Lactose specific rotation

Anhydride. Since /1-lactose is less soluble than the a-isomer above 93.5°C, the crystals formed from aqueous solutions at temperatures above 93.5°C are / -lactose these are anhydrous and have a specific rotation of 35°. /1-Lactose is sweeter than a-lactose, but is not appreciably sweeter than the equilibrium mixture of a- and /1-lactose normally found in solution. 

The specific rotation, [a]o°, of lactose in solution at equilibrium is +55.4° expressed on an anhydrous basis ( + 52.6° on a monohydrate basis). The specific rotation is defined as the optical rotation of a solution containing 1 g ml"1 in a 1 dm polarimeter tube it is affected by temperature (20°C is usually used indicated by superscript) and wavelength (usually the sodium D line (589.3 nm) is used indicated by subscript). 

The specific rotation of lactose varies with the solvent. It is higher in glycerol than in aqueous solutions but lower in alcoholic or acetone solutions (Nickerson 1974). Not only the specific rotation but also the equilibrium ratio of a to /3 may be changed by the nature of the solvent. For example, upon dilution with water, concentrated solutions of lactose in methanolic calcium chloride show a high (1.3) initial to final rotation, regardless of whether the a- or /3-isomer was used originally in preparing the solution (Domovs and Freund 1960). 

---