Isotopically induced variation in the stability of FMN-wrapped carbon nanotubes.

Isotopic, hydrogen-to-deuterium substitution has been an invaluable tool in the characterization of small molecules and biological nanostructures. The natural variability of most inorganic nanomaterials has hindered the use of isotopic substitution in gaining meaningful insights into their structure. The ideal helical wrapping of a flavin mononucleotide (FMN) around (8,6)-SWNTs (single-walled carbon nanotubes) is presently utilized to probe isotopically dependent intermolecular interactions. The facile proton-to-deuterium exchange of the imide group of FMN enabled us to alter the intermolecular stability of the helix depending on the surrounding solvent (i.e., H2O vs D2O). Our studies show that FMN-dispersed (8,6)-SWNTs exhibit greater stability in D2O than in H2O. The higher complex stability in D2O was verified on the basis of (i) FMN helix replacement with SDBS (sodium dodecylbenzenesulfate) and (ii) thermal- and (iii) pH-induced helix dissociation. This is in agreement with the previously observed stronger amide H-bonding of proteins in D2O, and to the best of our knowledge, it demonstrates the architectural fidelity of FMN-wrapped SWNTs, which is expected to enhance the assembly repertoire of carbon nanotubes further.