Resolving the Regioregularity of Poly(N-n-hexyl-N'-phenylcarbodiimide) via Nitrogen-15 Labeling

Synthesis of pyrimidines from dinitrogen and carbon

The element nitrogen and nitrogenous compounds are vital to life. The synthesis of nitrogen-containing compounds using dinitrogen as the nitrogen source, not through ammonia, is of great interest and great value but remains a grand challenge. Herein, we describe a strategy to realize this transformation by combining the heterogeneous approach with the homogeneous methodology. The N₂ molecule was first fixed with carbon and LiH through a one-pot heterogeneous process, forming Li₂CN₂ as an 'activated' nitrogen source with high efficiency. Then subsequent homogeneous treatments of Li₂CN₂ to construct the organic synthon carbodiimide and the RNA/DNA building block pyrimidines were fulfilled. By using ¹⁵N₂ as the feedstock, their corresponding ¹⁵N-labeled carbodiimide and pyrimidines were readily obtained. This homogeneous-heterogeneous synergy strategy will open a new chapter for N₂ transformation.

Novel approaches in selective tryptophan isotope labeling by using Escherichia coli overexpression media

NMR-based investigations of large protein complexes require optimized isotopic labeling schemes. We report new methods to introduce stable isotopes into tryptophan residues; these are fine-tuned to the requirements of the particular protein NMR experiment. Selective backbone labeling was performed by using a new α-ketoacid precursor as an additive in cell-based overexpression media. Additionally, we developed synthetic routes to certain isotopologues of indole with (13)C-(1)H spin systems surrounded by (12)C and (2)H. The corresponding proteins, overexpressed in the presence of these precursor compounds, can be effectively analyzed for conformational changes in tryptophan residues in response to external stimuli, such as interaction with other proteins or small molecules.

Helical polycarbodiimide cloaking of carbon nanotubes enables inter-nanotube exciton energy transfer modulation

The use of single-walled carbon nanotubes (SWCNTs) as near-infrared optical probes and sensors require the ability to simultaneously modulate nanotube fluorescence and functionally derivatize the nanotube surface using noncovalent methods. We synthesized a small library of polycarbodiimides to noncovalently encapsulate SWCNTs with a diverse set of functional coatings, enabling their suspension in aqueous solution. These polymers, known to adopt helical conformations, exhibited ordered surface coverage on the nanotubes and allowed systematic modulation of nanotube optical properties, producing up to 12-fold differences in photoluminescence efficiency. Polymer cloaking of the fluorescent nanotubes facilitated the first instance of controllable and reversible internanotube exciton energy transfer, allowing kinetic measurements of dynamic self-assembly and disassembly.