CHEMISTRY OF SYNTHETIC RECEPTORS AND FUNCTIONAL-GROUP ARRAYS .18. APPROACHES TO QUANTITATIVE SUPRAMOLECULAR CHEMISTRY - HYDROGEN-BOND-BASED MOLECULAR RECOGNITION PHENOMENA AND SIGMOIDAL BEHAVIOR IN MULTICOMPONENT MIXTURES

In this paper we present NMR data from continuous titration competition experiments and describe a method for quantitative analysis of these data. Host 1 is evaluated and shown to have an extremely high affinity for adenine derivative 4 (K(a) = 505 000 +/- 100 000 M-1). Two different experiments showing sigmoidal solute response are presented and analyzed. The methods discussed here promise greater accuracy than the formulae previously used for competition experiments because exact terms for all equilibria are included and the methods can be extended to analyses of more complex supramolecular ensembles. For example, a competition experiment here required adding host 1 to a 1.0 mM solution of 9-ethyladenine (4) in the presence of 20 mM dimethyleneurea (3). Before any host is added, 10% of the 9-ethyladenine is bound to dimethyleneurea and 12% of the dimethyleneurea is present as the dimer. These equilibria will obviously contribute to the observed chemical shifts for the solutes during the titration, and failure to consider these equilibria in calculations will lead to inaccurate results. These unnecessary inaccuracies can be avoided by using the methods detailed here.