Tritopic Bis-Urea Receptors for Anion and Ion-Pair Recognition.

This work reports on the synthesis and characterization of three tritopic receptors and their binding properties toward various anions, as their tetrabutylammonium salts, and three alkali metal-acetate salts by UV-vis, fluorescence, 1H, 7Li, 23Na, and 39K NMR in MeCN/dimethyl sulfoxide (DMSO) 9:1 (v/v). Molecular recognition studies showed that the receptors have good affinity for oxyanions. Furthermore, these compounds are capable of ion-pair recognition of the alkali metal-acetate salts studied through a cooperative mechanism. Additionally, molecular modeling at the density functional theory (DFT) level of some lithium and sodium acetate complexes illustrates the ion-pair binding capacity of receptors. The anion is recognized through strong hydrogen bonds of the NH- groups from the two urea sites, while the cation interacts with the oxygen atoms of the polyether spacer. This work demonstrates that these compounds are good receptors for anions and ion pairs.

Electrophilic assistance to the cleavage of an RNA model phopshodiester via specific and general base-catalyzed mechanisms.

Kinetics of transesterification of the RNA model substrate 2-hydroxypropyl 4-nitrophenyl phosphate promoted by Mg(2+) and Ca(2+), the most common biological metals acting as cofactors for nuclease enzymes and ribozymes, as well as by Co(NH(3))(6)(3+), Co(en)(3)(3+), Li(+), and Na(+) cations, often employed as mechanistic probes, was studied in 80% v/v (50 mol %) aqueous DMSO, a medium that allows one to discriminate easily specific base (OH(-)-catalyzed) and general base (buffer-catalyzed) reaction paths. All cations assist the specific base reaction, but only Mg(2+) and Na(+) assist the general base reaction. For Mg(2+)-assisted reactions, the solvent deuterium isotope effects are 1.23 and 0.25 for general base and specific base mechanisms, respectively. Rate constants for Mg(2+)-assisted general base reactions measured with different bases fit the Brønsted correlation with a slope of 0.38, significantly lower than the slope for the unassisted general base reaction (0.77). Transition state binding constants for catalysts in the specific base reaction (K(‡)(OH)) both in aqueous DMSO and pure water correlate with their binding constants to 4-nitrophenyl phosphate dianion (K(NPP)) used as a minimalist transition state model. It was found that K(‡)(OH) ≈ K(NPP) for "protic" catalysts (Co(NH(3))(6)(3+), Co(en)(3)(3+), guanidinium), but K(‡)(OH) ≫ K(NPP) for Mg(2+) and Ca(2+) acting as Lewis acids. It appears from results of this study that Mg(2+) is unique in its ability to assist efficiently the general base-catalyzed transesterification often occurring in active sites of nuclease enzymes and ribozymes.