Journal of Inclusion Phenomena and Macrocyclic Chemistry 38: 267–276, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 267 Determination of Binding Constants for Cyclodextrin Complexes with Alkanols by the 1 H NMR Measurements of Longitudinal Relaxation Time Using Tetramethylammonium Chloride as an Internal Reference JALALUDDIN AHMED, TATSUYUKI YAMAMOTO and YOSHIHISA MATSUI ⋆ Faculty of Life and Environmental Science, Shimane University, Nishikawatsu, Matsue 690-8504, Japan (Received: 31 August 1999; in final form: 1 December 1999) Abstract. The longitudinal relaxation times (T 1 ) of 1 H NMR signals for a variety of alkanols in D 2 O markedly decreased with increasing concentrations (c) of α- and β -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 δ, 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. Key words: cyclodextrin, alkanol, inclusion complex, binding constant, NMR spectroscopy, longi- tudinal relaxation time, ROESY spectrum. 1. Introduction Cyclodextrins (CDs) provide hydrophobic cavities into which a variety of organic molecules are trapped to form inclusion complexes [1]. The formation of a CD inclusion complex is often studied by means of nuclear magnetic resonance (NMR) spectroscopy [2, 3]. For example, the complexation of CD with a guest molecule generally causes changes in chemical shifts (δ ) of 1 H and 13 C involved in CD or guest. The changes in δ with the concentration of CD or guest are numerically analyzed to afford the binding constant (K a ) of a CD complex. This so-called NMR shift titration [3] is very useful for the determination of K a . In this connection, we have previously reported [4] that accurate K a values are obtained by the use ⋆ Author for correspondence.