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Our efforts in the past year have emphasized NMR diffusometry to characterize how bile salt micelles differentially solubilize the atropisomers R,S-BNDHP.    Below is an illustration of S-BNDHP:

Figure 1. S-BNDHP (S-binaphthyl diylhydrogen phosphate). Hindered rotation about the bond connecting the naphthyl rings means there are two non-superimposable forms even though there is no stereogenic center (atropisomerism).

Our earlier work supports that the bile salts cholate and deoxycholate form anti-parallel dimers that interact selectively with R- and S-BNDHP.  The NMR data localize BNDHP in the hydrophobic interior of cholate and deoxycholate aggregates and show that the R,S-BNDHP enantiomers interact with different outer edge surfaces of the bile aggregates. 

Recently, we turned to NMR diffusometry as a hydrodynamic technique to observe how the binding of R,S-BNDHP by bile micelles may affect the size of those micelles.  Independent of chemical shifts and NOE's, the translational diffusion provides helpful new information in these systems.

Figure 2.  Plot of effective radii of cholate micelles with (closed symbols) and without (open symbols) BNDHP guest molecules.  Effective radii of cholate are calculated from NMR derived diffusion constants in combination with measured viscosities.

Diffusion data were obtained over a wide concentration range of 0 to 100 mM cholate in the presence and absence of the R,S-BNDHP analytes.  When interpreted via Stokes-Einstein analysis, one obtains effective radii which represent a weighted average of all aggregation states of the bile salt. (Figure 2):

-       The greatest difference in the bile salt radii occurs at ca. 20 mM cholate, a region we have previously found maximizes the presence of primary micelles.

-       The presence of S-BNDHP results in higher average aggregate sizes, consistent with micellar capillary electrophoresis experiments which show that the S enantiomer is retained longer than the R.

-       The radii of the bile aggregates in the presence of either R or S-BNDHP  (filled symbols) converge to the radii observed in the absence of any analyte (open symbols), showing that high concentrations of cholate begin to abolish the interaction with BNDHP.

-       Cholate undergoes a rapid increase in average size at about 30-40 mM, consistent with the onset of secondary aggregation.

In sum, we see direct hydrodynamic evidence for primary and secondary aggregation in these data, and are especially pleased to see that the effective aggregate size correlates directly to chirally selective solubilization.  We have also acquired  diffusometry data for deoxycholate  where similar correspondences were observed.  We look forward to continued work in the application of NMR diffusometry to the study of bile aggregates.