High resolution nuclear magnetic resonance spectroscopy of bile salts: individual proton assignments for sodium cholate in aqueous solution at 400 MHz.

The 400 MHz 1H-nuclear magnetic resonance spectrum of sodium cholate in dilute aqueous solution has been successfully resolved using a combination of decoupling, partial relaxation, and decoupled partial relaxation techniques. The individual carbon resonances in the 13C-NMR spectrum of sodium cholate have also have assigned. Assignments of individual methylene protons were made by consideration of the molecular structure of sodium cholate and the expected couplings and 1H-nuclear Overhauser enhancement experiments. Verification of the assignments of the methine protons was made by application of single frequency 1H-decoupled 13C-NMR. Variation of pH* from 6.0 to 11.0 did not alter the individual chemical shifts except for those between 2.12 delta and 2.30 delta, originating from the protons on the C23 position adjacent to the ionizable carboxyl group. The chemical shifts of the proton resonances were independent of concentration below 5 mM. Above 10 mM (micellar region), the proton chemical shifts were altered slightly and some band broadening occurred. These data are consistent with the formation of small micellar aggregates (up to N = 4) of cholate molecules.