Determination of binding constants for cyclodextrin complexes with alkanols by the 1H NMR measurements of longitudinal relaxation time using tetramethylammonium chloride as an internal reference

The longitudinal relaxation times (T-1) of H-1 NMR signals for a variety of alkanols in D2O markedly decreased with increasing concentrations (c) of alpha- and beta-cyclodextrins (CD). Tetramethylammonium chloride (TMA) used as an internal reference was available for evaluating an effect of solution viscosity on relaxation rates R(=1/T-1), since TMA showed no appreciable interaction with CD. Changes in the ratio of R for alkanol protons to R for TMA protons with c were analyzed by the curve-fitting method to give K-a. These K-a values agreed well with those obtained by the analysis of changes in delta, indicating that T-1 measurement is available for the determination of K-a for CD complexes. 2D ROESY spectra provided definite information on the molecular structures of CD complexes with alkanols.