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

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 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−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 o-benzoquinone, 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.