A - C (1 - 3), are characterized by a polyhydroxycholanic acid moiety, in which the 24-carboxylic acid function is found as an amide derivative ofD-cysteinolic acid. (Résumé d'auteur

Sterol composition of the "living fossil" crinoid Gymnocrinus richeri

The compn. of sterol mixt. from the living fossil crinoid G. richeri collected off Noumea (New Caledonia) was investigated. The free 3β-OH sterol mixt. was found to contain 14 components, Δ5 and ring satd. stanols, identified by GC-MS. Cholest-4-en-3-one, cholesta-1,4-dien-3-one, 5α-8α-epidioxy sterols, and 5α-ergosta-7,22-diene-3β,5,6β-triol were also present, their characterization being accomplished by EI-MS and 1H-NMR. The methanol ext. also contained sterol sulfates, which were identified by GC-MS after solvolysis to remove the sulfate group