Fluorescent water-soluble molecular clips. Self-association and formation of adducts in aqueous and methanol solutions

Abstract

We have synthesized the water-soluble molecular clip AC containing two anthracene sidewalls connected by semirigid aliphatic units to a benzene bridge which carries two sodium sulfate substituents. For comparison purposes, the analogous clip NC with naphthalene instead of anthracene sidewalls has been synthesized. In methanol solution both clips exist in their monomeric form, while in aqueous solution the anthracene clips AC self-assemble to give a highly stable dimer with log K_dim = 5.1 (for the naphthalene clip NC, log K_dim = 2.5). The ¹H NMR spectra of the dimers have evidenced the interaction of two intertwined clip molecules in a perpendicular arrangement. The non-associated clip AC exhibits a very strong blue fluorescence typical of the anthracene chromophoric unit. The (AC)₂ dimer has a broader absorption spectrum and a much weaker and red shifted emission band. Addition of acetylcholine leads to the disruption of the dimer (AC)₂ and the revival of the very strong fluorescence typical of the monomeric clip AC. Both clips interact with nicotinamide adenine dinucleotide (NAD⁺) in buffered aqueous solution at pH = 7.2. A ¹H NMR analysis shows that the naphthalene clip NC forms a 1 : 1 complex with NAD⁺ including the active site of the cofactor inside the clip cavity. In methanol, formation of a stable 1 : 1 adduct between AC and a fluorescent first-generation dendrimer containing four dansyl groups appended to a 1,4-diaminobutane core is driven by acid addition. In such adduct, a very efficient energy transfer takes place from the excited anthracene units of the clip to the dansyl chromophores of the dendrimer. The adduct can be reversibly disassembled by addition of base or of competitive guests.