Cooperativity in the assembly of H-bonded duplexes of synthetic recognition-encoded melamine oligomers

Recognition-encoded melamine oligomers (REMO) are synthetic polymers composed of repeating triazine-piperazine units and equipped with phenol and phosphine oxide side-chains. Short oligomers have previously been shown to form length- and sequence-selective H-bonded duplexes in non-polar solvents. Here, automated solid phase synthesis was used to prepare homo-sequence REMO with either twelve phenol recognition units or twelve phosphine oxide recognition units. The ends of the oligomers were functionalised with an azide and an alkyne group to allow investigation of duplex formation by covalent trapping with copper-catalysed azide-alkyne cycloaddition (CuAAC) reactions. The oligomers were also functionalised with a dansyl fluorophore or a dabcyl quencher dye to allow investigation of duplex formation by Förster resonance energy transfer (FRET). Covalent trapping showed that the duplex is the major species present in a 1 : 1 mixture of the phenol 12-mer and phosphine oxide 12-mer at micromolar concentrations in dichloromethane. FRET titration experiments showed that the association constant for duplex formation is greater than 10⁸ M^-1 in chloroform, and DMSO denaturation experiments showed that duplex formation is highly cooperative. The Hill coefficient for denaturation of the 12-mer duplex was 4.6, which is significantly higher than the value measured for the corresponding 6-mer duplex (1.9). This behaviour mirrors that observed for nucleic acid duplexes, where denaturation becomes increasingly cooperative as more base-pairs are added to the duplex.