Self-assembling ADADA helices formed by hydrogen bonding.

A computational investigation of the electronic properties of an experimentally prepared ADADA helix indicates that the helix is held together with four strong hydrogen bonds as well as many other weak interactions. Determination of the electronic energy changes, as well as thermodynamic parameters, suggests that helix formation is a favorable process, driven by the formation of the hydrogen bonds. For instance, the unsubstituted helix has an electronic binding energy of -85.8 kJ/mol. Furthermore, the strength of binding can be tuned to some extent by the careful selection of substituents. The hydrogen bonds are strengthened when the pyridine ring (H-bond acceptor) is substituted with an electron-donating group such as an amine, while electron-withdrawing groups on the thiazine ring (H-bond donor) are preferred. The most significant enhancement in binding is achieved when the helix is constructed from monomers that consist of contiguous hydrogen-bond acceptors or donors. This so-called AAAAA-DDDDD helix exhibits a binding energy almost 4-fold greater than that of the unsubstituted ADADA helix at -335.4 kJ/mol, a dramatic improvement over the ADADA helix.