Complexes of Alkaline and Ammonium Cations with Dopamine and Eumelanin Precursors: Dissecting the Role of Noncovalent Cation-pi and Cation-Lone Pair (sigma-Type) Interactions

Cation-pi 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-pi 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 NH4+) is sampled by means of accurate ab initio calculations. To this end, we resort to the mp2mod 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 104 arrangements. The mp2mod interaction energy landscapes reveal that, besides the expected cation-pi 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 sigma-type interaction with the oxygen's lone pairs. Due to the loss of aromaticity, the sigma-type/cation-pi strength ratio is remarkably large in o-benzoquinone, where cation-? 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.