Carbohydrates ionization constants

Ionization constants have been determined for numerous simple carbohydrates (10,13, i5, 25, 45), as well as for cellulose (32, 43), wheat starch (43), and alginate (43). Selected carbohydrates with their corresponding pK values are presented in Table I. The analytical methods involved in these determinations include conductimetry, potentiometric titration, thermometric titration, and polarimetry. Polarimetry was used by Smolenski and co-workers (45) to calculate a first and a second ionization constant for sucrose at 18°C (Ki = 3X 10"13 K2 = 3 X 10"14). 

Table I. Ionization Constants (Hydroxyl Group) of Carbohydrates in Water ...

As mentioned by Tipson,10 the question arises as to whether ultraviolet absorption spectra of the type described112-114 can serve as proof for the positional assignment of the carbohydrate moiety on the pyrimidine ring of nucleosides. The increasing usage of spectrophotometry in the isolation, and in structural and enzymic studies, of nucleosides and nucleotides warrants a detailed consideration of this technique. The studies described above,112 114 for example, may be open to question (despite the fact that the conclusions were correct), since the pH of the curves selected was not predicated upon the ionization constants of the compounds under investigation. A series of comprehensive spectral studies of nucleosides of nucleic acids, their parent pyrimidines, and related compounds have been reported,87-110 and much of the following discussion is derived from them. 

Figure 6.12 shows the detection of carbohydrates by amperometric detection at a constant potential (0.6 V versus Ag/AgCl) with a cylindrical copper wire electrode65 (25 pun in diameter). Because the pK values of most sugars are in the vicinity of 12-13, they are ionized at high pH and separated by CZE under such conditions, without prior derivatization or complexation procedures. 

The carbohydrate-binding sites of WGA are probably situated at the surface of the protein molecule. The pH dependences of association constants for lectin-chitotriose binding indicate that an ionizable group, pK = 3.9, is probably involved in complex-formation.494,504 This observation is especially noteworthy, inasmuch as protein carboxyl... 

MS, especially in combination with advanced separation techniques, is one of the most powerful and versatile techniques for the structural analysis of bacterial glycomes. Modern mass spectral ionization techniques such as electrospray (ESI) and matrix-assisted laser desorption/ionization (MALDI) provide detection limits in the high atto- to low femto-mole range for the identification of peptides and complex carbohydrates. Structural characterization of these trace level components can be achieved using tandem MS. This provides a number of specific scanning functions such as product, precursor ion, and constant neutral loss scanning to... 

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