Unraveling the Hidden Role of the Counteranion in “Cation in a Cage” Systems

Abstract

The current work showcases general principles at play in systems consisting of cations present inside molecular cages. Such systems, relevant to chemistry and biology, have been carefully investigated by computational methods. The important Ge(II)-encapsulating cage systems have been studied first. The very fact that such compounds exist appears highly unlikely, given the highly reactive nature of the Ge(II) dication. Our studies reveal what really occurs in solution when such complexes are formed: the Ge(II) dications are actually present as [Ge−X] + (where X is the "non-coordinating" counterion employed in such systems) during entry and subsequent existence at the center of the cage. Hence, what is actually present is a "pseudomonocation". Interestingly, such pseudomonocationencapsulated cages are seen to be equally relevant in systems of biological importance, such as for dicationic s block-based ionophores. In explaining such cases, the concept of "isoionicity" is introduced, demonstrating that the counterion-coordinated dications are isoionic with a monocation, such as Li(I), isolated in the same ionophore.

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“…. ., An, in relative importance [21]. First, during a pairwise comparison, the decision maker must repeatedly answer the question of which of the two elements is more important and which is more important for the Ck criterion.…”

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“…Valinomycin (Figure 2) is an ionophore that selectively binds the K + cation over the Na + (▵▵E binding =5.7 kcal/mol) via predominantly electrostatic stabilization, as validated by the calculated Wiberg Bond Index (WBI) values (Table S6, ESI), for transport across membranes. In our earlier work, we had shown that counter anions present in biological systems (like Cl − ) can remain coordinated to a dication inside ionophore cages [41] . In the case of the valinomycin ionophore, counter anions (ClO 4 − ), along with the Ba 2+ dication, have been characterized experimentally to be present inside a synthetic cage complex [42] .…”

“…In our earlier work, we had shown that counter anions present in biological systems (like Cl À ) can remain coordinated to a dication inside ionophore cages. [41] In the case of the valinomycin ionophore, counter anions (ClO 4 À ), along with the Ba 2 + dication, have been characterized experimentally to be present inside a synthetic cage complex. [42] Here, we have examined the possibility of a similar scenario for the valinomycin ionophore, where counter anions present in biological systems (like Cl À ) would remain inside the macrocycle along with the pertinent bound cations, like K + and Na + .…”

“…56 NBO, Mulliken and Merz‐Kollman (MK) charges were used to calculate the net electrostatic force on the anionic chloride center [57] . The net force was calculated according to the protocol described in a previous report [41] . Furthermore, the strength of the electric field was calculated by employing the TITAN package, using the calculated NPA charges as the input [58] .…”

“…[57] The net force was calculated according to the protocol described in a previous report. [41] Furthermore, the strength of the electric field was calculated by employing the TITAN package, using the calculated NPA charges as the input. [58] All the Wiberg bond indices (WBI) reported are the maximum values reported between the central cation (K + and Na + ) and the heteroatoms (N, Cl and O).…”