Simulations of Nanocylinders Self-Assembled from Cyclic β-Tripeptides

This paper examines the self-assembly of cyclic β-tripeptides using density functional theory. On the basis of literature precedents, these cyclic peptides were expected to self-assemble into cylindrical structures by stacking through backbone−backbone hydrogen bonding. Our calculations show that such stacking is energetically favorable, that the association energy per cyclic peptide decreases (becomes more favorable), and that the overall macrodipole moment of the cylindrical assembly increases with the number of stacked rings, for up to eight rings. For a structure in which two peptide ring units are joined through a single side chain−side chain covalent linker, the association energy between the two rings is favorable, albeit less so than for the unlinked rings. Significantly, the association energy in the dimers is only weakly dependent on the length (above a certain minimum) and conformation of the covalent linkers. Finally, as a plausible route for controlling assembly/disassembly of nanocylinders, we show that, for a pair of rings, each bearing a single amino-functionalized side chain, protonation of the amino group results in a strongly positive (unfavorable) association energy between the two rings.