Self-association of cromolyn sodium in aqueous solution characterized by nuclear magnetic resonance spectroscopy

The major objective of this study was to investigate and characterize the solution properties of cromolyn sodium (in D(2)O or D(2)O/H(2)O phosphate buffer at pH 7.5) using nuclear magnetic resonance (NMR) spectroscopy. The self-association of cromolyn molecules was examined primarily via one-dimensional (1)H and (13)C, and two-dimensional homonuclear NOESY NMR. Significant spectral shifts were observed for a majority of cromolyn (1)H and (13)C resonances, and are attributed to inter-molecular ring-stacking association accompanied by intra-molecular conformational changes. The critical self-association concentration was determined to be 10 mg/mL at pH 7.5 and 25 degrees C by measuring the chemical shift of a specific cromolyn (1)H resonance. The observed magnitude and sign changes of NOESY correlations indicate the formation of cromolyn aggregates with restricted molecular mobility. Mesomorphic liquid crystal formation is suggested by uniformly pronounced line broadening in concentrated cromolyn solutions; the transition concentration was approximately 60 mg/mL at 25 degrees C, which is consistent with literature findings based on other techniques. A stronger tendency toward association was observed at lower temperature but aggregation appeared to be independent of pH. Lastly, it was concluded that self-association of cromolyn is promoted by the presence of monovalent cations as a result of reduced electrostatic repulsive forces.