Network forming properties of various proteins adsorbed at the air/water interface in relation to foam stability

A series of proteins was studied with respect to their ability to form a network at the air/water interface and their suitability as foaming agents and foam stabilizers. Proteins were chosen with a range of structures from flexible to rigid/globular: beta-casein, beta-lactoglobulin, ovalbumin, and (soy) glycinin. Experiments were performed at neutral pH except for glycinin, which was studied at both pH 3 and pH 6.7. The adsorption process was followed with an automated drop tensiometer (ADT). Network forming properties were assessed in terms of surface dilational modulus (determined with the ADT), the critical falling film length (L(still)) and flow rate (Q(still)) below which a stagnant film exists (as measured with the overflowing cylinder technique), and the fracture stress and fracture strain measured in surface shear. It was found that glycinin (pH 3) can form an interfacial gel in a very short time, whereas beta-casein has very poor network-forming properties. Hardly any foam could be produced at the chosen conditions with glycinin (pH 6.7) and with ovalbumin, whereas beta-casein, beta-lactoglobulin, and glycinin (pH 3) were good foaming agents. It seems that adsorption and unfolding rate are most important for foam formation. Once the foam is formed, a rigid network might favor stabilizing the foam.     

Labile coodination dendrimers

Addition of anionic benzylsulfate dendrons to dynamic mixtures of Ag+ and triphosphine ligands results in the assembly of loosely-bonded cage-core dendrimers.     

Mechanisms of sulfanyl (RS) migrations: synthesis of heterocycles

Thiiranium (episulfonium) ions had been acknowledged as reaction intermediates for many years, but it was not until 1977 that Nicolaou demonstrated systematically that these reactive heterocyclic cations could be trapped by carboxylic acids to give lactones. In the years that followed this report, extensive research greatly extended the scope of this reaction, particularly with regard to the methods for generating thiiranium ions, the types of nucleophiles that are compatible with this reaction, and the selectivity involved in the cyclization reactions. For many years we have been using thiiranium ions for the synthesis of saturated heterocycles. Whereas Nicolaou's method relied on electrophilic sulfenylation of alkenes, we have generated thiiranium ions by displacement of a leaving group with neighboring-group participation by a sulfanyl group. Many of the examples we have reported are of cyclizations that are reversible and so where two (and in some cases more) products can result, the outcome of the reactions provides fundamental information about the relative stability of different heterocyclic ring systems. This Review will begin with a brief introduction to sulfanyl participation as a method for generating thiiranium (and thiolanium) ions, and will go on to explore the idea of using sulfanyl migrations in synthesis. Initially, emphasis will be placed on mechanisms of [1,2] sulfanyl migrations: we will look specifically at [1,2] sulfanyl migrations (usually PhS) with elimination, substitution, and cyclization. Emphasis will then shift to the factors that affect the outcome of cyclization reactions. In particular, we will cover cyclizations with hydroxy nucleophiles and examine situations in which there are more than one hydroxy nucleophile present. We will also examine cyclizations with other nucleophiles, namely amines and sulfides. After our discussion of [1,2] sulfanyl migrations, we will look very briefly at the scope of [1,4] sulfanyl participation, before finally drawing up some guidelines that (we hope) will help other organic chemists take advantage of the rearrangement reactions that the sulfanyl group has to offer.