Molecular recognition in micelles: The roles of hydrogen bonding and hydrophobicity in adenine-thymine base-pairing in SDS micelles

Article abstract of DOI:10.1021/ja00070a007

This paper describes a new class of molecular receptors 2 that bind adenine derivatives 3 in aqueous solution by means of hydrogen bonding (base-pairing). The receptors are supramolecular assemblies of thymine groups embedded in micelles, which self-assemble when (thyminylalkyl)ammonium salts 1 [Thy-(CH₂)_n-N⁺Me₃] are mixed with aqueous sodium dodecyl sulfate (SDS) solutions. NMR titration studies and Job's method indicate that binding occurs with 1:1 adenine-thymine stoichiometry in a fashion consistent with base-pairing within the micelles. In the absence of SDS, base-stacking occurs in preference to base-pairing. Three factors that were anticipated to be most significant in binding were examined: the lipophilicity of the adenine substrate, the position of the thymine group within the micelles, and the role of SDS in binding. To study these factors, the length of the alkyl group on adenine 3 (m = 1, 2, 3, and 4), the length of the alkylammonium chain of thymine 1 (n = 4, 6, 8, and 10), and the concentration of SDS (0-40 mM) were varied. The lipophilicity of the adenines was found to have the greatest effect upon the measured association constant K_obs. A binding model is proposed in which adenine 3 first partitions between the bulk aqueous solution and the interior of the micelle and then base-pairs to the thymine group within the micelle. In accordance with this model, a linear relationship is observed between log K_obs and log K_ow (where K_ow is the octanol-water partition coefficient of adenine 3).