No abstract

Rhamnolipid B and its precursor A, both microbial and membranous constituent, reversibly changed morphologies of their molecular aggregates from vesicles under acidic conditions, to lamella, lipid particles, and finally to micelles under weakly acidic conditions within a narrow pH range of about 5–7.

Static and dynamic light scattering were measured for monodisperse polyethylene oxide) (PEO) in water and methanol at 25 °C. PEOs of three different molecular weights ranging from nominal Afw = 2.6 X 104 to 9.96 X 10s were used. For all of these molecular weights, angular dependence of Zimm plots showed no downturn at low angles not only in water but also in methanol, indicating no large aggregates of PEO in both solvents. Furthermore, apparent weight-average molecular weight " , radius of gyration Rgapp-and hydrodynamic radius Rhm did not change over 3 weeks. This observation indicates that molecularly dispersed PEO is stable and aggregate-free property is inherent in these solvents.

Mass discrimination effects in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) were quantitatively investigated using equiweight and equimolar mixtures of uniform polystyrene (PS) oligomers. Uniform PS oligomers were separated by preparative super-critical fluid chromatography (SFC) from commercial standard PS samples. The separated PS oligomers, with degrees of polymerization n = 2±25, have absolutely no molecular weight distributions. Equiweight and equimolar mixtures of uniform PS oligomers were accurately prepared by weighing by microbalance, and their spectra were recorded using a MALDI-TOF mass spectrometer. In the lower molecular weight region (less than about 103 ) the oligomers with lower molecular weights give lower mass spectral intensities, with no correlation with laser power. In contrast, higher laser powers yield a decrease of mass spectral intensities in the higher molecular weight region. These results clearly show that mass discrimination effects occur at lower and higher molecular weights depending on the laser power, and provide quantitative information about the discrimination. Using the data on equiweight and equimolar mixtures of PS oligomers, it was possible to calibrate the MALDI-TOF mass spectral data for an analysis of molecular weight distribution of a standard monodisperse PS sample with number-averaged molecular weight of 10 3 , and to compare it with the molecular weight distribution measured by analytical SFC. The result from the calibrated MALDI-TOF mass spectrum, however, does not agree perfectly with that from the SFC results, because undetectable peaks in MALDI-TOF mass spectra at lower and higher molecular weights could not be included in the calibration of peak intensities.

A pure sample of an oligo(ethylene glycol) (OEG), tetrapentaconta(ethylene glycol) (H0-(CH2CH20)54H (OEG54)), has been prepared by stepwise synthesis coupled with preparative size exclusion chromatography (SEC) used as a separation method. The purity of the OEG54 sample has been checked by analytical SEC and differential scanning calorimetry. Furthermore, the molecular weights (MW) of the OEG54 sample and a pure sample of octadeca(ethylene glycol) (OEG18) previously prepared have been determined by mass spectroscopy, light scattering (LS), vapor pressure osmometry, 13C NMR, and NMR. The observed MW values of the OEG samples are in good agreement with those expected from their chemical formulas. In particular, fairly good agreements observed in LS and NMR measurements show that these pure OEG samples can be used as ideal MW standard samples.