Preparation and solubility of phosphorylated β-cyclodextrins

beta-Cyclodextrin (CD) was phosphorylated with phosphoryl chloride in aqueous alkaline media at different temperatures and pH values. The phosphorylated cyclodextrin (PCD) were characterized by phosphorus contents and positions of substitution as determined by P-31-NMR spectroscopy. Reaction of CD with equivmolar POCl3 for 3 hr at pH 12 and 45 degrees C yielded in a PCD with a phosphorus content of 5.67%. The ratio of mono- and diphosphate esters increased when the reaction temperature was raised from 25 to 60 degrees C. The monoesterified phosphate groups were mainly located at C-6 of the anhydroglucose units when the reaction pH was 11 or 12. Reactions at pH 10, however, led to a higher degree of substitution at C-2 than at C-6. Phosphorylation enhanced the water solubility of CD. Solubility of a PCD (5.65% phosphorus) was 35% at pH 8 and 25 degrees C. Simultaneously, solubility of the PCD in 25% ethanol in water was much greater than unsubstituted CD (22.3 vs. 2.8%). The PCD enhanced the water solubility of nonpolar compounds, such as beta-carotene.