Two-dimensional gel permeation chromatography (2-D GPC) correlation studies of the aggregate–aggregate interactions in acid-catalyzed triethoxysilyl-terminated polystyrene systems: weak catalysis by HNO<Subscript>3</Subscript>

Time-resolved gel-permeation chromatography (GPC) profiles were measured over a long period of time (t=0–768 h) for the HNO₃-catalyzed reaction mixture of a well-defined polymeric silane coupling agent, triethoxysilyl-terminated polystyrene. The results showed that HNO₃ catalyzes the rate of polymerization, but only to a small extent. Two-dimensional (2-D) GPC correlation spectra were calculated from time-resolved GPC profiles and were used to examine the mechanism of polymerization. The resolution enhancement obtained by 2-D correlation provided ample evidence for existence of four monomeric components. It has been demonstrated that dynamic variation in the population of these components occurs over a long reaction time, reflecting the mechanism of a slow polymerization process.     

New Aqueous Gel Based on Soluble Cyclodextrin/Vitamin A Inclusion Complex

Rameb (randomized dimethyl--cyclodextrin) was mixed with vitamin A propionate (PVA) (molar ratio 10/1 in water) and a water soluble complex was formed and studied by HPLC and NMR (structure, concentration and stability of PVA). Then solution was used to form an aqueous gel. The skin absorption of PVA through stratum corneum, epidermis and dermis of human skin (on modified Franz cells) was assayed by HPLC and was compared to that of a reference gel (or oil) without cyclodextrin. The solution obtained contains a maximum of 10 mg/mL PVA (if saturated with Rameb) and the PVA can remain stable up to 90 days in solution, and up to 1 year if freeze-dried (storage at 4 °C, in the dark). The results of the different experiments of skin distribution were statistically analyzed and show that when complexed with Rameb, the amount of PVA that penetrates each skin layer is significantly higher than pure PVA. The results also show that PVA cannot pass through the dermis and enter the circulation.     

Quantum-mechanical studies of environmental effects on biomolecules VI.Ab initio Studies on the hydration scheme of the phosphate group

Ab initio SCF computations using the STO 3G basis set have been performed on the hydration scheme of the dimethylphosphate anion (DMP^?) within the “supermolecule” approach. This model compound represents the phosphate group in nucleic acids, phospholipid components of membranes and a number of other important biological structures. In the first place the principal hydration sites for the fixation of a water molecule have been established and this was followed by the determination of the polyhydration scheme of the first solvation shell, which may involve up to six water molecules. The effect of hydration on the relative stabilities of thegg,gt andtt forms of (DMP^?) was evaluated. The study was extended to the computation of the energies of interaction in the second and third hydration shells and the conclusion was reached that organized hydration around (DMP^?) is essentially limited to two hydration shells with possible residual organization of a third shell around specific sites. The structure of the bound water was investigated. Charge transfer is observed from (DMP^?) to the bound water, the perturbation extending essentially to the first and with decreasing intensity, to the second hydration shell.     

Synergistic insecticidal mode of action between sesquiterpene lactones and a phototoxin, alpha-terthienyl

The synergistic insecticidal action of characteristic defensive substances produced by the plant family Asteraceae was investigated under controlled laboratory conditions. Sesquiterpene lactones isolated from Asteraceae that may form, through a Michael addition process, conjugates with glutathione were administered in a meridic diet to a herbivorous insect, Manduca sexta. By administering sesquiterpenes, variable in vivo reduced glutathione levels were observed in the insect larvae. When the Asteraceae-derived photooxidant alpha-terthienyl was co-administered, lipid peroxidation and larval mortality were significantly enhanced in the treated groups of insects with lowered in vivo glutathione levels.     

