Electrospray mass spectrometry and gel permeation chromatography of oligo- and polysaccharides dissolved in N,N-dimethyl acetamide/lithium chloride.

摘要

Electrospray (ES) is presently the softest ionization method available in mass spectrometry. Utilizing ES, a series of maltooligosaccharides consisting of maltose-maltoheptaose, dissolved in N,N-dimethyl acetamide with lithium chloride (DMAc/LiCl), was studied. ESMS allowed detection of lithiated oligosaccharides, as well as numerous neutral LiCl attachments onto the sugars. These neutral attachments were observed only with LiCl, and to a much lesser extent with LiBr, but not with other salts. Lithiation of maltose was studied via semi-empirical computer modeling. Heat of formation studies indicated that lithium coordination to maltose seems to occur in a trivalent fashion, and is not necessarily ring-bridging. ES experiments also showed that solvation of lithium cations of the chloride salt occurred more readily with N,N-dimethyl propionamide than with DMAc or with N,N-dimethyl formamide (DMF), based on mass spectrometrically determined ion abundances. This has been attributed to an inductive, electron-donating effect on the part of the alkyl groups of the solvents. Evidence to support this was provided by computer modeling of electrostatic potentials and charges of the amide solvent molecules. ES intensities of the solvated alkali cations of alkali chlorides dissolved in DMAc and DMF are in accordance with the model that the ion escaping the droplet in ESMS is not the naked ion, but an ion-solvent molecular cluster.; Over the last three decades gel permeation chromatography (GPC) has developed into the technique of choice for polymer characterization. With it the dilute solution behavior of several polysaccharides was investigated. Applying the concept of universal calibration afforded calculation of not only molecular weight averages and distributions, but also of parameters defining each polysaccharide's dilute solution characteristics. Incorporation of a multi-angle laser light scattering detector into the experimental set-up permitted observation of solution aggregates of the various pullulan samples analyzed. Experimental data agreed with polymer theory, as the branched polysaccharides, predicted to have tighter coil densities in solution and, as such, to occupy a smaller hydrodynamic volume than linear polysaccharides of equal molecular weight, calculated as having smaller radii of gyration and lower intrinsic viscosities than their linear counterparts.