Thermodynamics of the Quasi-Epitaxial Flavin Assembly around Various-Chirality Carbon Nanotubes

Establishing methods to accurately assess and model the binding strength of surfactants around a given-chirality single-walled carbon nanotube (SWNT) are crucial for selective enrichment, targeted functionalization, and spectrally sharp nanodevices. Unlike surfactant exchange, which is subject to interferences from the second surfactant, we herein introduce a thermal dissociation method based on reversible H+/O-2 doping to determine SWNT/surfactant thermodynamic stability values with greater fidelity. Thermodynamic values were reproduced using molecular mechanics augmented by ab initio calculations in order to better assess g pi-pi interactions. This afforded detailed quantification of the flavin binding strength in terms of pi-pi stacking (55-58%), with the remaining portion roughly split 3:1 between electrostatic plus van der Waals flavin mononucleotide (FMN) interdigitation and H-bonding interactions, respectively. Quasi-epitaxial pi-pi alignment between the near-armchair FMN helix and the underlying nanotube lattice plays a crucial role in stabilizing these assemblies. The close resemblance of the thermal dissociation method to helix-coil and ligand-binding transitions of DNA opens up a unique insight into the molecular engineering of self-organizing surfactants around various-chirality nanotubes.