Molecular recognition analyzed by observing intramolecular interconversionwith EPR spectroscopy

Molecular recognition between Rebek's cleft R and 4-(2-benzimidazolyl)-amin omethyl-2,6di-tert-butyl-phenol (Bi) is investigated by means of EPR spectr oscopy. Their specific intermolecular interactions are distinguished from n on-specific interactions represented by mere protonation on addition of ben zoic acid. Both interactions are related to intramolecular interconversions at the C-alpha-atom of Bi*, where a two-jump mechanism is found for free a nd protonated Bi* and a two-site exchange for the complexed ligand. While N MR spectroscopy indicates the existence of 1:1 and 1:2 receptor-substrate c omplexes, the faster time scale of EPR spectroscopy allows to identify one 1:2 as well as three 1:1 associates and to elucidate their formation condit ions. Based upon the corresponding hf parameters, binding modes and sites a re proposed for the key-in-the-lock system.