Cationic amino-containing N-isopropyl- acrylamide-styrene copolymer particles: 2-surface and colloidal characteristics

 In a previous paper [1], the synthesis of various polystyrene– poly[NIPAM] core–shell latexes bearing cationic amidino and/or amino charges has been described. Several colloidal properties of these cationic latexes have been charac-terized such as: particle size, surface charge density, electrophoretic mobility and finally colloidal stability. Due to the poly[NIPAM]-rich layer in the shell, it was found that temperature played a significant role on all these properties, a LCST around 33 °C being exhibited. In addition, ionic strength was also found to affect the colloidal behavior of these latexes, the largest effect being observed with latexes having both amidino and amino surface charges. The critical coagulation concentra-tions (CCC) of the various latexes above and below the LCST were determined, highlighting the contribution of electrostatic and steric repulsive forces to the stability of these particles. Received: 20 January 1998 Accepted: 8 June 1998     

Perfluorocycloalkyl(aryl) diazenes from heptafluoronitrosocyclobutane and nonafluoronitrosocyclopentane

The perfluoronitrosocycloalkanes, heptafluoronitrosocyclobutane and nonafluoronitrosocyclopentane, are convenient precursors to a family of new perfluorocycloalkyl(aryl) diazenes. With aniline and o-aminobenzamide, $\text{CF}{}_2\text{(CF}{}_2\text{)}{}_{\mathrm{x}}\text{C}$FNN$\text{C(CH)}{}_4\text{C}$H and $\text{CF}{}_2\text{(CF}{}_2\text{)}{}_{\mathrm{x}}\text{C}$FNN$\text{C(CH)}{}_4\text{C}$C(O)NH₂ (x = 2,3) are formed. Additionally, heptafluoronitrosocyclobutane gives $\text{CF}{}_2\text{(CF}{}_2\text{)}{}_2\text{C}$FNN$\text{CCFCFCHCFC}$F and $\text{CF}{}_2\text{(CF}{}_2\text{)}{}_2\text{C}$FNN$\text{C(CH)}{}_4\text{C}$NH₂ with 2,3,5,6-tetrafluoroaniline and o-phenylenediamine     

Photoinduced electron transfer between chlorophyll a and gold nanoparticles

Excited-state interactions between chlorophyll a (Chla) and gold nanoparticles have been studied. The emission intensity of Chla is quenched by gold nanoparticles. The dominant process for this quenching has been attributed to the process of photoinduced electron transfer from excited Chla to gold nanoparticles, although because of a small overlap between fluorescence of Chla and absorption of gold nanoparticles, the energy-transfer process cannot be ruled out. Photoinduced electron-transfer mechanism is supported by the electrochemical modulation of fluorescence of Chla. In absence of an applied bias, Chla cast on gold film, as a result of electron transfer, exhibits a very weak fluorescence. However, upon negatively charging the gold nanocore by external bias, an increase in fluorescence intensity is observed. The negatively charged gold nanoparticles create a barrier and suppress the electron-transfer process from excited Chla to gold nanoparticles, resulting in an increase in radiative process. Nanosecond laser flash experiments of Chla in the presence of gold nanoparticles and fullerene (C60) have demonstrated that Au nanoparticles, besides accepting electrons, can also mediate or shuttle electrons to another acceptor. Taking advantage of these properties of gold nanoparticles, a photoelectrochemical cell based on Chla and gold nanoparticles is constructed. A superior performance of this cell compared to that without the gold film is due to the beneficial role of gold nanoparticles in accepting and shuttling the photogenerated electrons in Chla to the collecting electrode, leading to an enhancement in charge separation efficiency.     

Lanthanide complexes for second order nonlinear optics: evidence for the direct contribution of f electrons to the quadratic hyperpolarizability

This study descibes for the first time the nonlinear optical (NLO) properties of multipolar lanthanide complexes, Na3[Ln(dipic)3]. The "metal-induced" NLO activity enhancement is confirmed as a general property of f-block elements, and the direct participation of f electrons to the hyperpolarizability beta is strongly supported by experimental data.     

Spontaneous resolution under supramolecular control

The spontaneous resolution of enantiomers is an intriguing and important phenomenon in a number of research areas. Non-covalent interactions can play a key role in the process which can now be observed not only in crystals, but in liquid crystals, self-assembled monolayers, self-assembled fibres, and supramolecules self-assembled in solution. The evidence gathered in all of these areas is important for explaining the transfer of chirality from molecule to bulk, and in particular the spontaneous resolution of enantiomers.