Duplex-Forming Oligocarbamates with Tunable Nonbonding Sites
In biopolymers such as proteins and nucleic acids, monomer sequence encodes for highly specific intra- and intermolecular interactions that direct self-assembly into complex architectures with high fidelity. This remarkable structural control translates into precise control over the properties of the biopolymer. Polymer scientists have sought to achieve similarly precise control over the structure and function of synthetic assemblies. A common strategy for achieving this goal has been to exploit existing biopolymers—known to associate with specific geometries and stoichiometries—for the assembly of synthetic building blocks. However, such systems are neither scalable nor amenable to the relatively harsh conditions required by various materials science applications, particularly those involving non-aqueous environments. To overcome these limitations, we have synthesized sequence-defined oligocarbamates (SeDOCs) that assemble into duplexes through complementary hydrogen bonds between thymine (T) and diaminotriazine (D) pendant groups. The SeDOC platform makes it simple to incorporate non-hydrogen-bonding sites into an oligomer's array of recognition motifs, thereby enabling an investigation into this unexplored handle for controlling the hybridization of complementary ligands. In the first part of this work, we successfully synthesized monovalent, divalent, and trivalent SeDOCs and characterized their self-assembly via diffusion ordered spectroscopy, 1H-NMR titration, and isothermal titration calorimetry. Our findings revealed that the binding strength of monovalent oligomers with complementary pendant groups is entropically favorable and independent of monomer sequence. The results further showed that the hybridization of multivalent oligomers is cooperative, that their binding enthalpy (ΔH) and entropy (TΔS) depend on monomer sequence, and that sequence-dependent changes in ΔH and TΔS occur in tandem to minimize the overall change in binding free energy. In our second investigation, we synthesized a new series of recognition-encoded SeDOCs, which utilized a photo-crosslinkable moiety to enable on-demand, covalent stabilization of hydrogen-bonded duplexes. Though photo-crosslinking experiments were unsuccessful, this worked showed that we can install structurally diverse pendant groups without impacting hybridization. Overall, this body of work is a promising step in the development of recognition-encoded ligands for producing advanced materials.