NMR diffusion spectroscopy as a measure of host-guest complex association constants and as a probe of complex size

The complexes of cyclohexylacetic acid and cholic acid with beta -cyclodextrin were studied by NMR diffusion coefficient measurements. The diffusion coefficient of the 1:1 cyclohexylacetic acid/beta -cyclodextrin complex, K-a = 1800 +/- 100 M-1, is slightly slower (3.23 +/- 0.07 x 10(-6) cm(2) s(-1)) than that of beta -cyclodextrin (3.29 +/- 0.07 x 10(-6) cm(2) s(-1)). The diffusion coefficient of the 1:1 cholic acid/beta -cyclodextrin complex, K-a 5900 +/- 800 M-1, is significantly slower (2.93 +/- 0.07 x 10(-6) cm(2) s(-1)) than that of beta -cyclodextrin. The results indicate that caution should be exercised when studying host-guest complexation by the so-called 'single point' technique. A novel data treatment is introduced which takes into account the diffusion behavior of all of the species when determining K-a. Experimentally determined diffusion coefficients of complexes are also a useful probe of the size of host-guest complexes.