~(13)C NMR and electrospray ionization mass spectrometric study of sucrose aqueous solutions at high pH: NMR measurement of sucrose dissociation constant

摘要

The ~(13)C NMR technique is used for the measurement of the first dissociation constant of sucrose (HL) in highly alkaline solutions. In 1.0 M NaCl/NaOH medium and for 25℃, the concentration dissociation constant (pK_1) was 13.1 ± 0.3; and, for 60?, pK_1 = 12.30 ± 0.05. The β-D-fructofuranosyl ring was found to be responsible for dissociation. The NMR data reveal no clear evidence of the second dissociation step below pH 14, either at 25℃ or at 60℃. In the solutions with 4-10 mol dm~(-3) NaOH content the ~(13)C NMR technique indicated the chemical shift changes, treated as the second dissociation step of sucrose and a sodium complex formation. A very rough estimation, for variable ionic strength, gives the value: pK_2 ～ 15.8 ± 0.8. The anionic species L~- and NaH_(-1)L~- have been registered by electrospray ionization time-of-flight mass spectrometry (ESI-ToF MS) for 0.01 M sucrose solutions with initial pH 13.