Binding of Acetylcholine to a Cyclophane Host. Influence of Water and Reliability of NMR Measurements of Small Association Constants

Abstract

A detailed analysis of the association of acetylcho- line (ACh) and tetramethylammonium (TMA) picrates with the cyclophane tetraester 1b in CDC13 at T = 296K was undertaken to assess the reliability of measurements of small association constants by NMR shift titrations. Results showed that, with simple expedients and careful treatment of data, binding constants of 13 and 29 M-1 for ACh and TMA respectively can be measured through standard nonlinear regression procedures with ±1% accuracy. Systematic errors caused by the interfer- ence of water at low host concentration were shown to be the major source of inaccuracy. The plot of residuals of experimental data points from the regression curve was used to monitor systematic deviations while the shift of the water signal was used as a probe of processes occurring in solution.