Dynamic anion recognition by macrocyclic polyamines in neutral pH aqueous solution: development from static anion complexes to an enolate complex

Abstract

Multiprotonated macrocyclic polyamines are useful host molecules for anion guests at neutral pH in aqueous solution. An intramolecular uracil anion complex with a diprotonated macrocyclic tetraamine recently provided a unique example of electrostatic stabilization of the uracil N¹ anion at neutral pH, which may be relevant to the facile glycosylation and deglycosylation of uracil at N¹ in DNA. Macrocyclic polyamine complexes with Zn²⁺ possess strong anion affinities and hence can deprotonate weak acids at neutral pH to bind with the resulting conjugate bases: e.g. H₂O → HO^–, ROH → RO^–, ArSO₂NH₂ → ArSO₂NH^–, RCONHR′ → RCON^–R′, RCONHCOR′ → RCON^–COR′. The Zn²⁺–conjugate base complexes act as catalytic nucleophiles (i.e. HO^––Zn²⁺, RO^––Zn²⁺), fluorescence sensors (ArSO₂NH^––Zn²⁺), and thymine or barbital recognition hosts, which are often found in zinc–enzyme functions. Enolate anion complex formation has recently been observed in intramolecular interaction of carbonyl oxygen with Zn²⁺.