Carbohydrates polarimetry

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). 

Polarimetry takes advantage of the optical activity of carbohydrates. The high selectivity of this procedure makes it especially suitable in the case of complex food extracts, where other components would interfere with ultraviolet or refractive index detection. However, a major disadvantage is its lower sensitivity. The use of immobilized enzymes (51) with detection by fluorescence or electrochemistry has also been applied in fermentation juices (52) and other particular cases. 

Polarimetry is extremely useful for monitoring reactions of optically active natural products such as carbohydrates which do not have a useful UV chromophore, and samples for study do not need to be enantiomerically pure. Nevertheless, compared with spectrophotometry, the technique has been applied to relatively few reactions. It was, however, the first technique used for monitoring a chemical reaction by measuring a physical property when Wilhemy investigated the mutarotation of sucrose in acidic solution and established the proportionality between the rate of reaction and the amount of remaining reactant [50]. The study of a similar process, the mutarotation of glucose, served to establish the well-known Bronsted relationship, a fundamental catalysis law in mechanistic organic chemistry. 

Carbohydrates in nature are optically active and polarimetry is widely used in establishing their structure. Measurement of the specific rotation gives information about the linkage type (a or (3 form) and is also used to follow mutarotation. Nuclear magnetic resonance spectroscopy (NMR) can be used to differentiate between the anomeric protons in the a- or /3-pyranose and furanose anomers and their proportions can be measured from the respective peak areas. 

Polarimetry of the brucine salts of L-guluronic acid and D-mannuronic acid released from alginates by acid hydrolysis has been used to measure the ratio of these acids in the native alginate. Nitroxide spin-labels have been investigated as molecular probes to determine the microviscosity of aqueous solutions of carbohydrates such as alginate. ... 

---