23Na and 35/37Cl as NMR probes of growth and shape of sodium taurodeoxycholate micellar aggregates in the presence of NaCl

The growth of the aggregates of the dihydroxylated bile salt sodium taurodeoxycholate (NaTDC) upon NaCl addition and the involvement of the counterion were investigated by NMR spectroscopy of monoatomic ionic species. 23Na T1 values from 0.015, 0.100, and 0.200 mol kg1 NaTDC solutions in D2O, at variable NaCl content, proved to be sensitive to the transition from primary to secondary aggregates, which occurs in the former sample, and to intermicellar interaction. Some 79Br NMR measurements were performed on a 0.100 mol kg1 NaTDC sample added by NaBr in place of NaCl for comparison purposes. The 23Na, 35Cl, and 37Cl double quantum filtered (DQF) patterns, from the 0.100 mol kg1 NaTDC sample, and 23Na ones also from the 0.200 mol kg1 NaTDC one, in the presence of 0.750 mol kg1 NaCl, are a clear manifestation of motional anisotropy. Moreover, the DQF spectra of 23Na and 37Cl, which possess close quadrupole moments, display a striking similarity. The DQF lineshapes were simulated exploiting the Scilab environment to obtain an estimate of the residual quadrupole splitting magnitude. These results support the description of NaTDC micelles as cylindrical aggregates, strongly interacting at high ionic strengths, and capable of association with added electrolytes.