Structure Determination of Derivatized Disaccharides by Tandem Mass Spectrometry and Molecular Modeling.
Date of Award
Doctor of Philosophy (PhD)
Roger A. Laine
This research entails the synthesis of derivatized disaccharides, mass spectrometry studies for structural elucidation of the synthetic disaccharides and molecular modeling of the native and derivatized disaccharides. Peralkylated (methyl to pentyl), peresterifed and tert-butyl-dimethyl silylated linkage isomeric glucose-glucose disaccharides were synthesized to eventually be used in mass spectrometry studies. In this dissertation, mass spectrometry was used to examine anomeric pair discrimination of two sets of peralkylated (methyl to pentyl) disaccharides (maltose/cellobiose and isomaltose/gentiobiose). This was carried out by electrospray ionization and collision experiments on a triple quadrupole mass spectrometer. In addition, effects of alkyl substitution on cleavage of the glycosidic bond were observed. Collision energy offset voltages were plotted versus Product/Parent (D/P) ion ratios generating a trend observed for both the 1→4 and 1→6 alpha and beta isomers. The methyl derivative had the lowest D/P ion ratio followed by ethyl, propyl, butyl, and pentyl. Collision energy is converted to rotational-vibrational modes in competition with bond cleavage represented by the slope of offset energy versus D/P ion ratio. Rotational freedom at the glycosidic linkage is hypothesized to play a major role in this phenomenon. Molecular modeling was carried out using MM3 to validate differences observed in the rates of cleavage in the mass spectrometer. A flexible residue calculation was used involving rotation of the monomeric residues around the glycosidic bond having torsion angles &phis; and psi. Three dimensional energy plots of &phis;,psi and energy were plotted with the volumes of the wells taken as an indication of the conformational freedom of motion around the glycosidic bond. Two dimensional &phis;-psi energy surfaces of both the 1→1, 1→2, 1→3, and 1→4 native and permethylated disaccharides were also plotted to study their characteristics. The calculated freedom of motion volumes for the beta-(1→4) were smaller than for the alpha-(1→4) bonded glucoses, which may account for the higher rate of bond cleavage for the beta-(1→4) linkage. Additionally an interesting outcome was noted from the difference maps between the permethylated and native disaccharides, which were plotted to study the effect of the methyl groups on the energy surfaces.
Mendonca, Sanford Louis, "Structure Determination of Derivatized Disaccharides by Tandem Mass Spectrometry and Molecular Modeling." (2000). LSU Historical Dissertations and Theses. 7378.