Photoluminescent toroids formed by temperature-driven self-assembly of rhodamine B end-capped poly(N-isopropylacrylamide).

In this paper, self-assembled polymeric toroids formed by a temperature-driven process are reported. Rhodamine B (RhB) end-capped poly(N-isopropylacrylamide) (PNIPAAm) demonstrating a lower critical solution temperature (LCST) is prepared. In a two-phase system, the polymer in the aqueous phase could move to the chloroform phase on raising the temperature above its LCST. This temperature-driven process results in the formation of polymeric toroids in the chloroform phase, and the strategy affords a new pathway to toroidal self-assembly of polymers. Moreover, the photoluminescent behavior of the RhB end-capped PNIPAAm species formed by the process is also studied and discussed.

Hydrophobic contribution of amino acids in peptides measured by hydrophobic interaction chromatography.

The adsorption behaviors of amino acids in short chain peptides were examined. Each amino acid, aliphatic or charged, was inserted between the two tryptophans of a peptide, GWWG. The capacity factors of these peptides on an Ocytl-Sepharose column were measured. The adsorption enthalpies, entropies, and the number of repelled water molecules after adsorption were estimated to analyze the contribution of each different amino acid to its hydrophobic adsorption. The peptides inserted with aliphatic amino acids owned the highest capacity factors but released the least amount of adsorption heat among all the peptides under examination. It was found that the hydrophobic contribution of aliphatic amino acids was derived from the entropy gain by repelling the ordered water surrounding them. The insertion of negatively charged amino acids greatly reduced the capacity factors but still repelled a significant number of water molecules after adsorption. This indicated that the water molecules surrounding ionic amino acids were not orderly aligned. The dehydration cost energy but the water repelling did not offer enough entropy to drive the adsorption. Subsequently, lower retention was obtained from the peptides inserted with negatively charged ionic amino acids. The insertion of lysine increased the adsorption enthalpy but repelled no water molecules after adsorption. It was speculated that the inserted lysine still interacted with hydrophobic ligands but disturbed the interaction between ligands and adjacent tryptophans. Therefore, the adsorption enthalpy increased and the capacity factors decreased. Different amino acids contributed to hydrophobic interaction in different ways. The simultaneous analysis of capacity factor, adsorption enthalpy, adsorption entropy, and the number of repelled water molecules facilitated the understanding of the adsorption processes.

Atom Transfer Radical Addition/Polymerization of Perfluorosulfonic Acid Polymer with the C-F Bonds as Reactive Sites.

This work reports the first demonstration of the chemical reactions on the C-F groups of perfluorosulfonic acid polymers. The Nafion chains show chemical reactivity for atom transfer radical addition onto multiwalled carbon nanotubes and ability to serve as a macroinitiator for atom transfer radical polymerization. The C-F groups and mainchain -CF2 groups have been demonstrated, under a study with 19F NMR, as the active sites responsible for the reactions. The results could certainly extend both the scopes of chemistry and application of perfluorosulfonic acid polymers as well as the windows of atom transfer radical addition/polymerization to fluorinated compounds.