Direct Backbone Structure Determination of Polyisocyanodipeptide Using Solid-State Nuclear Magnetic Resonance

The success of polyisocyanides as the basic scaffold for functional materials has been attributed to its rigid and highly stable structure. It has been demonstrated that these polymers can be stabilized by formation of inter side chain hydrogen bonds (Cornelissen et al. Science 2001, 293, 676) which in turn has resulted in development of several polyisocyanide based functional materials. Despite the success of the material, the exact structure and conformation of these polymers has been subject of discussions over many years and several structural models based on evidence from different analytical and theoretical methods have been proposed. This study determines the structure of the isocyanide dipeptide polymer using solid-state NMR spectroscopy. Two-dimensional separated-local-field and double-quantum single-quantum spectroscopic methods have been employed to obtain structural constraints for the polymer backbone. These constraints were used to build a molecular model and subsequently subjected to molecular dynamics simulations. The backbone structure of the polyisocyanide is determined to be a 15(4) helix with hydrogen bonding interactions between n and n + 4 side chains.