Complexes of Alkaline and Ammonium Cations with Dopamine and Eumelanin Precursors: Dissecting the Role of Noncovalent Cation−π and Cation–Lone Pair (σ-Type) Interactions

Abstract

Cation−π interactions and their possible competition with other noncovalent interactions (NCI) might play a key role in both dopamine-and eumelanin-based bioinspired materials. In this contribution, to unravel the delicate interplay between cation−π interactions and other possible competing forces, the configurational space of noncovalent complexes formed by dopamine or eumelanin precursors (o-benzoquinone, DHI and a semiquinone dimer) and three different cations (Na + , K + , and NH 4 + ) is sampled by means of accurate ab initio calculations. To this end, we resort to the mp2 mod method, recently validated by us for benzene−, phenol−, and catechol−cation complexes, whose computational convenience allows for an extensive exploration of the cation−molecule interaction energy surface, by sampling a total of more than 10 4 arrangements. The mp2 mod interaction energy landscapes reveal that, besides the expected cation−π driven arrangements, for all considered molecule−cation pairs the most stable complexes are found when the cation lies within the plane containing the six-membered ring, thus maximizing the σ-type interaction with the oxygen's lone pairs. Due to the loss of aromaticity, the σ-type/cation−π strength ratio is remarkably large in obenzoquinone, where cation−π complexes seem unlikely to be formed. The above features are shared among all considered cations but are significantly larger when considering the smaller Na + . Besides delivering a deeper insight onto the NCI network established by the considered precursors in the presence of ions, the present results can serve as a reference database to validate or refine lower level methods, as, for instance, the force fields employed in classical simulations.

Solvent effects on catechol's binding affinity: investigating the role of the intra-molecular hydrogen bond through a multi-level computational approach

“…17 Furthermore, the possible rotation of one or both hydroxyl groups was found to play an important role also in aggregate formation, with stacked catechol dimers being enforced by a network of several HBs, settled between the two monomers. 28 Finally, in more recent computational studies carried out by some of us, 29–31 the formation/breaking of internal HBs in catechol was found to be decisive in balancing cation–π and chelating σ-type interactions between catechol and neighboring metal and organic ions. As a matter of fact, even reducing the “dimensionality” of the problem to a single catechol molecule in the gas phase or solvated in a simple liquid, thus not considering other players as large bio-molecules, surfaces, ions and possible aggregates, the details of the interplay between solute's structure and solvation patterns remain not completely clear and therefore a subject of vibrant research.…”