Search results for "Quaternary"

Probable root structures and associated trace fossils from the Lower Pleistocene calcarenites of favignana island, southern italy: dilemmas of interp…

2012

Two types of large, branched structures from the Lower Pleistocene (Calabrian) high-energy calcarenites of Favignana Island are described: Faviradixus robustus gen. et sp. nov. and Egadiradixus rectibrachiatus gen. et sp. nov. They may be interpreted as root structures of large plants, trees and trees or shrubs, respectively. The former taxon co-occurs with the marine animal trace fossils Ophiomorpha nodosa , Ophiomorpha isp., Thalassinoides isp. and Beaconites isp. The interpretation as root structures although tentative is probable and can be related to short emergence episodes for the formation of E . rectibrachiatus or to longer emergence, responsible for the discontinuity at the base o…

Molecular Structure of the Arthropod Hemocyanins

1992

Hemocyanin is an extracellular, blue protein that occurs in high concentrations in the blood of many arthropods, including spiders, scorpions, horseshoe crabs, crustaceans, and at least two centipedes. Serving as an ### oxygen carrier, it is functionally equivalent to hemoglobin, but performs reversible oxygen binding between two copper ions. Hemocyanin is composed of a number of subunits that assemble in an extremely large macro-molecular entity. These particles, which are similar in size to viruses or ribosomes, exhibit a complex allosteric behavior during oxygen binding. There is growing evidence that this functional plasticity has evolved upon, and answers to, ecophysiological constrain…

Polymer-coated reversed-phase packings with controlled hydrophobic properties

1992

Abstract We have designed and synthesized novel reversed-phase packings of non-porous and porous polymethacrylate-coated silicas. By varying the hydrophobicity of the polymer coating, selective unfolding of polypeptides may be achieved, thus enabling manipulation of the chromatographic profile. This study characterizes these packings through their employment for separations of model synthetic peptides of defined secondary, tertiary and quaternary structure. Thus, the packings were applied to the reversed-phase separation α-helical amphipathic and non-amphipathic peptides of the same amino acid composition but different sequences. In addition, selective unfolding of model to-stranded α-helic…

On the stability of the 24-meric hemocyanin from Eurypelma californicum.

1998

The stability of the 24-meric hemocyanin from Eurypelma californicum towards various denaturants (GdnHCl, urea, urea derivatives and salts of the Hofmeister series) indicates that the quaternary structure is stabilized by hydrophilic and polar forces. Thus, the interaction between the seven different subunit types of this cheliceratan hemocyanin is comparable with that of the closely related crustacean hemocyanins. In contrast, no significant influence of divalent ions such as Ca2+ and Mg2+ on the stability is observed at pH 8.0 and pH 8.5 but not at pH 7.0. Studies, both in the presence of urea and GdnHCl indicate that the denaturation process consists of a dissociation of the oligomeric s…

Bimodal Effect of Amphiphilic Biocide Concentrations on Fluidity of Lipid Membranes

1996

Abstract Using the spin label method (ESR) it has been shown that biologically active, amphiphilic compounds (quaternary ammonium salts -AS) containing polar heads with single and double positive charge caused, at low concentrations, decrease fluidity of liposome membranes formed with egg yolk lecithin (EYL). At higher concentrations an increase in fluidity was observed. With compounds having a single positive charge minimum fluidity of membrane structure occurs in the range of 1 to 3%, with compounds containing double positive charge -in the range of 4 -6 % . That effect does not depend on polar head size and length of alkyl chains of the AS used. Analysis of the electrostatic interaction …

A five-step synthesis of (±)-tylophorine via a nitrile-stabilized ammonium ylide.

2012

The Stevens rearrangement of a nitrile-stabilized ammonium ylide is the key step of a very short and practical synthesis of the phenanthroindolizine alkaloid (±)-tylophorine. The method requires only five linear steps and is devoid of any protecting group manipulations.

Organocatalytic enantioselective synthesis of trifluoromethyl-containing tetralin derivatives by sequential (hetero)Michael reaction–intramolecular n…

2017

The enantioselective synthesis of tetralin derivatives bearing a trifluoromethylated all‐carbon quaternary stereocenter has been accomplished through a synthetic sequence comprising an organocatalytic β‐functionalization of ortho‐1‐trifluoromethylvinyl‐(hetero)aromatic conjugated aldehydes followed by the intramolecular nitrone 1,3‐cycloaddition reaction (INCR). Both nitromethane and N‐Cbz‐hydroxylamine were employed as nucleophiles in the initial organocatalytic conjugate addition step, which provided the chiral information required for the subsequent diastereoselective INCR. Although diastereoselectivity was moderate, good yields and enantioselectivities were, in general, obtained. Intere…

J. Christoffers, A. Baro: Quaternary Stereocenters – Challenges and Solutions for Organic Synthesis.

2006

ChemInform Abstract: K2AuPS4 (I), Tl2AuPS4 (II), K2AuAsS4 (III), and KAu5P2S8 (IV): Syntheses, Structures, and Properties of Quaternary Gold Thiophos…

2010

Mediterranean coccolith ecobiostratigraphy since the penultimate Glacial (the last 145,000years) and ecobioevent traceability

2015

The Mediterranean Sea is a miniature ocean ideal to test the response of marine ecosystems to amplified orbital and suborbital climate changes. Here we present coccolith data from a Sardinia Channel gravity core (Arcose C_33) analysed over the last 145,000 years, with a mean resolution of about 900 years. The study highlights that regional phytoplankton assemblages underwent significant modifications between the penultimate glacial and the last interglacial, as well as between the last glacial and the Holocene. The N ratio palaeoproductivity index suggests reduced productivity levels and the development of a deep nutricline during the last interglacial and the Holocene. Within the last glac…