Salt tolerance of an aqueous solution of a novel amphiphilic polysaccharide derivative

An aqueous solution of an amphiphilic polysaccharide derivative, hydrophobically (stearyl alkyl group) and hydrophilically (sulfonic-acid salt group) modified hydroxyethylcellulose (HHM-HEC), showed increased viscosity, elasticity, and thixotropic properties in response to the addition of monovalent and divalent salts. Furthermore, the HHM-HEC solution had a transparent appearance at a NaCl concentration of 7 wt %. Since it showed superior salt tolerance to HEC, we focused attention on the two substituents of HHM-HEC and prepared HEC derivatives, namely, hydrophobically modified hydroxyethylcellulose (R-HEC), hydrophilically modified hydroxyethylcellulose (S-HEC), and nonmodified hydroxyethylcellulose (HEC). In addition, we used oscillatory, thixotropic, and fluorometric methods to compare the rheological properties of HHM-HEC with those of other derivatives in the presence of NaCl and ZnCl2, and attempted to elucidate the respective roles of the two substituents of HHM-HEC solution in the salt-tolerance mechanism. As the NaCl concentration in the HHM-HEC solution increased, the values of the elastic modulus G' and the viscous modulus G' increased, and, moreover, the relative intensities of the first (I1 = 372 nm) and the third (I3 = 383 nm) vibronic bands of the pyrene monomer emission spectrum (the I1/I3 ratio) decreased. These results suggested that the added salt strengthened the three-dimensional network structure of the HHM-HEC polymer by the formation of cross-linkages through the association of hydrophobic substituents. This hydrophobic substituent was therefore essential in allowing HHM-HEC to exhibit a high viscosity in a salt solution. Although the R-HEC solution showed a higher viscosity than did the HHM-HEC solution in the absence of added salts, it became cloudy and lost its viscosity at high NaCl concentrations, apparently because of the shrinkage of its network structure. This signified that the hydrophilic substituent was essential for the sufficient solubility of HHM-HEC to show its rheological properties in a salt-rich solution. We propose to explain how the viscosity of HHM-HEC increases in the presence of salts as follows: Added salts weaken the electrostatic repulsion between the hydrophilic substituents, thereby enhancing the interactions of hydrophobic substituents and consequently increasing the rigidity of the HHM-HEC solution.     

Improved synthesis of the A-G ring segment of brevetoxin B

An efficient synthesis of the A-G ring segment 2, a key intermediate for the total synthesis of brevetoxin B (1), was achieved in 37 steps and 5.0% overall yield. The intramolecular allylation of the O,S-acetal 22, prepared from the ABC ring segment 15 and the FG ring segment 17, was carried out using AgOTf as a Lewis acid to give the desired compound 23, predominantly. Ring-closing metathesis of 23 with the Grubbs catalyst 12 afforded the heptacyclic ether 25. Selective hydrogenation of the E ring olefin of 25 was performed by diimide reduction to afford 2.     

Diagnostics of laser-ablated carbon plumes by photoionization using a tunable laser

Tunable laser pulses at wavelengths from 250 to 880 nm were fired at plumes produced by laser ablation of a carbon target in He atmosphere. We observed an electrical current due to photoionization, which roughly reflected the behavior of carbon clusters in the plume. The photoionization current had dynamic temporal variations, comprising a rapid increase at the beginning (T_D<0.2 ms, where T_D is the time after a YAG laser pulse for laser ablation irradiates the target), a gradual increase for 0.2D<3 ms, and a slow decrease for T_D>7 ms. The increasing phase of the photoionization current was synchronized with the decreasing phase of C₂ radical density. For He gas pressures lower than 0.8 Torr no photoionization current was detected. The growth rate of the photoionization current was higher for a higher He gas pressure.